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PRACTICAL INJECTION MOLDING

PLASTICS ENGINEERING Founding Editor

Donald E. Hudgin

Professor Clernson University Clernson, South Carolina

1. Plastics Waste: Recovery of Economic Value, Jacob Leidner 2. Polyester Molding Compounds, RobertBums of Electrically Conducting 3. Carbon Black-Polymer Composites: The Physics Composites, edited byEnid Keil Sichel 4. The Strength and Stiffness of Polymers, editedby Anagnostis E. Zachariades and RogerS. Porter 5. Selecting Thermoplastics for Engineering Applications, Charles P. MacDemott 6. Engineering with Rigid PVC: Processability and Applications, edited byl. Luis Gomez 7. Computer-Aided Designof Polymers and Composites, D. H. Kaelble 8. EngineeringThermoplastics: Properties and Applications, edited by James M. Margolis 9. Structural Foam: A Purchasing and Design Guide, BruceC. Wendle IO. Plastics in Architecture: AGuide to Acrylic andPolycarbonate,Ralph Montella 11. Metal-Filled Polymers: Properties and Applications, editedby Swapan K. Bhattachatya 12. Plastics Technology Handbook, ManasChanda and Salil K. Roy 13. Reaction Injection Molding Machinery and Processes,F. Melvin Sweeney 14. Practical Thermoforming: Principles and Applications, John Florian 15. Injection andCompressionMoldingFundamentals,edited by Avraam l. lsayev 16. Polymer Mixing andExtrusion Technology, NicholasP. Chefernisinoff 17. High Modulus Polymers: Approaches to Design and Development, edited by Anagnostis E. Zachariades and RogerS. Porter 18. Corrosion-Resistant Plastic Composites in Chemical Plant Design, John H. Mallinson 19. Handbook of Elastomers: New Developments and Technology, edited by Ani/ K. Bhowrnick and HowardL. Stephens 20. RubberCompounding: Principles, Materials, andTechniques, Fred W. Barlow 21. Thermoplastic Polymer Additives: Theory and Practice, edited by John T. Lutz, Jr. 22. Emulsion Polymer Technology, RobertD. Athey, Jr. 23. Mixing in Polymer Processing, editedby Chris Rauwendaal

24.HandbookofPolymerSynthesis,Parts A and B, edited byHans R. Kricheldorf 25. Computational Modelingof Polymers, edited by Jozef Bicerano 26.PlasticsTechnologyHandbook:SecondEdition,RevisedandExpanded, Manas Chanda and SalilK. Roy 27. Prediction of Polymer Properties, Jozef Bicerano 28. Ferroelectric Polymers: Chemistry, Physics, and Applications, edited by Hari Singh Nalwa 29. DegradablePolymers,Recycling,andPlasticsWasteManagement, edited by Ann-Christine Albertsson and Samuel J. Huang 30. Polymer Toughening, edited by Charles B. Arends 31. Handbook of Applied Polymer Processing Technology,edited by Nicholas P. Cheiemisinoff and PaulN. Cheremisinoff 32. Diffusion in Polymers, edited by P. Neogi 33. Polymer Devolatilization,edited by Ramon J. Albalak 34. Anionic Polymerization: Principles and Practical Applications,Henry L. Hsieh and Roderic P. Quirk 35. Cationic Polymerizations: Mechanisms, Synthesis, and Applications, edited by Krzysztof Matyjaszewski 36. Polyimides: Fundamentals and Applications, edited by Malay K. Ghosh and K. L. Mittal 37. Thermoplastic Melt Rheology and Processing,A. V. Shenoy and D. R. Saini 38.PredictionofPolymerProperties:SecondEdition,RevisedandExpanded, Jozef Bicerano 39. Practical Thermoforming: Principles and Applications, Second Edition, Revised and Expanded,John Florian 40. MacromolecularDesign of PolymericMaterials, edited by Koichi Hatada, Tatsuki Kitayama, and Otto Vogl 41. Handbook of Thermoplastics, edited by Olagoke Olabisi 42.SelectingThermoplasticsforEngineeringApplications:SecondEdition, Revised and Expanded, Charles P. MacDermott and Aroon V. Shenoy 43. Metallized Plastics: Fundamentals and Applications, edited by K. L. Mittal 44. Oligomer Technology and Applications,Constantin V. Uglea 45. Electrical and Optical Polymer Systems: Fundamentals, Methods, and Applications, edited by Donald L. Wse, Gary E. Wnek, Debra J. Trantolo, Thomas M. Cooper, and Joseph D. Gresser 46. Structure and Properties of Multiphase Polymeric Materials, edited by Takeo Araki, Qui Tran-Cong, and Mitsuhiro Shibayama 47. Plastics Technology Handbook: Third Edition, Revised and Expanded, Manas Chanda and Salil K. Roy 48. Handbook of Radical Vinyl Polymerization, Munmaya K. Mishra andYusuf Yagci 49. Photonic Polymer Systems: Fundamentals, Methods, and Applications, edited by Donald L. Wse, Gary E. Wnek, Debra J. Trantolo, Thomas M. Cooper, and Joseph D. Gresser 50. Handbook of Polymer Testing: Physical Methods, edited by Roger Brown 51. Handbook of Polypropylene and Polypropylene Composites, edited by Harutun G. Karian

52. Polymer Blends and Alloys, edited by Gabriel 0. Shonaike and George P. Simon 53. Star and Hyperbranched Polymers,edited by Munmaya K. Mishra and Shiro Kobayashi 54. Practical Extrusion Blow Molding, edited by Samuel L. Belcher 55. PolymerViscoelasticity:StressandStraininPractice, Evaristo Riande, Ricardo Diaz-Calleja, Margarita G. Prolongo, Rosa M. Masegosa, and Catalina Salom 56. Handbook of Polycarbonate Science and Technology,edited by Donald G. LeGrand and John T. Bendler 57. Handbook of Polyethylene: Structures, Properties, and Applications, Andrew J. Peacock 58. Polymer and Composite Rheology: Second Edition, Revised and Expanded, Rakesh K. Gupta 59. Handbook of Polyolefins: Second Edition, Revised and Expanded, edited by Cornelia Vasile 60. Polymer Modification: Principles, Techniques, and Applications, edited by John J. Meister 61. Handbook of Elastomers: Second Edition, Revised and Expanded, edited by Ani1 K. Bhowmick and Howard L. Stephens 62. Polymer Modifiers and Additives, edited by John T. Lufz, Jr., and Richard F. Gmssman 63. Practical Injection Molding,Bernie A. Olmsfed and Martin E. Davis

Additional Volumes in Preparation

PRACTICAL INJECTION MOLDING

Bernie A. Olmsted Consultant Springfield, Massachusetts

Martin E. Dauis Consultant Prescott, Arizona

m MARCEL

D E K K E R

MARCEL DEKKER, INC.

-

NEWYORK BASEL

ISBN: 0-8247-0529-7 This book is printed on acid-free paper.

Headquarters Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 100l6 tel: 2 12-696-9000;fax: 2 12-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4,Postfach 8 12, CH-400I Basel, Switzerland tel: 41-6 1-26 1-8482; fax: 4 1-6 1-26 1-8896 World Wide Web http://www.dekker.com The publisher offers discounts on thls book when ordered In bulk quantities. For more Information, wrlte to Special SaledProfesslonal Marketing at the headquarters address above.

Copyright 02001 by Marcel Dekker, Inc. All Rights Reserved. Nelther this book nor any part may be reproduced or transmitted In any form or by any means, electronlc or mechanlcal, including photocopying, microfilming, and recording, or by any mformatlon storage and retrieval system, wlthout permlsslon I n writmg from the publisher. Current printing (last diglt): I O 9 8 7 6 5 4 3 2 1

PRINTED IN THE UNITED STATES OF AMERICA

Foreword The Society of Plastics Engineers is pleased to sponsor Practical Injection Molding by BernieA. Olmsted and MartinE. Davis. Practical Injection Molding provides fundamental, a yet comprehensive, coverage of injection molding concepts. A practical, yet state-of-the-art, approach is used throughout. Theory is presented in such a fashion that the reader will gain a sound understanding of the basic principles. Case studies, drawings and charts are used very well to further illustrate the point.

The authors have kept true to their audience and have touched on each important aspect without getting into too much detail. SPE, through its Technical Volumes Committee, has long sponsored books on various aspects of plastics. Its involvement has ranged from identification of needed volumes and recruitmentof authors to peer review and approval and publicationof new books.

Technical competence pervades allSPE activities, not only in the publication of books but also in other areas such as sponsorship of technical conferences and educational programs. Michael R. Cappelletti Executive Director Society of Plastics Engineers Technical Volumes Committee: Robert C. Portnoy, Chairperson

...

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“Why read this book?” The purpose of this book is to provide operating personnel, those with “hands on” responsibilities, with a better understanding of the basics of injection molding. The resulting benefit is a more informed group of people doing a betterjob of molding plastic parts and, equally important, enjoying the successof their improved accomplishments. The mechanics,operatorsandset-uppersonnelinmolding operations are in key positions to control product quality and improve operating performance. Unfortunately, many of these people have not had the benefit of formal training nor the opportunity to attend seminars or workshops that would enhance their ability to perform job. theirMost have learned fiom their predecessors who probably had even fewer educational opportunities.Productqualityandoutputmayimprovedramatically through the educationof this group of personnel and . . . . . there may even be increased enlightenmenton the part of their supervisors and managers. Inadditiontothosepeopleinvolvedactivelyin the injection molding process, there are others who could also benefit fiom the book. Sales personnel responsible for providingthe plastics materials, molding machineryandauxiliaryequipmentmayfindthisbookbeneficialin rounding out their knowledgeof the entire molding environment. Vo-tech instructors, and perhaps even college professors, may view this book as a good overall reference for understanding the molding process. There are many factors that influence the successful molding of a finished product in additionto an understanding of the injection molding machine itself. They include a basic knowledge of the raw materials, the plastic pellets, which are converted from a solid to a melt and back to a solid product with various shapes and features. Additives are combined with the plastic pellets to produce a certain cosmetic appearance or provide increased mechanical properties in the finished product. It is also vital to understand how the plasticis melted and the role playedby the screw, the barrel and the machine’s temperature controls. The mold, with cavities and cores that form the shape of the finished product, is a critical element in the V

v1

molding of apartthat is freefromdefectsandachievesdimensional integrity. Most injection molding machines manufactured today utilize sophisticated electronic controls that enable the successful molding of a productwith minimum a of manualintervention. The operator’s understanding of these controls and how they automate the production process can make the difference between a profit or loss in this highly competitive industry. This book will deal with each of these factors, and others, in a manner that is straightforward and easily understood. Case studies from actual molding experiences will help to enhance the understanding of the material in someof the chapters. We want this book be to a practical guide for those involved in injection molding and not a highly technical reference for those with advanced plastics technology backgrounds or education. Bernie A. Olmsted / Martin E. Davis

Acknowledgments

I wish to express my thanks to those people who have made this book a reality. First, thanks to my wife, Barbara, for her patience, encouragement and help with the typing. Thanks also to my daughter Cynthia and her husband, Philip J. Mayher, for the editing and much of the computer work. And thanks, too, to my daughter, Nola, whose initial help on the computer kept me going while she offered constant encouragement. Her husband, Robert Reis, was a help with a review of much of the material. And sincere thanks toMartinDavis of WestlandCorporationfor his interest, encouragement and for his willingness to co-authorthe book with me.

Bernie A. Olmsted

I have known Bernie for a number of years during which he has educatedmeinthepracticalapplication of technicalknowledgetothe fundamentals of injectionmolding.Hisdepth of experienceandbroad knowledge of the subject have been helpful to me personally and to the company I founded, Westland Corporation. I am pleased to contributeto the completion of the book in the hopes that it will offer newcomers to our industry, and those of us in the industry who are still learning, a practical guide to the basicsof injection molding. Many thanks to several friends who critically edited the book, including Dave Larson, President of Westland Corporation, Robert L. Reis, CQE, of GE Plastics, and Virgil Rhodes, a Senior Mold Builder for a well regarded tooling firm. The CAD engineers atWestlandCorporation,especiallyWayneHook,helpedwiththe illustrations, for which we are most grateful. A special thanks to John W. Bozzelli of Injection Molding Solutions forhis detailed review. His efforts have contributed significantly to the technical validity of the book. Most important, I must thankmywife,Vicki,who has patientlyhelped me participate in this effort.

Martin E. Davis

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Contents 1 Introduction ........................................................................................... 1.1 Elements of the Injection Molding Process............................ 1.2 Summarizing the Elements ..................................................... 2 Plastics.................................................................................................. 2.1 Thermoplastics........................................................................ 2.2 Thermosets.............................................................................. 2.3 Crystalline vs. Amorphous Materials.................................... ............................. 2.4 How Plastics Affect the Molding Process 2.4.1 Melting Characteristics .......................................... 2.4.2 Thermal Conductivity............................................ 2.4.3 Shear Sensitivity.................................................... 2.4.4 Viscosity (Melt Index)........................................... 3 Additives............................................................................................. 3.1 Fillers and Reinforcements................................................... 3.2 Plasticizers............................................................................ 3.3 Stabilizers.............................................................................. 3.4 Flame Retardants .................................................................. 3.5 Colorants............................................................................... 3.6 Adding the Additives ............................................................ 4 Loaders and Dryers............................................................................. 4.1 Hopper Loaders and Conveying Systems............................. 4.2 Dryers.................................................................................... 4.2.1 Hot Air Dryers....................................................... 4.2.2 Dessicant Dryers .................................................... 5 hjection Unit ...................................................................................... 5.1 The Barrel ............................................................................. .............................................................. 5.2 End Cap and Nozzle 5.3 Heater Bands......................................................................... 5.4 Non-Return Valve................................................................. 5.5 Screw..................................................................................... 5.5. I Length-to-Diameter Ratio......................................

1 1 6 8 8 9

10 11 11 12 13 14 16 16 17 17 18 18 18 21 21 22 22 23 26 29 30 33 34 38 39

1x

X

5.5.2 Screw Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.5.3CompressionRatio . . . . . . . . . . . . . . . . . 40 5 S.4 Helix Angle . . . . . . . . . . . . . . . . . . . . . . . 41 5.5.5 Drive Design . . . . . . . . . . . . . . . . . . . . . . 41 5.6 Injection Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 45 6ClampUnit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Hydraulic Clamp System . . . . . . . . . . . . . . . . . . . . . . . . . . 46 6.2 Hydro-Mechanical System . . . . . . . . . . . . . . . . . . . . . . . . . 47 6.3 Clamp Unit Specifications . . . . . . . . . . . . . . . . . . . . . . . . . 50 51 6.4 Ejector System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 7.1MoldComponents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 7.2 Types of Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 7.2.1 Cold Runner Molds . . . . . . . . . . . . . . . . . . . . . . . 62 7.2.2 Hot Runner Molds . . . . . . . . . . . . . . . . . . . . . . . . 64 7.2.3 Other Mold Types . . . . . . . . . . . . . . . . . . . . . . . . 66 7.3 Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 7.4 Projected Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 7.5 Mold Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 7.6 Mold Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 8Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 8.1 Processing Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 8.2 Control Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 8.3 ProcessControl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 9 Robotics and Granulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 9.1Robotics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 9.2 Granulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 10 Getting 10.1 Mold Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 10.2 ProcessMethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 10.3 Process Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 10.4 Mold Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 10.5 ProcessDocumentation . . . . . . . . . . . . . . . . . . . . . . . . . . 95 11AnOverview - The Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 11. l The Cycle - Defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 11.2 The Importance of Cycle Time . . . . . . . . . . . . . . . . . . . . 98

xi l 1.3 The Greater Importance of “Good Production” . . . . . . . 100 12 The Ten Keys to Successful Molding . . . . . . . . . . . . . . . . . . . . . . 102 102 12.1 Adequate Mold Venting . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Proper Mold Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 12.3 Using the Right Screw . . . . . . . . . . . . . . . . . . . . . . . . . . 106 12.4 Selecting the Appropriate Valve . . . . . . . . . . . . . . . . . . 111 113 12.5 Controlling the Heat Profile . . . . . . . . . . . . . . . . . . . . . . 12.6 Using Back Pressure Wisely . . . . . . . . . . . . . . . . . . . . . 119 120 12.7 Controlling the Injection Rate . . . . . . . . . . . . . . . . . . . . 12.8 Managing Screw RPM and Residence Time . . . . . . . . . 124 128 12.9PerformanceMeasurement . . . . . . . . . . . . . . . . . . . . . . 12.10 Preventive Maintenance Program . . . . . . . . . . . . . . . . 130 13 Thermoset Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 132 13.1ThermosetMaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2MachineModifications . . . . . . . . . . . . . . . . . . . . . . . . . 134 13.3 Processing Modifications . . . . . . . . . . . . . . . . . . . . . . . . 135 137 14Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.1 Safety Requirementsof ANSI B 15 1.1. . . . . . . . . . . . . . 137 14.2 Safety Rules to Follow . . . . . . . . . . . . . . . . . . . . . . . . . . 140 143 15 Recognizing Molding Problems . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1 Process Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 15.1.l Brittleness of Parts . . . . . . . . . . . . . . . . . . . . . 143 15.1.2 Bubbles and Voids . . . . . . . . . . . . . . . . . . . . . 144 145 15.1.3BurnedMaterial . . . . . . . . . . . . . . . . . . . . . . . 15.1.4 Cloudy or Hazy Parts . . . . . . . . . . . . . . . . . . . 146 15.1.5 Drool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 15.1.6Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 149 15.1.7 Flow Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.8 Gate Blush . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 15.1.9 Inadequate Color Mixing . . . . . . . . . . . . . . . . 150 15.1.10Jetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 15.1.11 Knit Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 15.1.12 Part Deformation . . . . . . . . . . . . . . . . . . . . . . 153 15.1.13 Poor Screw Recovery . . . . . . . . . . . . . . . . . . 154 15.1.14 Short Shots . . . . . . . . . . . . . . . . . . . . . . . . . . 155

xii 15.1.15SinkMarks . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.16Splay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.17 Warped Parts . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 Component Wear Problems . . . . . . . . . . . . . . . . . . . . . . 16Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 OtherMoldingNotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.1SpecialMoldingProcesses . . . . . . . . . . . . . . . . . . . . . . 17.1.1 TwoColorMolding . . . . . . . . . . . . . . . . . . . . 17.1.2TurretMolding . . . . . . . . . . . . . . . . . . . . . . . . 17.1.3GasAssistMolding . . . . . . . . . . . . . . . . . . . . 17.1.4PowderInjectionMolding . . . . . . . . . . . . . . . 17.1.5IntrusionMolding . . . . . . . . . . . . . . . . . . . . . . 17.1.6 Other Molding Processes . . . . . . . . . . . . . . . . 17.2 Molding Operation Items . . . . . . . . . . . . . . . . . . . . . . . .

156 157 158 159 165 186 186 186 188 189 189 191 191 192

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 A InjectionMoldingMaterials . . . . . . . . . . . . . . . . . . . . . . . 194 B Properties of Common Plastics . . . . . . . . . . . . . . . . . . . . . 196 C Recommended Plastic Drying Data . . . . . . . . . . . . . . . . . . 199 DUsefulData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 Abbreviations and Symbols . . . . . . . . . . . . . . . . . . . . 200 Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 E Velocity Control on Injection Molding Machines . . . . . . . 203 F Procedure for Application of Bolt Torque on NozzleAdaptors(EndCaps) . . . . . . . . . . . . . . . . . . 209 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

211

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

212

1 Introduction This book is intended to provide the “hands on” injection molding personnel,themachineoperators,techniciansandmechanics,withan improved understanding of the basics of injectionmolding.Although injection molding has been in use in the United States since the1930’s, the operatingpersonnelhavegenerallylearnedtheprocessfromtheir supervisors,whoeitherlearnedfromtheirpredecessors or gained the knowledge by “trial and error.” There have been few seminars, workshops or books that have offered educational opportunities to the operator withno college degree or formal training in plastics technology. This book will explore the elements of the molding process at the most basic levelthe in hope that it will contribute to the productivity job andsatisfaction of those “hands on’’ personnel. The residual benefit to the managers and owners of molding operations . . . . improved profitability.

1.1 Elements of the Injection Molding Process The elements involvedin illustrated in Figure 1.

the injectionmoldingprocessare

-

Injection Unit

Clamp Unit

Mold

i

Plastic I -

Controls Unlt

Temp Controller

Figure l

Grlnder

Injection molding elements.

2

When persons not previously acquainted with the injection molding process first view an injection molding unit comprised of all of these elements, they are impressed with all the wires, hoses, tubes and pipes that connect the elements together. It is vital that the operator or technician understand the basicsof how each elementof this seemingly complex unit works and the role each plays in the successful molding of a part. With the exceptionof robotics and additivefeeders, all of the other elements are critical to the injection molding process. Robotics equipment is used in some of the more automated molding operations to provide improved overall efficiency butis not essential tothe successful molding of a part. Additive feeders are used to proportionthe addition of colorants or other additives to the plastic pellets. While the feeder itself does not affect the process (if functioningproperly),thequantityandtype of additives may change some important processing parameters. The injection molding machineitself consistsof the clamp unit, the injection unit, the control unit and a hopper. As we will learn later, the hopper becomes modified to include a loader, dryer and in some cases, an additive feeder.

Figure 2

Schematic of an injection molding machine

The injection molding machine, also referred to as a press, is purchased from a manufacturer, such as Cincinnati Milacron, Van Dorn Demag,

3

HPM and various other U.S. and foreign manufacturers. The machine is illustrated inFigure 2. The diagramshows the clamp unitclosed without a mold. The injection unit heats, melts, pumps and injects the plastic into the mold when the moldis “closed.” The control unitmonitors and,as the name implies,controls the functioning of the injection unit andthe clamp unit. The mold is mounted within the clamp unit and this unit opens the mold to allow plastic parts to be ejected and holds the mold closed when melted plastic is being injected. Each of these units will be the subject of an entire chapter later in the book. The mold is purchased from a mold maker, whose capabilities may include computer-aided-design (CAD) and computer-numericallycontrolled (CNC) milling machinesthat help automate the manufacture of complex molds. Usually these companies are small, employing less than 100 persons andrequire highly competent machinists and engineers. The mold consists of two halves, the core half (or male part shape) and the cavity half (or female part shape). Because the core is made to be a little smaller than the cavity into which it the fits,area between thecore and the cavity represents the part. This area is filled with melted plastic, then cooled and ejected from the mold to become the plastic part, as illustrated in Figure 3. Part

Thefigure is simplifed to illustrate only the formationof the parts. In practice, molds may contain a large number of cavrties and cores and include other mechanisms that permit the formation of quite complex parts.

Figure 3 Illustrative drawing of an injection mold

4

In order to solidify the plastic part in the moldso that it can be removed The cooling is (ejected), it isusuallynecessarytocoolthemold. accomplished by circulating cool water through cooling channels that are machined into the mold itself.The water is cooled by a chiller, which can either be a free standing unit nearby the press or by a partof a temperature controller system that may serve several molds in several injection molding machines.Temperaturecontrollersmaytaketheform of chillers,as described, or in other cases, heating units (for thermosets), heat exchangers, and various typesof water and oil temperature controlling devices. The molder’s raw material, plastic, is usually purchased in pellet form. Eachpellet is aboutthe size of a small kitchen match head or approximately one-eighth of an inch or three millimeters in diameter. In some cases, plastic is purchased in powder form which is a little more coarse than flour but not as coarse as salt. Very few plastics, other than thermoset materials, which willbe discussed later, are bought or used in powder form today.Plastic is typically purchased from the manufacturer or a distributor andis delivered in 55 lb. bags (25 kg), in drums (about200 lbs. or 100 kg), in 1,000 lb. or about450 kg gaylords (big boxes) or in rail cars. Many typesof plustic are available to use in the production of plastic products. Each type offers different mechanical properties in the molded form. Perhaps more importantto the molder, allplastics do not melt in the same way nor at the same temperature, further complicating the molding process.

Moving the plastic from its storage to the pressis the function of the loader. A loader may be as simple as a vacuum powered unitwith hoses that pull pellets from a gaylord and deposit them in the hopper.This unit is called a hopper loader and sits beside the press with hoses that can access gaylords thatarenearby. A loaderunitmaybeascomplicated as a pneumatic materialhandlingsystemthatconnectsalargenumber of injection molding machines to storage silos (large, enclosed metal bins) and

5

distributes a variety of plastics throughout the plant. Loaders or material handling equipment are typically sold by the same type of manufacturers’ representative that sells other auxiliary equipment. Unfortunately, mostplastics are hygroscopic (meaning the plastics are able to absorb and retain moisture). If the moistureis not removed to a certain level, the plastic parts produced will contain cosmetic or structural defects and the injection unit components may suffer corrosive wear. As a result, many plastics are processed through a dryer. There are several of either hot types of dryers available today, but all involve the circulation air or dehumidified hot air through the pellets before they are allowed to enter the injection unit. Some dryers are quite small and fiton top of the to dry press as a part of the hopper assembly and others are large enough the plastic that might feed several machines. These units are typically sold by the manufacturers’ representatives who handle a line of dryers for a particular dryer manufacturer. After the plastic parts are removed (ejected) from the mold, they may be droppedontoa conveyor (orremoved by robot) forfurther transporting to an inspection or packaging area. Conveyors come in all sizes and shapes and help automate the parts movement throughout the plant. In addition to conveying parts, they may also convey runner systems and parts that have defects to a grinder. The grinder is used to grind up the runners (see next paragraph) and defective parts into a form (typically flake-like pieces slightly larger in size than pellets) that be canmixed with new pellets (referred to as virgin material) in quantities up to 50% (and sometimes more) for remelting again into plastic parts. The ground-up material is referred to as regrind. Runners are solid plastic branch-like structures that represent the small channels in the mold through which the plastic must travelto enter the mold cavities. They are ejected along with the plastic parts and may be ground up and recycled into the virgin material the same as defective parts.

6

1.2 Summarizing the Elements Thisintroductorydiscussion of injectionmoldingelements is designed to acquaint the operator with all of the pieces to the molding puzzle. Each element will be discussed in further detail in a succeeding chapter of the book. A knowledge of how the elements fit together should help when the basicsof each element are pursued further.The table on the next page summarizes these elements for the reader’s review.

7 I

I

I

I

I

Element

I

I

Function

I

Plastic

Raw material usedto mold parts

Loader

Transports plastic from storageto machine hopper

Dryer

Removes moisture from plastic before entering the injection unit

Additives ~~~~

l

to the plastic Colorants, lubricants or other ingredients added pellets

~

Additive Feeder

Adds specified quantity of additive (colorant or other additive) to the plastic being fedto injection unit

Injection Unit

Part of the injection molding machlne that heats, melts and injects the plastic into the mold

I

Part of the injection molding machine that contains the mold and holds it closed during injection and opensit when the parts are ejected Control Unit

Part of the injection molding machine that controls all elements of the molding process

Mold

A combination of rectangular tool steel plates that form and support the cavities and cores and allow the injection of melted plastic into the cavities

Chiller

A type of refrigeration unit that cools the water that circulates through the moldto speed the solidificationof the plastic part

Conveyor

Transports finished plastic partsto areas for other operations or to a point where they may packaging and conveys the runners enter a grinder for recycling

Grinder

Grinds runners and defective parts into a regrind that may be added to virgin material and recycled

Robots

Used to pick parts out of the mold or perform movement functions

~~

Injection Molding Elements

2 Plastics Although a technical discussionof plastics is clearly not within the intended scope of this book, anyone attempting to learn the basics of injection molding must fist have an understanding of the raw materials used.There are two types of plastic used in injectionmolding.Most injection molding is performed using thermoplastic material. However, some injection molding uses thermoset material. There is considerable difference between thetwo types.

2.1 Thermoplastics

The majorityof thermoplastics are made fiom petroleum and have the unique physical property of being able to be melted, solidified, and remelted again without signiscantly changing the chemistry of the material (provided they are kept clean and not contaminated). By grinding up the solidified thermoplastic and remelting it, the material can usually be reused, with or without mixingitwithvirgin (unprocessed)material [l]. Dependinguponhowmanytimesandunderwhat conditions the thermoplastic material has been melted and solidified (its heat history), some of its properties may be diminished. As a result, most thermoplastic that is reused (and referred to as regrind) is mixed with virgin material where the regrind represents less than 50% of the resulting mixture. There are some cases, however, where plastic products are molded fiom 100% regrind. These are instances where the mechanical andor the cosmetic properties of the resultingparts are not critical. Thermoplasticstypically have long propernames that relateto their basic chemistry type. The chemistry ofthermoplasticsis rather complex and may be studied firther by reference to some excellent books[2]. Persons who are not acquainted with the chemistryof plastics or how plastics are manufactured can refer to them by their “short name.” All plastics have been given an alphabeticsymbol that is a “short name” for each plastic’s longertechnicalname.Someofthemorecommonplasticsandtheir symbols include the following.

Sym Long Name

Polyethylene Polypropylene Polystyrene Acrylonitrile Butadiene Styrene Polyamide (nylon) Polycarbonate Polymethylmethacrylate (acrylic) Polyoxymethylene (acetal) Polyvinylchloride Styrene Acrylonitrile

bo1

PE PP PS ABS PA PC PMMA POM PVC SAN

Many of the long names begin with “poly,” which simply means many or multiple. The smallest repeating unitin the chemical structure of plastics(which is amolecule) is called a monomer. Whenseveral monomers are combined or joined, the resulting chemicalstructure is called apolymer. So it becomes clear that polyethylene, polypropylene and the others beginningwith the letters “poly”areplasticsconsisting of a combination of monomers.A fiuther understanding of the molecular structure ofplastics is not required, but is available in several references[3]. A listingof the more common thermoplastics, showing their proper name and symbol (and other important data) is included at the conclusion of this book as Appendix A.

2.2 Thermosets Thermosets are plastics that undergo a chemical change when heated to a certain temperature. These materials,once solidified, cannot be remelted or reused. Any attempt to remelt thermosets simply results in burning or decomposition of the material rather than returning it to a moldable melt. Thermosets cannotbe reprocessed or welded.

10

The chemical change thatoccurs in thermosets is often referredto be dissolved by organic solvents without decomposition. It is not surprising that thermoset products are well suited for electrical, construction and household applications where resistance to temperature and various types ofwear is critical. The raw materials for thermosets are somewhat different than thermoplastics. Base materials include phenol (acoal tar derivative), formaldehyde and urea. It is not important to remember these materials, but rather to understand that thermosets are entirely different than thermoplastics, both in how theyare made and,as will be illustrated, in how they are processed.

as curing or cross-linking [4]. Curedthermosetpolymerscannot

2.3 Crystalline vs. Amorphous Materials We have learned that plastics are composed of small molecules (called macromolecules) which have been joined together to form longchain moleculesthat are referredto as polymers. In the solidstate, some of the polymer molecules are arranged in a very orderly, repetitive pattern and are called crystaffine. Others are structured in a veryrandom are arrangement without anyorder or regularly repeating pattern, and they called amorphous. Without a detailed explanation, let us simply state that the greater the degree of crystallinity of a polymer, the more orderly its structure becomes [3].

I

Crystalline

Amorphous

Figure 4 Illustrative drawing of polymer molecules

11

As can be seen in Figure 4, the crystalline polymer molecular chains are more ordered and repetitive, whereas the amorphous molecules are random, looking something like a plate of spaghetti. Although it is nearly impossible to achieve total crystallinity in polymers [4], for simplicity, we shall refer to the group of plasticsthat are semicrystalline as being crystalline (asopposed to amorphous) materials.

The important thing to remember is that thephysical properties of these two types of plastics are quite different. This is very important to the part designer who must select the best material for each unique produc Although this book is not intended to include part design, we believethe molder should have some of idea the properties of the more common plastic materials, whichare shown at the conclusionof the book as Appendix B.

2.4 How Plastics Affect the Molding Process More importantto the molder, however,is the factthat crystalline and amorphous materials react quite differently during the molding process. There are at leastthree vital differences in the way the two types of material respondto the melting and molding process.

2.4.1 MeltingCharacteristics The first major processing difference between crystalline and amorphous materials is the way they melt [ 5 ] . As heat is applied, both types of materials soften somewhat at first, but the amorphous material continues to soften gradually until it w li flow. The softening point is referred to as the glass transition temperature (or T, ). Amorphous materials have no defined melting point.

In contrast, the more highly crystalline materials remain in a relativety solid state until the temperature reaches their melting point. The melting point of plastics is labeled T, . As we w lisee, this difference in the way the materials melt is an important factor in how the materials are molded.

12

The listing below has an added column to indicate whether each material is crystalline(C) or amorphous (A). Note: A simple way to determine whether theplastic is amorphous or crystalline is to check the Suppliers’ Data Sheet. If a melt temperature (Ta is given, the material is crystalline. If a softening or glass transition temperature (Td is given, it is amorphous. (See Appendix A for expanded list.) Long Name Polyethylene Polypropylene Polystyrene Acrylonitrile Butadiene Styrene Polyamide (nylon) Polycarbonate Polymethylmethacrylate (acrylic) Polyoxymethylene (acetal) Polyvinylchloride Styrene Acrylonitrile

Sym bo1 PE PP PS ABS PA PC PMMA POM PVC SAN

IYJE

C C A A C A A C A A

2.4.2 Thermal Conductivity The ability of plastics to absorb heat(referred to as thermal conductivity) is quite low, about two to three times lower than metals[ 6 ] . The low rate ofheat absorption influencesthe speed with which plastics can be heated, melted and molded. Thesecond important difference in how plastics are molded is the difference in heat absorbing ability between crystalline and amorphous materials. Amorphous materials have much less ability to conduct heat than crystalline materials. In fact, as the crystallinity increases,the ability to conduct heatalso increases. Stated another way, you cannot add more heat to amorphous materials and expect themto melt any faster! In fact, iftoo much heat is applied to amorphous materials, they w liburn and degrade. Although not intended to represent any scale of values, Figure 5 illustrates the difference in the way thetwo types of material canabsorb heat.

/

Crystalline

I

0 /

0 0 ’

morphous

Figure 5 Heat absorption characteristics of cytalline vs. amorphous polymers

2.4.3 Shear Sensitivity After considering the first two differences between the two types of materials, the third difference becomes easily understood. Amorphous materials are moresensitive to shear. Shear occurs when plastic pellets are compressed, rubbedtogether causing fiction or are signilicantly agitated during the molding process. High shear results in rapidly increasing the temperature ofthe material whilebeing molded which amorphous polymers do not tolerate well [ 5 ] . From these considerations, it can be concluded that amorphous materials should be gradually (not abruptly) heated when changing themfrom a solid to a melt. Excessivemelt temperatures insome materials (especially amorphous materials) can cause residual molded-in stresses (upon cooling) that detract fkom part appearance or reduce the mechanical strength of the parts. Unfortunately, in many cases the lossof mechanical properties (such as impact strength) cannot be determined until the partis subjected to impact tests or fails when performingin its intended use. In later chapters, you wil learn how these three factors (melting characteristics, thermal conductivity and shear sensitivity) are controlled in the molding process. We must remember that all materials

14 [7]. have a maximum limiting shear rate, beyond which they will degrade

2.4.4 Viscosity (Melt Index) Another propertyof both crystalline and amorphous materials that affects the molding process isviscosity. Viscosity may be defined as the In otherwords, if a meltedplasticis resistance of afluidtoflow. considered viscous, it is thick (like molasses) and will not flow easily. The viscosity of a melted plasticcan be measured and given a rating called a Melt Index (MI). A high melt index means that the melted plasticis thin and watery (andhas a low viscosity). The lower the melt index, the more thick and viscous the melt is and the less easily it will flow. The melt is less than one index of plastics range from a fractional MI, meaning itthat (l), to more than a hundred(100). Most common materials have a MI in the range of 2 to 12. There are various test methods and parameters for measuring Melt Index. When comparing materials,it is important that the method and parameters are the same. The viscosity of a plastic is important to the molder. Materials with a very high MIor very low viscosity are more difficult to pushor inject and, in some cases, more difficult to mold. Incidentally, itis good to remember that Melt Index is also a measure of molecular weight. A higher MI indicates a lower molecular weight for a given polymer family. This will also be discussed further in later chapters. CASE STUDY NO. l : Check New Materials

In the study of plastic materials, it is important that the reader become aware that the same materialfrom two different manufacturers may not process alike. In fact, two lots of the same material from the same source mav not process alike. An example follows: A very large user of a fairly common material, high density polyethylene (HDPE), purchased a rail car of the identical grade of HDPE they had been using from a second source and unloaded the

15

material into their bulkstorage tank (silo). After processing with material from that silo for a few days, the quality of the melt changed andthe same colors they had been achieving could notbe maintained. Despite changes in the processing profile, the reject rate became so intolerable that the entire remainder of the material in the silo had to be dumped. This was a very expensive lesson for an experienced molding operation. The lesson is: Whenever material sources or different lots of material are about to be used test the process with the new material before proceeding and before commingling the old material with the new.

CASE STUDY NO.2: Plastics Meltine Illustration If you were ableto take a pellet of an amorphous material (such as acrylic) anda pellet of a crystalline material (such as nylon) and put them on a skillet that couldbe heated sufficientlyto melt the materials, two totally different results would occur. The pellet of acrylic would soften, soften fbrther and gradually reach the point where it would flow. In contrast, the nylon pellet would not visibly soften and, after a period of heating, it would rather quickly change to a complete melt. Moreover, if the heat of the skillet was significantly increased, the acrylic would degrade andburn rather than changing more rapidlyto a fluid state. It is likely that the nyloncouldsurvive the excessiveheat(unless unreasonably high) and reach the molten state more quickly. The lesson is: Amorphousmaterialsaremorelikelythan crystalline materials to degrade and burn if overheated during the "melting"process Avoid excessive heat when processing amorphous materials, regardless of the heat source.

3 Additives In addition to the raw plastic materials discussed in the last chapter, there are a number of other ingredients thatmay be added to the plasticto mod% its properties. These other ingredientsare referred to as additives andinclude plasticizem, fillers, reinforcements,stabilizem, flame retardants, colorants, lubricantsand manyothers. Although it is not vital to remember all of the various types of additives, it is essential that the molder understand the need for some of the additives and their impact on the molding process. The more important and commonly used additives are discussed inthe following pages.

3.1 Fillers and Reinforcements Fillers and reinforcements are added to the plastic primarily to increase thestifkess ofthe resulting plastic part and/or increase the other mechanical properties of the part. Additives that are used to increase the mechanical strength ofthe part are usually referredto as reinforcements [4]. Some of the more commonfillers include calcium carbonate (which is basically powdered limestone),talc (another powdered mineral that hasa slippery or soapy feel), carbon black (which is used as a black colorant and, more importantly, as a protector against UV radiation) and silica (a very small, spherical-shaped mineral). Although classitied as fillers, calcium carbonate andsilica, in someforms, might be considered as reinforcements and can be quite abrasiveto the metal surfacesin the injection unit andthe mold. As one might guess, the addition of such substances to the plastic increases the melt viscosity of the material and, as we w li learn, can sigmficantly affectthe molding process and shrinkage. Most of the reinforcements added to plastics take the form of small fibers, powdersor flakes. The most common reinforcementaisglass fiber that is like a very h e mono-ent [.009 to .013mm (.00037 to .00052") in diameter]. The strands are cut to short lengths (less than 6.35mm or .250"in most cases) and addedto the plastic when it is formulated.Glass fiber reinforced nylon and polyester materials are quite common and are

17

quite strong, often replacing die cast metal parts. The glass reinforced material requires special molding considerations and is very abrasive to the metal surfaces with which they come contact. into As ifthese are not severe enough, fibersare also madefiom carbon, graphite, and metal, enhancing the resulting part strength and even M h e r complicating the molding of the parts. Some metal fiber reinforcements are added to provide electrical conductivity. In addition to powder and fiberforms, some reinforcements are in the form of a flake. These flake reinforcements include those made fi-om mica (a lightweight mineral),glass and aluminum.

3.2 Plasticizers In contrast to some of the fillers discussed previously,plasticizers are usedto reduce the stifbess ofthe plastic part, makingit more flexible. In achieving the increasedflexibility,plasticizersmayalsoreduce the viscosity of the melt and h c t i o n as a lubricant. Plasticizersare frequently used in producing parts made fi-om PVC, increasing part flexibility to a “rubbery” feel [4].

3.3 Stabilizers PVC is a common object ofbothheatstabilizers and UVstabilizers. If PVC is processed at too high a temperature, degradation wil occur and may be associated withthe release of hydrochloric acid. This may result in a lossof properties in the molding process, potential severe damage to the metal surfaces in the injection unit and the mold and, if excessive heat occurs, can create a safety problem for the molder. Heat stabilizers are combined withthe plastic to help prevent degradationfiom excessive heat in processing. We wil later learnthe importance of the proper control of heat duringthe molding of all plastics, including PVC.It is also important that the stabilizersbe compatible withthe resins to which they are added to avoid chemical or viscosity problems in molding. Uvstabifizers are also important because they increasethe molecular stabilityof plastics that are exposed to light. UV stabilizers help increase the “weatherability” ofplastics exposed to sunlight in outdoor environments.

3.4 Flame Retardants Because so many of the products and equipmentthat people use in their everyday livesare now made of plastic, the degree to which each is ofthe home, automobile, boats flammable is very important. Major portions be and airplanesare now constructed with plastics and these plastics must as resistant to burning and smoke generation as possible. As a result,jlame retardants and smoke suppressantsare added to many plastics to control the undesirable effects that can resultfkom combustion. Flame retardants are added directlyto the material when formulated allowing the molder to purchase flameretardant grades of material. Unfortunately, most of these additives are corrosive to the metal surfaces to which they are exposed, requiring specialprotective coatings to be used inthe injection unit and the mold. They also may accelerate resin degradation and restrict the heat profile.

3.5 Colorants One of the advantages of manufacturing parts fiom plastics is avoiding the need to paint the resulting product. Colorants may be added to the plastic allowing the entire part to be colored, not just the surface. In order to accomplish this,pigments and dyes are added to the plastic, either as a part of the formulated plastic pellet, an additive pelletof colorant or as a liquid thatis added to theplasticafterit enters theinjectionunit. Pigments are not soluble in the plastic melt but are mixed in by a dispersion process described later, whereas dyes are soluble and provide maximum color strength andbrilliance at minimum cost [8]. Thetechnology supporting thecoloring of plasticissomewhatcomplexandmaybe explored by the reader in greater depth by reference to any of several books on the subject.

3.6 Adding the Additives Most of the additives discussed in this chapter are added to the plastic (virgin material) by additivefeeders or blenders. There are different

19

types of feeders and blenders used inthe molding process. Some can mix and add onlytwo different typesof solid material whileothers may be able to mix and add up to five types of materials, including pellets, powders, granulated material and even liquids. Most of these units are mounted directly on the injectionmoldingmachine,inaddition to the standard hopper. The more complex units are floor-mounted and use pneumatics to the reader to take the mixed materialto the machine. It is not important for understandhowtheseblendersandfeeders are constructed, it is only important to understand how they operate and what they are designed to do. It is recommended that, other than colorants and foaming agents, none of the additives discussed here be added to the material at the molding machine. Preferably, it should be done by the people compounding the material.

CASESTUDYNO. 3: The Effect of Additives on Processing Each of the additives discussed above has a different and important effect on the injection molding process. One molder found that the change fiom processing a non-reinforced materialto one reinforced withfiberglass fibers required that his screws needed to be manufactured fiom a more wearresistantmaterial,havedeeperflightsandlesscompression. In addition, he learned that the heat profile needed to be greatly altered to achieve the proper quality melt. Another molder discovered that the addition of carbon blackto the HDPE he was processing required a screw that more aggressively mixed and shearedthe material or the resulting melt had “windows”or unmixed areas that would adversely affect the “weatherability” of the resulting product. was adding titanium dioxide to his material to Yet another molder alter the color of his parts. TiOZis a commonly used additiveto achieve a very white coloration of the parts produced. However,in a very short time, the molderdiscovered severe wear inhisbarrelandscrew.Research revealed that TiOZ can be very abrasive and requiresthat the injection unit

components (screw, barrel, valve and end cap), and sometimes the runners and gates inthe mold, be made from more wear resistant materials.In this case, the components were replaced with different, wear resistant materials and the wear previously experiencedwas no longer a problem. Another molder was using silicone as an additive in his process. Silicone isknown for its chemical and physiological inertness and has good physical and electrical properties that don’t change significantly from very low to very high temperatures. Silicone is also water-repellant and antiadhesive and is used as a lubricating agent,an anti-foaming agent and foam stabilizer. After processing for a time, the molder began to observe tiny metallic particles embeddedin the parts. After considerable research and experimentation, it was learned that silicone erodes certain flight hardsurfacing materials usedon the flights of screws. The screws were rebuilt and chrome-plated completelyover the entire surface of the flights. The problem was eliminated. The last example involves a molder who found that the addition of a colorant to his material required a screw with a mixing device to achieve the properly colored part andthe better the mixing device, the less of the expensive colorant was required. The lesson is: Any time an additive is added to a material, there will be an important eflect on themoldingprocess that will likely require changes in the processing profile anHor equipment. Be alert for these effects and take corrective action before costly remedies are needed!

21

4 Loaders and Dryers The next step in the molding process is to dry the plastic (if required) and move it fiom its storage location to the injection molding machine hopper. Thisis accomplished by one or more piecesof equipment categorized as dryers and loaders.

4.1 Hopper Loaders and Conveying Systems As we learned inthe first chapter, the equipment may be as simple as a vacuum powered hopper loader with hoses that pull pellets fiom a gaylord or as complex as a pneumatic material handling system.

Pneumatic systems offer simple, reliable cost andeffective solutions to transporting the plasticto the machine, whether in pellet, flake, granular of a power unit, a or powder form. A vacuum conveying system consists system controller, material pick-up devices and receivers that are connected by tubing for both vacuum supply and material transport. The technical specifications of this type of equipment are not essential to an understanding of its use. In simple terms,the system controller senses when there is a need to convey more plastic and calls uponthe power unit to accomplish the task Thepick-up devices are designed to allow a simple adjustmentof the aidmaterial ratioat the pick-uppoint. Material receivers are required at each drop-off or destination point and are located near sensors that tells the system controller to actuate the vacuum system[g]. In recent years, portable self-contained hopper loaders are in greater use. These units have their own blower and motor and are no longer located on top of the press, but rather are cart-mounted configurations located at the floor level. A central systemhas a main controller, one large motorhlower, a single stand-alone filter and dust collector and a filterless receiver on each molding machine. Compressed-air blowback filter cleaning and stainless-steel constructionare optional features available.

Another option is a portable combinationdryedloader system that allows the drying of the material off-line and is moved into position when the dried materialis scheduled to be used [ 101.

4.2 Dryers All plastics used in the molding process, including regrind, are affected by moisture to some degree.Ifthe moisture is not removedfrom the plastic, it can cause defects in the molded product, such as splay marks (streaks) and brittleness. Plastic materials are considered to be either hygroscopic or non-hygroscopic. Those considered to be hygroscopic absorb the moisture within the pellet (or flake) and cause a molecularbondwith the material.Includedamong the hygroscopic materials are: ABS, PMMA (acrylic), FEP, PA (nylon), PBT, PC, PET, PPO,PVC, S A N , PSU and PE1 (see Appendix A for technical names). In addition to causing part defects, moisture that is allowed to remain in these materials can unite with other elements to produce corrosivesat processing temperatures. The result is a premature corrosive wearon the surfaces of the injection unit components.This type of wear w l ibe discussed in a later chapter. Non-hygroscopic plastic materialsdo not absorb moisture, butthe moisture in the air adheres to the surface of the pellets or flakes and can cause some of the same types of processingproblemsobservedwith hygroscopic materials. Non-hygroscopic materials include polyethylenes, polypropylenes and polystyrenes. Often it is advisable to dry these materials as well.

4.2.1 Hot Air Dryers There are two major typesof dryers usedin the molding process, hot air dryers and dessicant dryers. Although it is not necessary to understand the details of how these dryers are made or how to size their operating capacities, it is helpll to know, in simple terms, how each type is used and how they performthe drymg hction.

The hot air dryer basically consistsof heaters and an air blower and are typicallymounted on top of the injection unit as a dryer hopper, replacing the standard hopper. Ambient air (air inthe processing room)is pulled into the dryer-hopper, heated and then blown up through the plastic pellets in the hopper. The hot air evaporates the moisturein the plastic and then movesit out of the hopper back intothe room air[ 1 l].

4.2.2 Dessicant Dryers The dessicant dryer utilizes small beads (referredto as dessicant) thatcanabsorb a lot of moisturewithout undergoing anysignificant structural change. The dessicant dryers operate much like the hot air dryer, pulling the moist air fiom the plastic pelletsin the drying hopper into the dryer througha filter, thenthrough a layer of the dessicant beads (called the dessicant bed), which absorb the moisture, and finally to a heating unit where the dryair is brought up to a specified temperature. This dry airis then circulated through the plastic pellets in the drying hopper. Thedry air becomes more moistas it leaves the plastic andthe closed-loop processis repeated untiltheproperlevel of moistureallowedfortheplasticis achieved. When the dessicant bed becomes nearly saturated with moisture,the air flow is diverted to another dessicant bed and the process continues. The original bed is regenerated (ridof moisture) and is ready for use again. A table designatedas Appendix C describes the drying requirements for some of the more common plastics. The primary consideration in choosing a dryer is throughput or pounds per hour. There are basically fouroptions f?om which to choose a dryer configuration:(1) a totally machine mounted unit; (2) a portable unit; (3) a larger less portable unit that sits near the press; and (4) a central of dessicant drying system[121. These units offer various controls, numbers beds and other options that we w li not address here. However, these features are important and should be carefblly studied when making a dryer selection.

24 CASE STUDY NO. 4: Conveying is not Free of Problems When conveying material, most of the force used to convey is a vacuum. If the material being conveyed is notofall similar size and weight, the vacuum will pull more of the lighter weight material ahead of the heavier.Whether the lighterweightmaterialis an additive,regrind or simply fines,there w l i be an undesirable effecton the parts being molded. The same problem can happen in a hopper where the force is gravity. Clearly the heavier material that is closerto the center of the hopper w li drop first, leaving dissimilar material aroundthe periphery. The lesson is: Be mindfd of the problems that can arise in the conveying and feeding of material and use additive feeders or other similar devices that insure a constant feeding of homogenous material.

CRTE STUDY NO. 5: The Proper Removal of Moisture Later in the book, we wil discuss the use of vented barrels as a means of removing moisture fiom hygroscopic materials without drying them Simply stated, the materials are not dried and enter the heated barrel through the hopper. A special screw performs a prelirmnary compression and shearingof the material which generates steamas the heated plastic is rid of its moisture. A hole in the barrel, called the vent port, allows the steam to escape, thereby circumventing (no pun intended) the presumed need to dry the material. Despite many successhl vented applications, thereare an equalor greater number that have tried venting and have revertedtoback drying the material before itis processed. The problems that can result with venting, if not carehlly monitored, include a partiallyor totally clogged vent port which allows undried melted plastic to proceed into the mold producing faulty parts. The question is, how much can theport vent be blocked before parts problems occur? Even if the vent port is cleaned religiously, it will partially block,fiom time to time, andthe resulting parts are not madefiom the same degreeof moisture-fiee plastic.

25

When drying material,be sure to observe the supplier’s rangeof the recommended percentageofmoisturethat should be allowed inthe material. It is often forgotten,butmaterial can be over-driedwhichcancause subsequent degradation and the inability of the material to flow properlyin the melted condition. The lesson is: If moisture contamination in the Pam must be avoided, drying ofthe plastic before moldingis essential. Always dry to the supplier’s recommendation and don’t forget that any regrind must be added before the drying process is applied

26

5 Injection Unit The injection unitof an injection molding machine (I") of the elements shownin the schematic drawing below: Barrel ( a l s oknown as a cylinder) Non-returnValve EndCap Hydraulic or Electric Screw Drive

Non-retum Vahre

I \ I

Nonle

consists

Screw Nozzle HeaterBands Hopper

Heater

\

End Cap

Il

I

Barrel

Screw

I

/

Screw Drive & Injection Cylinder

Figure 6 Elements of an injection unit of an IMM

The injection unit is perhaps the most important part of the injection molding machine because if it fails in its functions, the moldingof quality plastic parts will not occur. Not only does the injection unit have primary responsibilityfor the molding of good plasticparts, it contributes significantly to the efficiency of the molding process. The injection unit receives plastic pellets, conveys, heats and melts them and then injects the melt through the nozzle into the mold where the plastic part is formed. Each of the elements of the injection unit contributeto this process. The hopper holds the plastic pellets whichare gravity-fed through the feed hole in the barrel. The screw has helical (spiral) flights which, when thescrew is rotated, cause the plastic pelletsto move forward inthe

27 barrel. The barrel, which housesthe screw, has heater bands surrounding it which heatthe barrel andthe plastic inside based on temperature controls which take readings fiom the thermocouples positioned in the barrel wall. The temperature controls are set for a specified temperature and the thermocouples tell the controls whether the requested temperature has been reached, If the temperature is not sufficiently high,the thermocouples w li call uponthe controls to supply more heat. The temperaturescalled for and the actual temperatures, as measured by the thermocouples, may beseen on the machine control panel. The screw also provides some of the heat to melt the plastic pellets, by squeezing and shearing the pellets againstthe screw flights and the barrel vital h c t i o n of the screw and how wall as the pellets move forward. This it is designed to perform this h c t i o n is covered in Section 5.5 of this chapter. The screwdrive is typicallyahydraulicmotordrive,although electric drive units are becoming quite common. The construction of the screw drive unitis beyond the scope of this book, however,it is simply a h c t i o n of converting hydraulic (or electric) powerto mechanical powerto turn the screw. As the screw turns, the plastic moves forward and becomes a melt

which ultimately reaches the end of the barrel. The melt then proceeds through the non-return valveand the end cap. For reasons wewill explain later, the melted plastic cannot move forward through the nozzle. As a result, the pressure of the melted plastic builds up in front of the screw and forces the screw backward. The screw drive only indirectly causes the screw to move backward. Because of the back and forward motion, some referto the screw as the "reciprocating screw." The control on the injection molding machine can beto set allow the screw to move backward only a specified distance. The distance is referred to as stroke and is measured in inchesor millimeters. ( I v o t e : All injection is approximately equal molding machines have a muximum stroke that tofour (4) times the diameterof the boreof the barreL)

28

When the specified distance is achieved, a signalis given and the screw drive, injection cylinder and non-return valve perform their additiona hctions. The screw drivestops the screw from rotating and the injection cylinder causes the screw to move forward like a ram. The non-return valve closes to prevent any movementof the melt back intothe screw. The injection unit then completes its primary function. With the pressurizing of the hydraulic injection cylinder,the screw moves forward, causing the injection of the plastic forward through the nozzle into the mold. Although the melted plastic could not moveforward into the mold during the screw rotation, injection is now possible because the previous plastic that was in the mold has now been removed (ejected). The mold ejects the parts &om the prior shot and closes just prior to the injection of the new shot. Shot is the term applied to the amount of melted plastic that is injected intothe mold. Theshot sizeis the quantity of melt injected into the mold, measured in ounces or grams. The screw hydraulic system holdsthe screw in its forward position to permit the packing of the melt into the mold cavities. After a specified time interval (controlled by the machine), the screw drive again beginsto rotate and the entire process is repeated. Thisprocess, beginning with the screw intheforward position, then reciprocating backward, pausingfor the opening ofthe mold and ejection, and moving forward to inject the moltenplastic of the next shot, is referred to as a cycle. The time required to complete one cycle is appropriately calledcycle time. Cycle times can vary fiom a few seconds to several minutes, depending upon a varietyof factors which are discussed more hlly in a later chapter. The screw drive performs some additional fimctions that have a bearing on the efficiency of the process and the quality of the melt. The screw drive can exert a forward pressure (or resistance to its backward movement)while the screw is rotating,causinga greater mixingand shearing motion insidethe barrel. This forward pressure is curiously called backpressure and is fi-equently used to help meltthe plastic andto increase the mixing action of the screw. Some back pressure is good; a lotof back pressure canbe bad.

5.1 The Barrel The barrel,also referred to as the cylinder, is the cylindrical housing in which the screw rotates. It consists of a shell orbacking which isthe thick outer wall to provide strength. The shell may be lined with a variety of different materialsto provide wear resistance. Some of the linings are cast inside the shell and form a metallurgical bondof the lining to the shell. In other cases, the shellmay be lined with wear resistant tool steel that can be removed for relining. The back end of the barrel fits into the casting of the injection molding machine. It is typically securedin place by one of three methods. The barrelmay haveaflange with bolt holes which allows the barrel to be inserted into the casting and then secured by large bolts going through the flange and into the casting.A second method that is similar to the flange, is the use of a split ring groove and removable flanges. The barrel is fit into the casting, the two flanges are placed in the groove and then bolted to the casting. The third method involves a threaded end on the barrel which is inserted into andthrough the casting andis secured by a large nut on the interior of the casting.

Figure 7 Three commontypes of mounting e n d of a barrel

30 Another method used more recently involves sliding the barrel downward into the casting and securing it with a flange-like piece that fits downon top ofthe barrel. Most machine manuhcturers use a versionof one of these methods and it is only important to the reader to be able to recognize the basic designof each. All injection machine barrels have feedahole that is located near the back end of the barrel (see Figure 6). The feed hole is located directly under the hopper and is the opening through which the unmelted plastic pellets are gravity-fed into the feed channels of the screw. You should also note that the barrel is equipped with thermocouple holes along its length into which threaded thermocouplesare placed to sense the temperatureof the barrel.

Although most barrel shells are made &oman alloy steel in the 4000 series(typically 4140 or 4150) AIS1 designation,there are several alternative linings available which resist wear. The alternatives maybe grouped into three types that relate to how the linings are manufactured. They include:nitrided barrels, cast bimetallic barrels andtool steel-lined barrels [ 5 ] . An understanding of the metallurgical considerations involved in each lining type is not withinthe scope of this book. It is important to remember that the linings of all barrels are not the same and they have a bearing on the wear life of the barrel before it must berepaired. Because the screw fits very snugly intothe bore of the barrel, it is vitalthatboth the screwand the barrel be quitestraight. A lack of straightness in either of the components can causethem to wear prematurely.

5.2 End Cap and Nozzle It is easier to understand the function of the end cap and nozzleby referring to the illustration at the top of the next page (see Figure 8).

31

figure 8 N o d e , valve and barrel assembly

Figure 8 shows the forward (nozzle end) of the injection unit, including the barrel, screw, heater bands, end cap and nozzle. Although no displayed in detail, the non-return valve is also shown. The construction of the valve and how itfunctions will be discussed later in this chapter.

of the barrels Although Figure 8 illustrates a bolt-on end cap, many and related end caps are threaded, allowing the end cap tobe threaded onto the end of the barrel. The non-return valve is screwed into the end of the screw so that the rear seat of the valve is flush with the register of the screw. The nozzle is threaded into the end cap and the nozzle tip is likewise threaded into the nozzle. The end cap, also referredto as a barreladaptor, is manufactured from very strong steel to be able to withstand the injection pressures of the molten plastic as it leaves the barrel and goes throughthe nozzle and into the mold. The end cap is a transition point where the shot of plastic contained in the end of the barrel is directed into the narrow passageway referred to as the nozzle. Because of the high pressure exerted during injection, ranging from a normal level of 113 to 155 N/mm2 (16,000 to

32 22,000 psi), the bolt-on end caps typically use 10 to 16 holes with bolts that are threaded and are rated very strong. When bolting the end cap to the barrel, it is very important to gradually tighten each bolt using a pattern which alternates opposite sides of the bolt circle. In addition, a degree of tightness isusuallyspecifiedby the machine manufacturer that is best achieved using a torque wrench. Over-tightening is as damaging as not tightening enough. Damageto the lining of the barrel can occur if the end cap is tightened too tight. The second transition point between the barrel and the mold is the n o d e . The nozzle is a “tube which provides a mechanical and thermal connection fiom the hot barrel to the much colder injection mold with a minimum pressure and thermal loss [7].” Some nozzles do not have an interchangeable nozzle tip, as illustrated in Figure 9. But in either case,the end ofthe nozzle tipor the nozzle(ifno tip is present) typically has a radius of either %” or X”. The radiused (rounded) end of the nozzle tip (or nozzle) fits into a part of the injection mold referredto as a sprue bushing. It is very important that the fit between the nozzle tip and the sprue the nozzle bushing is correct (See Figure 9).The incorrect fit could allow to back away duringthe high pressureof injection allowing plasticto leak.

N d e Tip

Sprue

Bushing

I

conedm

(Nonle Tip

not sealed off) l

Imrrect Fit

Figure 9 Illustration of the fit of a nozzle t@ to the sprue bushing

33

Some nozzles have a straight bore and others a tapered bore, depending upon the requirements of the plastic material being processed. The nozzle is threaded into the end cap and usually has a thermocouple and a small heater band to control the temperatureof the plastic at the nozzle. The nozzle tip is used to enable thematching of the orifice in the nozzle to the openingin the mold sprue bushing. Rather than maintaining an inventory of a large number of nozzles, a stock of nozzle tips can accomplish the same objective. There are at leastthree typesof nozzles used in injection molding. One type hasan open channel like the nozzle Figure in 9. A second typeof nozzle involves a valve that closes the nozzle (after the injection pressure diminishes) using a spring.The third typeis a shut-off nozzle in which the closing of the valve is accomplished by pneumatic or hydraulic pressure. Shut-off nozzles prevent drool, a problem discussed in Chapter 15. A majority of the nozzles in use are the first type. Basedon our experience, there are many pneumatic shut-off nozzles being utilized and only a few spring-loaded nozzles.

5.3 Heater Bands Electrical heater bands are used for heating the injection molding barrel and the nozzle. The heater bands vary from approximately 1 to 14” ormoreinwidth,andmanufacturersofferavarietyofconstruction variations.Several of thebandsarepositionedaroundthebarreland respond to pyrometer settings that are based on the readings from the thermocouples in the barrel. You can usually read the “set temperature” and the “actual temperature”of the barrel on the control gauges. ‘I

The most common type of heater bands aremica-irzsulated and can operate up to 371°C (700°F) (or higher under optimum conditions) with recommended wattage of 20 to 35 watts per square inch. They are reliable, efficient and offer multiple choices of construction characteristics and

34

electrical ratings. The wear life of mica-insulated bands is usually notas long as ceramic or mineral-insulated bands.

Ceramic-insulated heater bands offer improved efficiency, longer life and increased operating temperatures up to 1500°F. Recommended wattage ranges fkom 25 to 70 watts per square inch and, under some conditions, as high as 100 watts per square inch.Mineral-insulated heater bands are offered by most suppliers and they operate with much the same temperatures and ratingsas the ceramic-insulated bands. Heater bandsare also made fromother materials, including cast aluminum, bronze or brass. is very Thedetermination of heaterbandsizingcalculations important but not a subject of this book. It is important, however, that efficientheaterbands, operating withintheirratedcapacities,supply conductive heat to the barrel (and therefore, the plastic) to assist in the melting of the plastic. As we w lilearn later, the heat suppliedby heater bands should ideally supply 50% ofthe total heat neededto melt the plastic. The remaining 50% is supplied by the shear heat generatedby the screw.

In addition to the barrel heater bands, the nozzle requires a heater band to help maintain the proper temperatureof the melted plastic until it is injected into the mold. Tube andcartridge heaters (in additionto band heaters) are also used to maintain proper nozzle temperatures. The nozzle heater is controlled independentlyof the barrel heater bands.

5.4 Non-Return Valve S i m p l y stated, the non-return valve is placed at the end of an injection screwso that melted plasticwil not flow backward into the screw channels during injection. When the screw comes forward during injection, the valve closes. When the screw rotates backward, the valve opens to allow melted plasticto flow through the valve into the area in fkont of the screw.

35 There are several types of valves used in injection molding. The most common type is referred to as a ring valve. The ring is forced backward by the pressure of the melted plastic inh n t of the stud during injection thereby closing the valve, and is forced forward (open) by the forward movement of the meltedplasticwhen the screw is rotating backward. It is easier to understand the operation of the ring valve by looking at theillustration inFigure 10.

Screw Forwsrd in lnjcd Position

Screw Back In Rotate Position (Valve Open]

Figure l 0 Three piecering typ of non-rehanvalve

The non-return valve illustrated inFigure 10 is a threepiece ring valve. Pressure in fiont of the ring moves it back as the screw starts forward during injection, closingthe valve by shutting off against the rear seat. When the valve is open with the ring in its forward position andthe screw is rotating backward, the melted plastic flows forward, across the rear seat, under (inside)the ring and through the flutes. You w linote that the ringdiameter is slightly largerthan the end ofthe screw and moves back and forth in close proximity(.0015" or .038mm) to theinside diameterof the barrel. With the clearance beingso small, no appreciable amountof melted plastic can flow backward over the ring. It is also apparent that as the outside diameter (OD) of the ringbecomes worn, the efficiencyof the valve diminishes. Also, if the barrel becomes worn,there is room for the ring to expand and break underthe inten& injection pressures.

36 Some manufacturers make afourpiece ring valve, which includes a rear seat, a front seat, a stud and the ring.In the three piece ring valve, the ring can rub against the stud as the screw rotates, causing some wear. The fiont seat of the four piece valve is made of more wear resistant material. It fits againstthe stud at the point wherethe ring would typically come into contact with the stud, thereby absorbing the wearnormally sustained by the stud. Because the stud is the most expensive pieceof the ring valve,the less expensivefiont seat canbe replaced more economically than the stud. There is another style of ring valve thathas gained some popularity in the United States. It is a locking style ring valve. It kctions exactly the same as the ring valves illustrated, except that the ring is locked into place inthe stud and willrotate with the stud and the screw. In the standard ring valves, the stud, which is screwed into the end of the screw, rotates with the screw. However, the ring "free-wheels" and can stay in place, rubbing against the stud. The locking style avoids this fiiction against the stud, but does cause more wear againstthe inside diameterof the barrel as the ring rotates. Although the ring valve is the most commonly used valve in injection molding, there are some other valve types with which the reader should be familiar. The ballcheck valve is manufactured intwo styles, a side-discharge and a fiont discharge. Both styles utilize a steel ball rather than a ringto move back and forth, opening or closing the valve to the flow of melted plastic. These valves are illustrated in Figure II.

As you can see, both types of ball check valves requirethe melted plastic to enter the valve through four ports in the side (at the rear end)of the valve, making 90" a turn. The plastic must make another 90" turn toward the front of the valve to proceed forward. In the side discharge valve, the plastic must make another 90" turn, around the ball, to exit out the four ports on the side of the valve (near the forward end). The front discharge valve allows the plasticto move forward, aroundthe ball, and outthe front end of the valve.

Outlet Port8

Ball Seat Inlet pbrts Screw

Outlet Port Ball Stat Inlet Ports S m w

J I

Front Discharge Ball Check Valve [in open position]

Figure I I Front and side discharge ball check valves

In the closed positionfor both types of ball check valves, the ball shuts off against the conical shaped opening at the rear center of thevalve, allowing the valve to come forward during injection without allowing any plastic to flow backward. Although the ringand ball check valvesare the most commontypes of non-return valves,some other valve designsare available that utilize a different mechanism to achieve shut-off. One type uses a spring (which compresses during injection and expands during screw rotation) and small piston to accomplish the opening and closing of the valve. The piston moves back and forward to open or close a port (much likethe ball in a ball check valve) and is activated by the spring. Another type utilizes the presence or absence of pressure in fiont of the valve to move a piston which opens and closes the valve. However, this valve type is unique and the reader will likely see this typeof design in a limited numberof situations.

Each of the valve designs is well suited for certain types of plastic processing and notto others. The ring valveis a more fiee-flowing type of valve thanthe ball check valve and wouldbe used where the plastic being processed is more sensitiveto shear. Theuse of valves in various processing environments w li be discussed in more depth in a later chapter.

5.5 Screw The screw is housed inside the barrel and consists a shank of and a flighted length. The shank is designed to fit into the quicl of the screw drive, allowing the screw drive to turn the screw during screwrotation and causethescrew to goforwardduringinjection.Theflightedlength approximates 80% of the overall lengthof the screw and isthe portion of

Figure 12 Elements of an injection molding machine screw

the screw which receivesthe plastic pellets whichare gravity fed through the feed hole of the barrel. The flighted lengthalso conveys and melts the plastic as the screwrotates backward. Refer to Figure 6 at the start of this chapter which illustrates the screw in itsmostforwardposition,thepocket of the screw (circular beginning ofthe first flight) sits directly under the feed hole. Plastic pellets are gravity fed into the feed channel of the screw fiom the hopper and,as the screw rotates, the pellets are conveyed forward toward the meter section.

The reader will note that theflight depth (or channel depth) inthe feed section of the screw is considerably deeper than in meter the section. When the pellets are conveyed forwardthrough the transson section, they begin to be compressed. In fact,someauthoritiesprefer to call the transition section the compression section. The pellets continue to be compressed until they reachthe meter section, where once again (like the feed section), the root diameter is constant. The compressionof the pellets causes heatto build in the plastic as a resultof the shearing of the pellets againstthe flights, the inside lining of the barrel and againstother pellets. The shear heat, combined withthe conductive heat fiom the heater bands that surround the outside of the barrel, causes the pellets to change fiom a solid to a melt. The melted plastic goes through the valve (which is open) and builds up pressure in fiont of the valve. This pressure causes the screw to move backward, even as it continues torotate. If the screw is designed correctly andthe heater bands are set to achieve the proper temperatures, about 50% of the heat energy comes from the screw shear and 50% from the conductive heat of the heater bands. As we can see,the screw is a vital factor in the proper melting of the plastic.

5.5.1 Length-to-Diameter Ratio The length-to-diameter ratio (referred to as the LA9 ratio) is a measure of the length fiom the fiont edge of the feed openingto the fiont of the screw with the screw in the forward position. However, it is the practice in the industry to calculate the L/D ratio using the following formula. It should be recognized thatthe formula somewhatoverstates the actual working lengthof the screw because the flights underthe feed hole do not create any pressure or heat. L/D Ratio = Flighted Length + Outside Diameter

4n

Using the formula, ifthe flighted lengthofthe screw is 40 inches and the screw measures 2 inches in diameter, the L/D ratio is 20: 1. Most screws for injection molding machines have a1 L/D 20: ratio. Some manufacturers offer more thanone L/D ratio screw for their machines. Typicalratios for other length screwsare 24: 1,22: 1 and 18:1. The effectof lengthening the L/D ratio) will be discussed in a later flighted section (hence, increasing the section of this book.

5.5.2 Screw Profile The standard injection screw has three sections: thefeed section, where the plastic fist enters the screw and is conveyed along a constant root diameter; the transition section, wheretheplasticisconveyed, compressed and melted alongroot a diameter that increases with a constant taper; and the meter sectwn, where the melting of the plastic is completed root diameter. The meter and the melt is conveyed forward along a constant section is designed to allow the material to reachatemperatureand viscosity necessaryto flow into the mold and formparts [ 5 ] . The screw profile is the length, in diameters, of each of the three 10 sections of the screw. A 10-5-5 profile indicates a flighted surface with diameters lengthin the feed section,5 diameters in the transition section and 5 diameters in the meter section. Most new machines are supplied with a “General Purpose” (GP) screw that has a 10-5-5 profile. The GP screw is presumed to be capable of processing most typesof plastics.

5.5.3 Compression Ratio Another measure of screw being utilizedis the compression ratio. The channel depth in the feed section is deeper than in the meter section. Although the ratio of this differenceis actually thechannel depth ratio, it is commonly referred to as the compression ratio, which is calculated: Compression Ratio = Feed Channel Depth + Meter Channel Depth

41

Compression ratios for injection molding machine screws typically range fiom 1 S : 1 to 4.5:1. Most general purpose screws for thermophstic materials have a compression ratioof 2.5:l to 3.0:l. The screws usedto process thermoset materials typically have a1 .O: 1 ratio which means that the material is simply conveyed and not compressedor sheared. It is interesting to note that the bulk r a w , the ratio of the space occupied by plastic in pellet form to the space occupiedby the same plastic in melted form, is about 2 to 1. As a result, a screw with a compression ratio of 2:l causes a very gentle shearing of the pellets in the transition section of the screw, but not much compression.

5.5.4 Helix Angle Referring back to Figure 22, you will note that the helix angle of a screw is the angle of the screw flight relativeto a plane perpendicularto the screw axis. It is not important to understand the impact of the helix angle on the design of the screw. However, the reader should know that when the helix angleis 17.6568' (asis the case with most screws), it causes the distance fiom the fiont edge of one flightto the front edgeof the next flight to be equal to the diameter of the screw. This is referred to as a square pitch. A screw with a square pitch of 2 inches w lialso have a diameter of

2 inches. Except for screws with unusual designs for specific purposes, all injectionmoldingmachinescrewswilltypicallyhaveahelixangle of 17.6568' and a square pitch.

5.5.5 Drive Design The manufacturersof injection molding machines design the drive configurationof the shank to fit the quill (screw drive"female" connection) of their machine. Obviously, screw a that w lifit into one machine w li fitanothermanufacturer'smachine of the same size. There are many different drive designs for screws.

42 The designsusually fit intoone of thefollowingcategories: standard splines, involute splines,and keyways. It is not important for the reader to understand those categories, but it is important to know that a new screw for one type of machine will normally not replace a worn screw from another typeof machine. As a matterof interest, the drive design illustrated inFigure IO is afour tooth involute spline, typical to one (and only one) injection molding machine manufacturer.

5.6 Injection Pressure One of the most important performance parameters of an injection unit is the injection pressure that is exerted on the screw by the hydraulic (or electric) system to move the screw forward. This pressure forces the melted plastic that is in front of the screw throughthe nozzle and intothe mold. As soon as the melted plastic comes into contact with the cooler mold surfaces, it begins to cool and solidify. Accordingly, injection must be completed rapidly and with sufficient pressure so that the mold cavities are filled while the melt will still flow. The injection pressure must overcome the resistance of the viscosity As a result, of the material, the elements of the mold and its runner system. injection pressures in front of the screw may exceed 138 Mpa (20,000 psi and much higher-50,000 psi and above-in the newer high pressure systems), or a hydraulic pressure of 13.8 to 20.7 Mpa (2,000 to 3,000 psi), based on the size of the hydraulic cylinders involved. In many machines, a rule of thumb of 10 to 1 is applicable so that a hydraulic pressure of 13.8 MPa (2,000 psi) resultsin 138 MPa (20,000 psi)of injection pressure. Easyflowingmaterialsinjectedintoamoldwiththickwalled sections may require only 48.3 to 70.0 Mpa (7,000 to 10,000 psi) injection pressure, while a very high viscosity material being injected into thin walled sections of a mold with small gates might require more than 138 MPa (20,000 psi), even as high as 345 Mpa (50,000 psi).

43

A fkther discussion of injection pressures in the molding process w lifollow in later chapters. However, it is important for the reader to

understand that the pressure that is exerted on the melted plastic in front of the screw during injectionis considerable. Accordingly, the pressure that is necessary to hold the mold closed during injection, as discussed in the next chapter, is also significant.

CASE STUDY NO. 6: Minimizing the Cycle Time It is obvious thatifthe cycle time can be reduced, more parts can be made in the same space of time. The result: greater profitability! Efforts to reduce the cycle time to the absolute minimum occupy the time of many conscientious molders. There are a number of ways to ensurethat the cycletime is minimized.Most of them involveone or more of the Ten Keys to Successful Molding, as set forth in Chapter 12. However,some illustrationsofferedhere mayhelpinunderstandingthechapter just completed. A molderwithninety (90) injectionmoldingmachines,using identical molds producing exactly the same parts, desired to reduce their cycle time by ?4 second. Yes, one-half of a second. So many parts were being produced that a reduction of that amount of time could result in increased profitability of as much as five percent (5%). All the machines were new and research concluded that the process was optimally tuned to the fastest cycle possible. Further review disclosed that the screws being used, although new, were not completely optimum for the plastic being processed. A new screw design was developed with a resulting threefourths of a second (.75 second) reductionin cycle time. All screws were replaced to achieve the improved profitability. Another molder achieved a greatly improved reduction in cycle time by usinga differently designed non-return valve. Still others have improved cycle times by simply changing the barrel heat profile.

44

Of course, all of the controlled parameters must be properly set to provide the most rapid cycle time possible. These adjustments are discussed in a later chapterof the book. The lesson is: Minimizing the cycle time be can afunction of one or several things. Do E t count any optionsout, including new screw designs,valvedesignsandheatprofiles, in additiontothemore from adjusting processing parameters. traditional approaches resulting

CASE STUDY NO. 7: Selecting the Correct Screw A molder was processing polycarbonate(an amorphous material) with a general purpose screw that was very aggressive, that is, ita had short transition zone and a high compression ratio (you w l i learn about these terms in a later chapter). A screw of this type tends to shear the material being processed, creating high fictional heat and excessive heat in the transition zone of the screw. Because of the heat being developed, the molder was experiencing a reject rate in the parts molded of 19%, nearly all of which was the result of degradation, burning and resulting physical properties that wereout of tolerance. A screw that was designed for polycarbonate was supplied to the molder. The new screw had a long transition zone and a much lower compression ratio which resulted in lessshear, a more gradual heating of the material and without excessive heat atstage any in the melting process. The molder’s rejectrate dropped below2%. The lesson is: Amorphousmaterialsaremorelikelythan crystalline materials to degrade and burn if overheated during the “melting” process, regardless of the heat source. Treat amrphous materials ‘gently”and usethe proper screw.

45

6 Clamp Unit The clamp unit of an injection molding machine performs the following essential functions:(1) holds the mold;(2) closes the mold; (3) keeps the mold closed under pressure during injection; (4) opens the mold to allow the parts to be ejected; and(5) accommodates the ejector system which ejectsthe parts out of the mold. The clamp unit is illustrated inthe schematic drawing(Figure 13) showing the mold inthe open position with the moving platenas far to the left as possible.

The clamping mechanism provides the force to keep the mold closed during the injection and holding-pressure stages of the machine cycle. Although many variations have existed through the years, there are two basic types of clamping mechanism used on injection molding machines: hydraulic and hydro-mechanical. Manyinjectionmoldingmachine manufacturers offer both hydraulic and hydro-mechanical machines, and some now providea M y electrical machineas an option. Eachof the two more common clamping mechanisms will be illustrated and explained.

46

6.1 Hydraulic Clamp System The hydraulic clamp mechanismusesdirect acting hydraulic cylinders to achieve the closing and clampingof the mold. Figure 14 is a of this type of system with the mold inthe openposition. The typical design hydraulic oil reservoir is mounted in a position (some on top and some below the main cylinder) that allows the oil to flow by gravity or by pumping pressure into the main cylinder. Thehigh-speed cylinder is smaller in diameter and permits the rapid movementof the moving platento a point where the mold is nearly closed(m some cases, machines are designed with two small high-speed cylinders,cutting down on the cost of manufacture). During this movement, thepreflZ1 valve is open, allowingthe oil to flow into the hydraulic main cylinderbehind the main ram.

I

When the mold is nearly closed, the prefill valve closes. The hydraulic pressure, developedby hydraulic pumps, developsforce a on the main r m of about 20.7 MPa (3,000 psi). This force is used to achieve the final clamping of the mold, typically a relativelyshort stroke. Depending upon the size ofthe mainram,the 20.7 MPa (3,000 psi) cantranslate into a clamp tonnage of considerable magnitude.

47

For example, if the main ram is 540 mm (21.25") in diameter, it would have a surface area of about 229,032 m m 2 (355 square inchesor .785 x 21.25* = 354.48). If the injection molding machine hydraulic system permits a force of 19.43 MPa (2,817 psi), the clamp tonnage would be approximately 453,590 kg or 1,000,000 pounds (or 500 tons, 354.48 sq.in multiplied by2,817 psi).

6.2 Hydro-Mechanical System The hydro-mechanical clamp system uses lever arms, toggles or other mechanical devices to multiply the force exerted by the hydraulic system to achieve the desired clamping force. The most typical hydromechanical mechanismis referred to as a toggle clampsystem. Toggle Links Moving Platen [Open ~ositionlPlaten Clamp

Stationary Platen

Hydraulic Clamp Cylinder

R a r e I5 Injection molding machine toggle clamp unit

Figure 15 illustrates the functioning of a double toggle clamping unit. It is referred to as double toggle because of the two sets of toggle links bringing force to the moving platen, one set at the topand another set at the bottom.On smaller machines, typicallywith 45,359 kg (50 tons) of clamp force and less, asingle toggle clamp unitis often used. Because of the mechanical force enabled by the toggle links, the hydraulic cylinder tha

48

provides the force to the toggle links can be much smaller than on a hydraulic clampunit of comparable size. The mechanical advantagein the toggle system can range fiom 25:l to 50:l. This mechanical advantage reduces the diameter of the hydraulic cylinder requiredto achieve clamp force. It also permits the use of lower hydraulicpressures and hydraulicoil requirements. An example for the latest model of a U.S. manufactured machine: Machine T v ~ e

Clamr, Force Hydraulic

Hydraulic Toggle

725 tons 725 tons

Oil CaDacity 498 gal.

235 gal.

The toggle clamp unitin the closed positionis shown in Figure 16. Clamp

Platen

Toggle Links

Moving Piaten

Stationay Platen

Hydraulic Clamp Cylinder A p ' J 6 Toggle clamp unit in a closed position

The specifics of thedesign of hydraulic systems usedon injection molding machinesis beyond the scope of this book. What is important for the reader to remember is the incredible force that these systems generate to clamp and hold the mold closed. Injection molding machines in use today offer clamp tonnage from less than 27,000 kg (30 tons) to more than 4,500,000 kg (5,000 tons)!

49

Injection molding machinesare identified by their clamp tonnage and ounce capacity, for example,200 ton - 20 oz (567 g) machine. As we have learned, this means that the clamp unit can develop 200 tons of force to hold the mold closed during the injection and holding stages, and the injection unit is capable of generating a maximum shot size of 20 ounces (567 g) ofpolystyrene. Polystyrene isthe plastic material used for standard ratings of capacity. Both hydraulicandtoggleclampsystemshaveadvantagesand disadvantages and,as a result, some of the larger injection molding machine manufacturers offer bothtypes of machines. The relative meritof one vs. the other is not withinthe scope of this book so we w l ileave that debate to the molding machine manufacturers themselves. The clamp force is needed to withstand the opposing force of injection pressure as the screw comes forward. This pressure is typically between 16,000 psi and 22,000 psi m(1a )52 . Does this mean that a clamp force ofabout 20,000 psi(10 tons/ inch2)is required to offset the injection force? No, fortunately! If such force was required, even greater tonnage capacities wouldbe required to achieve successhl molding. Because of the viscosity and temperatureof the molten plastic,the runner system, gate size and temperatureof the mold, normal production can be conducted with about2% to 3 tons/inch2 of projected area in the mold. The term projected area is the area of the molded parts (including the runner systems) in the mold that is parallel to the platens [7]. The projected areawill be discussed firther in the next chapter on molds. One more interesting factabout clamping unitsis the effect of the clamping force on the tie bars. The clamping force is equally opposed by a stress on the tie bars (see Figure 14). Because the clamp force is so great, the tie bars actually stretch when the unit is fully clamped! For example, if the tie bars on a 500 ton clamp unit are 6 inches (152 mm) in diameter, whenM y clamped, those tie barsmay stretch as much as 1/16th of an inch (1.6 mm).

6.3 Clamp Unit Specifications In addition to understandins how the clamp unit functions, there are several specifications relatingto the unit that are equally important to the molder. The more important of these specifications are:

a. Mold Height MaximumMinimum - this specification defines the maximum and minimumtotal thickness of a mold (referred to as mold height) that canbe used in that particular clamp unit.

b. Maximum daylight - this is the maximum amount of space that is available between the movable and stationary platens when the clamp is completely open. It is approximately equalto the maximum mold height plus the clamp stroke. This specification can dictate the opening between the two halves of the mold that would allow a part to fall fiee fiom one mold half without damagefiom hitting theother halfof the mold.

c. Clamp Stroke - the distance that the movable platen is ableto move (open) is referred to as theclamp stroke. Thisspecification, combined with the maximum mold height, can serveto limit the height of a product that you can mold. When molding a container, the mold must open far emugh toremove the partfiom the mold without any damage to the part.

d. Pbten Sue - the horizontal and vertical measurement ofthe platem cm a determinant in the maximum size of the mold that can be mounted int b t p,articular clamp unit.

e. Disknw BFtwen Tie Bars - most clamp units havetwo upper and two bwer tie bars (also referred to as tie rods). Because most molds are mountedinto the unit fiom the top, thehorizontalmeasurement is perhaps more important than the vertical dimension. This specification, combined with the platen size and the maximummold height, serves to limit the overall size of the moldthat can be placed in the clamp unit.

51

Mwable

Stationary Platen

Platen

o-”

. . . . I

.-..

b - Clamp droks

- ..

c Mmmnnn closed

dayli&t

It should be noted thatthere are injection molding machines an the market that are considered “tie bar-less” machines which facilitate the mounting of and access to the mold. Most of these machines are of d e r clamp tonnages (under 500 tons). Two additional specifications, ejectorstroke (maximum) and ejector force, are discussed in the following paragraphs. Figure I7 may help the reader better understand the specificationsfor the clamp wit,

6.4 Ejector System

The method of ejecting parts out of the mold invobs bath the clamp unit and the molditsel€Hydraulic ejectioq is af0ature of all machines. However, there are some older,kjmliw molding machines still in service that use “mechanical ejectioq.’,’Bwause mast machinesbuilt during the last 20-25 years include hydravlic, ejection (except electric machines), the following explanations w libe limited to that type of system. The clampunit includes the hydraulic mechanismthat provides the force to push the part out of the mold. This mechanism isalso referred to as a hock-out (KO.) system. The knock-out system derives its force fiom

52

one (centrally located) ortwo (side mounted) hydraulic cylinders. In some cases, the force may even be pneumatic or electric, but those instances are the exception and usedonly in special circumstances. Although the mounting of the mold will be discussed in the next to chapter, it is important to thediscussion of theejectionsystem understand the standardized hole patterns that are included in both the moveable and stationary platens. The patterns include both mounting holes (for use in mountingthe mold to the platens) and knock-out holes (which arevital to the ejectorsystem). The pattern of holes was standardized by the Society of the Plastics Industry(S.P.I.) many years ago and includes severalsizes (of patterns and holes) depending upon the size of the injection molding machine itself.Figure 18 illustrates the patternof holes for a500 ton machine.

01:

:1

0

0

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0 0

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0 0 0 0 0

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Knock-out Holes

0 0

Q 0

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0 0 0 4

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Tie Bar Holes

0

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figure 18 Platen hole patternfor 500 ton injectionmoldmgmachme

To achieve ejection, ejector bars (also referred to as ejector rods) are inserted through the moveable platen and threaded into the ejector housing (sometimes referred to as the ejectorbox) that is mounted on the back side of the movable platen. The ejector housing is typically a steel plate that rideson guide rods. The hydraulic cylinder(s) that sits behind the ejector housing actuates the housing to move forward which, in turn, causes

53

the ejector bars to move forward. The ejector bar enters the back side of the mold and contacts the ejector prate in the mold, causing it to move forward.

The mold ejectorplate is in contact with oneof several possible devices, such as ejector pins, a stripper plate or anotherejection mechanism that,whenpushedfoward, contacts the moldedpart and ejects it out of the mold. Springs, mounted between the mold and the ejector plate, return the ejector plate to its normal position. All of the activity that occurs inside the mold w li be further explained inthe next chapter on molds. The important pointto understand about the ejector system involving the clamp unit is how the hydraulic force causes a forward action against the mold ejector plate. That action is illustrated inFigure I9 below.

II I

. The ejector systemshownabove is typical for manymolding machines. Othertypes of systems are used, involving:(1) a single hydraulic cylinder, (2) a push-pull action with the ejector bars bolted to the ejector plate, and(3) floating ejector bars. However, the principles involved in their operation are similar to thoseillustrated.

54

CASE STUDY NO. 8: Be Square! On many occasions, molders have experienced flashon the parts being produced yet a careful reviewof the mold and processing parameters fails to disclose a cause. Finally, a check of the squarenessof the platens, one to the other, revealed the source of the problem. In similar instances, the mold halves have been found to not be square, one to the other. so fundamental and obvious, such Although these maintenance checks seem problems do occur. The lesson is:Be absolutelysure that the machineplatens and the mold halves are square before processing. It may save considerable time and headache later!

CASE STUDY NO. 9: Be Careful! On several occasions during the past few years, we have read of cases where a machine operator has lost fingers and hands as a result of not paying heedto the safety rules involving the clamp unit. In some more violent accidentsthat have been describedin trade journals, men have been crushed to death when the clamp unit accidentally closed on their bodies. Although we cover this again in the chapter on safety, it bears repeating here.

All injectionmoldingmachines are equippedwithsafetiesthat prevent the accidental closingof the clamp unit. Yet there have been and will no doubt be many operators or maintenance men in the hture that on the clamp unit or disable the safeties to theoretically facilitate their work the mold while itis in the machine. The results canbe fatal! The lesson is: Do NOT &able machine safeties that affect the operation ofthe clamp unit, While working aroundthis unit anaYor the mold, turn the power off andproceed according to the safety rules that the machine manufacturersupplies.

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The injection mold is the element of the injection molding system that receives the molten plastic fromthe injection unit, formsthe shape of the desired plastic part, providesthe necessary cooling to solidlfL the part andejects the part.Regardless of the type,moldsconsist of several components, eachof which hlfills a vitalh c t i o n in the molding of parts.

7.1 Mold Components Molds consistof twohalves. Onehalf is the statwnary halfwhich the usually containsthe cavities or female portionsof the mold which forms molded parts. This halfof the mold is clamped to the stationary platenof the machine. Thecores or male portions ofthe mold are typically contained in the movable half of the mold which is clamped to the moveable platen of the machine. The separation betweenthe cores and the cavities, where the mold comes apart, is calledthe parting line. If you pick up most plastic parts and examine them closely, you can see a very small raised line in the plastic wherethe two halves of the mold came together. The flat steel sections that comprise a are mold referred to asplates, hence, molds may be referred to as a two-plate moldor a three-plate mold, indicating that the core sections, cavitysections and the runner system is contained within eithertwo plates or three plates in the mold. The runner system includesthe sprue or main channel through which the molten plastic enters the mold and the smaller channels, or runners, which carry the plastic fiom the sprue to the cavities and cores. As the molten plastic between the cores and cavitiescools and solidifies,so does the plastic in the sprue and runners. A two plate mold, which is one of the most common types of mold in use, is shown in Figure 20. It is referredto as a two plate mold because the parts the runner system are all contained within two plates, the A plate and the B plate. Because other types of molds involve the same principles thatare present in the two plate mold, its components will be reviewed to gain a basic understandingof how the mold works.

56

L* Rere 20 Ttro plate moldin a dosed position

Figure 20 shows the side view of a two plate molld in a closed position. Notice that in addition to the two plates referredto as the A and B Plates, which contain the parts and the runner system, there are three other plates that comprise the main elements of the mold. All of these plates, collectively, are referred to as the mold base. There are companies that manufacture standard-sized mold bases which can be used for molds of more simple construction. In this illustration,the parts are shallow bowls, whichare shown at the ends of the runner system. The very small opening where the molten plastic moves f h m the m e r system intothe part is referred to as the gate. There are many types of gates, some ofwhich w l l ibe illustrated laterin this chapter. Again, if you pick up a plastic part and look carefully, you w ill likely be able to identifl the gate through which the plastic flowed.

57

IR addition to the mold componentsdiscussed,several other components are identified in the illustration which fulfill the following hctions: Function Performed

Mold Component

rop c~arnpingPlate 'A" Plate

I ~~

Allcmrsthecavrtysidedtheroldtobeclampedt0the stationary platen of the injection molding machine.

Holds the cavity inserts. the sprue bushingand runner system. ~~~

2aVity Insert

Formsthefemalesideofthepart. Canbereplacedor repaired if worn or cracked without replacing the "A" Plate.

Part

ThespacebetweentheCavrtylnsertandtheCorelnsertthat is inwith d e np k t k and then coded and s d i i e d to form the desired part.

Core Insert

Forms the male side c4 the part. Like the Cavity Insert, can be repaired or replaced if requiredwithout replacing the"B' Plate.

Locdesvlecavrtysidedthemddtothestatiafyplatenof the machine. Containsthesprueand,liketheinserts,canbereplaced without replacing the "A" Plate.

Runner System

Accepts the molten plastic and directs it to the gate (small apeningt0thepartspace)whereitenterstoformthepart.

Coding Lines

Channels cut into the mold to permit the circulation of water to cod the part to a s d i state.

"B" Plate Similar the to

"A" Plate. Hokls the core inserts

and space for

qectorpins.

Leader Pins

Guidethecoreandcavltyhaksofthemddtocune tcgether quicklyand accurately.

Ejector Bar 8 Ejector Plate Provides the force to theEjector Plate to push the ejector pins forward to eject the pari off of the core.

* o r

Ejector RetainerPlateRetainsthe pins so that after retracted to their original position.

Ejector Housing

ejed~on.they can be

Together with the Support Plate and Support Pillars, forms the epxtor box which houses the ejectkm components.

The previous illustration shows the mold in a closed positionat the time the plastic is injected. When the mold opens, as required when the movable platen retracts, the parts and runner systemstay on the core half of the mold andthe spruepuUer pulls the sprue out ofthe sprue bushing. This is illustrated in Figure 21. The parts areready to eject andthe runner system andsprue must be separated fiom the parts.

Rgwe 21 Two plate mold m en open position

There are several methods of separating the parts fiom the sprue and runner system, One is a sprue picker, which is a pneumatic device similar to a robot, that goes into the mold opening and, before ejection, clamps onto the sprue and pullsthe sprue and runner system away fiom the parts, disconnecting at the gate. The other method is a conveyor that is designed to separate the parts fiomthe runner systemwhile conveying. The connection at the gate is usually very small and easily snappedOE

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Figure 22 shows the parts, runner system and sprue their in ejected position. This illustration assumes that a sprue picker was not used to separate the runner system andsprue from the parts.

Ejecto Pins

a

Rgure 22 Two plate mold shown at ejection

The design and manufactureof molds is a very specializedprocess and requires talented designers and craftsmen (mold makers). You have no doubt seen complicated plastic parts, such as cups with handles, plastic parts with threads so that they can be screwed onto another object and sophisticated medical parts (like syringes) that require a very precise fit to perform their intended task. Allof these parts, whichare designed by the Product Designer(who, in effect, dictates part of the mold design), must then be reviewed by Mold the Designer (who may work from a hand-made model or apartdrawing)who designs the type of moldandmold construction necessary to producethe part. Mold designis very complex and requires a knowledge of plastic materials, tool steels, mold construction and many other factors necessaryto make an efficient hnctioning mold.

The mold makers, who are craftsmen as well as excellent machinists, worlung with sophisticated equipment, then manufacture and assemble the mold according to design. The tolerances in many molds arevery demanding, often requiring components that do not deviate from the design as much as the thickness of the hair on your head.

In the design andmanufacture of the mold, there are many additional factorsto be considered. Because of their complex nature,they will only be mentioned here so that you may better appreciate the tasks involved. The runner systems needed to serve a mold that is making a number of parts in one shot can be quite elaborate. The designer may use a circular runner design or a symmetrical design, but all designs will normally be balanced, that is, the distance fromthe sprue to each parthas the same length and flow volume. Typical designs are shown below in Figure 23.

Parts

I \m

Rgure 23 Illustration of two bdancednuvler systems

The cross-section of the runner system can take one of several full-round, half-round, quarter-round, different forms, including trapezoidal, and modified trapezoidal. The full-roundandtrapezoidal forms are considered best because they provide the least contact withthe cool mold and, therefore, provide the best flow to the cavity. You also noted the gates in Figure 23. As with runner systems, gates can take a varietyof forms. They may be a tab gate,fan gate, ring gate, submarine gate, and others.It is not important thatyou know howor why these gates are used, but only to know that there is a variety of methods to gate a part.The gate is always smaller in cross-section than the of their runner so that the part may detach cleanly and easily. Because small size, gates also impart some shear to the melted plastic as it enters the cavity.Excessiveshear is notdesirableand can causecosmeticand physical imperfections inthe part. A closer look at Figure 23 shows a feature called a slug well. Because the initial surgeof material will cool as it goes through the sprue and runner, a “slug” of this cooler material may develop. The failure to “sidetrack” this slug could cause stresses when injected into the cavity [7]. An extension to portions of the runner system, where a comer exists, allows the slug to be “sidetracked” and remain in this slug well. In effect, the slug actually forms a rounded, insulated turn for the melt to follow. A large slug well is also designedjust below the sprueto accomplish the same purpose. There are severalslug wells in Figure 23.

The runnersystemsthathavebeendiscussedaboveare “cold runnersystems,” meaningthattherunnersarenotheated. The outer portion of the melted plastic in the runner system develops a “skin” of semi-solidified melt that insulates the center portion of the melt flow in the runner. In addition to cold runner systems, there are “hot runner systems in which the runners are heated by an external means to keep the plastic in the runner in molten form. These systems involve a different type of mold design which is discussed briefly in the next section of this chapter. ”

7.2 Types of Molds As suggested in the previous section, molds can be classified as either “cold runner molds” or “hot runner molds.” Within each classification, there are multiple types of molds that differ in the way in which they are constructed.

7.2.1 Cold Runner Molds Cold runner moldsare classified as such becausethe runner system is not heated and solidifies in the same manner as the part. The runner system is then ejectedalong with the part. Thetweplate mold, illustrated in the previous section(Figure 20), is probably the most commonly used cold runner mold design and draws its name because the parts and runner system are contained withintwo mold plates, generally requiring an external force to separate the parts fiom the runner system. Thistype ofcold runner mold is ideal forparts requiring large gates. The two-plate mold illustration (Figure 20) shows the parts being gated on the side or edge of the part. If the part was deeper, it would be more desirableto gate the part at the top center of the part. An alternative cold runner mold designis a three-plate mold which allows top center gating and differs fiom the two-plate mold bythe addition of a third plate referred to as a runner plate (Figure 24). A fourth plate (runner stripper plate) is fiequently added in lieu of using ejector pins. Instead of the runner system being located in the “B” plate as shown in Figure 20, it is contained in the added runner plate, allowing the desired center top gating of the part. When the mold opens, the parts are degated, leaving the runner system in the runner stripper plate. Continued movement of the core and cavity halvesof the mold, combined with some bolts(tie rods and stripper bolts) that restrict the movement of the runner stripper plate, result in stripping the runner system and sprueoff of undercut pins (sucker pins) located in the top clamp plate,allowing the runner system and plastic sprue to fall down out of the mold.

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PL2 I

PL.1 I

/

PL.3

Rgunr 24 T h e plate mold in a closed position. PU is parting line where m o l d ~ o p e n s . P I . 2andPL3 are secondandthirdpartinglinss

The three-plate mold is unique in that it opens in three places: (1) Parting line # l between the runner plate and the stripper plate;(2) Parting line #2 between the core and the cavity plates; and (3) Parting line #3 between the top clamping plate and the stripper plate.The mold opens first at P.L.l, breakrng the gateand leavingthe runner attached to thestripper plate because of the sucker pins. The moldthen opens at P.L.2, leaving the part on the coreawaiting ejection. The final opening at P.L.3 occurs as the mold opens further, strippingthe runner off of the sucker pins.

-

-

The three-plate mold in Figure 24 is shown withejector pins. As discussed on the previous page,a mold of this typewould fiequentlyutilize a stripper platerather than ejector pins.

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7.2.2 Hot Runner Molds In a hot runner m f d , the runners are maintainedin a heated condition to keep the molten plastic in a fluid state at all times. Sometimes the hot runner molds are referred to as “runnerless” molds. Hot-runner molds are similar to three plate molds, except that the runner system is contained in a runner plate that is never opened during the cycle. This heated runner plateis insulated fiom the rest of the cooled mold. Hot runner molds have several advantages over cold runner molds. First, there is no runner system and sprue molded as a by-product that must be ground up and reused. Second, sprue pickers or special conveyorsthat separate the runner system and sprue fi-om the part are not required. Third, a d o r m melt temperature can be maintained all the way from the nozzle to the cavity, insuring fewer deviationsin part quality dueto a melt that is not isothermal. Fourth, the shot capacity and clamp tonnage required thein injection molding machine are decreased by the size of the sprue and runner system which, although not a part, must be considered as a molded byproduct [ 1l]. There are some disadvantagesto hot runner molds, including ditficulty in controlling their temperature, inability to purge, making repair a time consuming process, and not being able to change color easily. There are two types of construction for what is referred to as “hot runner molds,” the insulated runner molds and true hot runner molds. The insulated runner m f d has runners that are very thick and are contained partly inthe top clamping (or backup plate) and partly in the “A” plate. The size of the runners allows the development of a thick “skin” of plastic around the outside perimeter of the runners which insulates the molten plastic on the inside ofthe runner. These moldsare oftenassisted by heated torpedoes that are inserted in each gate and are kept in a continually heated condition [7]. When these moldsare started up, the cold runner system must be removed completelyby separating the backup plate fiom the “A” plate and reassembling them.This type of mold in notas common as the true hot runner mold.

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The true hot runner ntotd (Figure 25) functions similarly to the insulated runner mold, however, the runner system is kept byhot heating the runner plate itselfor by using a heated manifold through which the runner system moves(hot maniford mold).The distinctionofthe hot runner mold is the ability to start up cold without intervention by an operator, as descriid above for insulated runner molds. The hot runner mold shownin Figure 25 illustrates the electrically heated manifold and insulated nozzle are thatcommon to this type of mold. The heated runner system within the manifold is insulated fiom the rest of the cooled mold.It should alsobe noted that, unlike the cold runner molds, the runner sectionof the mold does not open during the molding cycle. The parting line of the mold is noted in the illustration andthe part is held onto the core by undercuts as the mold opens. After opening,the part is ejected off of the core by ejector pins, a stripper plate or other ejection mechanism.

I

+"-U M

r

Rgwe 2.5 Hot runner mold in a closed position

66

7.2.3 Other Mold Types There are other types of molds that are defined by some type of special construction or function rather than by the type of runner system. These types can often use either hot or cold runner systems. They include: (a) StripperPlateMolds - wherethenamerelates to the construction of the ejection mechanism. Rather than use ejector pins, a separate plate with openings that, when the plate is pushed forward during ejection, actually strips the part offof the core, avoiding any ejector pin marks. This type of mold is popular for molding cups or cylindrically shaped parts. (b) Slide Molds - in this case the mold contains partial plates that are referred to as slides that move away from the part as the mold is opening. These slides are used to form a portion of the part that is not symmetrical. A typical example mightbe a coffee mug where the exterior of the mug involves a handle that would be sheared off at mold opening. Instead, a slideon each side of the mug forms the exterior of the mug and they pull away “sideways” as the mold opens. This side pull action be can accomplished with‘‘unglepin~~~ that mechanically cause the sliding action as the mold halves are opened, orcore by pull^'^ that do the same thing but are activatedbyhydraulicsasthemoldopens,withtheslideplates remaining closed against the core until the core and cavity plates are open. The part is ejected off of the core after the core pulls have completed their function. Core pulls are used to accomplish the forming of many part configurations and are essential to most custom molding operations.The core pulls are activated by the hydraulic system of the injection molding machine or pneumatics, which are available machine accessory options. (c) Unscrewing Molds - this type of mold is designed especially to produce threaded parts, such as caps for bottles. Typically the internal threads aremoldedbythecoreand,afterthemoldisopen,thecore rotates while the part is held in place until the core rotates itself of outthe part. This type of part, usually in larger sizes, can also be produced using

67

a “collapsible core” which mechanically folds inward allowing the part to be ejected. Both typesof construction are specialized and expensive. They are also very fascinating to watch! (d) Stack Molds - these molds essentially consist of two molds “stacked” together. The basic construction canbe two-plate, three-plateor a hot runner system. The center plate section forms cavities and/or coreson both sides and rides on a geared mechanism that opens two parting lines simultaneously. Twice as many parts may be produced in this type of mold. These molds are quite complex and require explanation beyond the scope of molds in use today, however, they all of this book. There are other types embody the principles used in those discussed above.

7.3 Ejection Although the ejection of parts was briefly reviewed in the sections on the clamp unit and mold components, this subjectis one that warrants further discussion. The type of ejection discussed in the earlier sections is described as hydraulic ejection which uses ejector bars that pass through holes in the moveable platen and, at a selected time in the cycle, are activated by machine hydraulics to push against the’ejector plate. As the ejector plate moves forward, it causes ejector pinsto contact the part and push it offof the core, completing the ejection of the part. Springs maybe used to return the ejector plate toits begin position or the ejector bars,if fastened to the ejector plate, can also return it to that position as the mold closes forthe next shot.

In the older machines,mechanical ejectionwas more common, and the ejector bars (also referred to as “bumpers”) used the mold opening As stroke, rather than hydraulics, to provide the action needed for ejection. the machine opens the mold, the ejector bars would contact a stationary par of the clamp mechanism positioned behindthe moveable platen. As the mold continued to open, the ejector bars, now stationary, would contact the ejector plate in the mold, causingit to move forward against ejector pins which pushed the part offof the core. This method of ejection, virtually obsolete inthe newer machines,is still used by older machines.

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Hydraulic ejection offers several advantages: (1) it can be actuated at any point in the mold opening stroke; (2) it offers the possibility of multiple ejection when operating in the automatic mode to insure complete ejection of the molded parts; (3) it allows forthe sequencing of core pulls which may require ejection beforeor after thecore pull action; and(4) the length, velocity and pressure of the ejection stroke can be regulated within design limits to conserve timein the cycle and prevent mold damage. Air cylinder ejection is sometimes usedto “blow” the part offof the core. Stack molds which may require ejectionfiom the stationary half of the mold(which has noejectionmechanism)canuseaircylinders to accomplish ejection. Syringe bulbs made fiom thermoplastic elastomers(a rubbery-like plastic that can stretch) can be molded over a knob-shapedcore and be blown off of the core rather than having to use a more expensive collapsible core. In some cases, air ejection is used in addition to hydraulic ejection to simply facilitatethe removal of the part fiom the mold. Previous discussions have referred to ejector pinsor stripper plates to removetheparts.Althoughthese two mechanisms are the most common, sometimessleeve ejectors are used to eject tubular parts. These ejectors are very delicate and are typically used wherenothere alternative is [l]. There are other morerefinedconsiderationsinvolvingejection, however, they should be reservedamore for technical book describing mold design.

7.4 Projected Area The projected area of a mold isthe area in the mold that will be filled with plastic at the mold’s parting line. The parting line of a mold is the primary opening of the mold where the core and cavity halves separate, allowing the parts to eject. Theprojectedarea has also been defined as “the area of the shadow castby the molded shot . . . on a plane surface parallel to the parting line [ 1l].” This includes the runner system. The projected area of the mold is used to establish the tonnage of an injection molding machine that is requiredto run a particular mold.

69

It is important to understand how to calculate the projected area of a mold. Figure 26 is a sketch of a simple moldthat produces rectangular flat piecesof trim tilethat could be used in a bathroomor kitchen.

Rpt.326 F'rojectedareaforafourca\rilymold

In this illustration, assumingthat therunner system occupies about 4 square inches (2,580 m m 2 ) and each part covers 24 square inches (15,484 m m 2 ) , the total projected area would equal approximately 100 square inches. (24x4 = 96 + 4 = 100) or 64,516 mm.' We learned in Chapter 6 that a clamp force of about 2% to 3 tons for every square inch of projected areais adequate for most molding conditions (based on a 20,000 psi rated machine). Accordingly, the projected area of the mold in this illustration should requirein the range of 250 to 300 tons ofclamp force. A 300 ton machine shouldcertahly be enough tonnage and very likely 250 tons would also be adequate.

711

It should be noted that thinnerpartstypicallyrequiregreater clamping force than thick-walled parts. Also, when molding with a higher temperature melt,a hotter mold, largergates or a fasterrate of injection will steel molds require a higher clamping pressure. Remember, however, that can be crushedby too much clamp pressure and some authorities estimate that mold destruction requires only 10 tons per square inch[ 1l].

7.5 Mold Venting Although relatively simpleto understand and basicto the molding process, venting is a subject that receives too little attention. Venting is simply the method of allowing the air that is trapped inside the cavity space (and runner system, ifapplicable)to escape. As the plastic is injected into the closed mold, ittraps any air that isin the cavity. If the air has no place to go, it w l i be compressed and, as a result, heated to a temperature sufficient to burn the plastic that is forming the part. Black or brown spots or streaks in the molded part offer evidence of inadequate venting. In addition, if there is sufficient air trapped in the cavity, the air may prevent the part from being completely filled, also causing a rejected part. Venting is accomplished by creating a verysmall gap or opening at the edges of the cavity so that air may be released out of the cavity rather than be trapped and burned. The openings are small enough, typically .0005" to .0015" (.00013 to .00038 mm) deep and .040"to .120" (.010 to .305 mm) long, to allow the air to escape but notthe molten plastic. Obviously, some molten plastic is much more viscous (thicker) than others.Forexample,nylonisveryrunnyand thin (non-viscous)and requires a ventthat is very shallow (.0003" to .0005"), whereas, acrylic and in the range polycarbonate are very viscous and can have vent gapsare that of .0015" to .0025" (.00038to .00064 mm). In addition to the vent gaps, there are usually vent relief grooves into which the vents exhaust. These mm) deep and.250" grooves are deeper thanthe gaps, typically .030" (.762 (6.35 mm) to .500" (12.70mm) wide, and mustrun out to the atmosphere to permit the complete releaseof the trapped air. The vents and vent relief

71

grooves are usually precision ground into the mold components. Usingpart of the previous illustration, Figure 27 shows the vents and vent relief grooves.

P ! -

27 Ilhrehation of typicalwriting of a mold cavity

7.6 Mold Cooling There are many control functions that are employed on injection molding machines and theyare discussed in Chapter 8. In addition, there is a h c t i o n that is not controlledby the machine, butrather by an external source. That isthe control over mold cooling. This is accomplished by the proper construction of cooling channels in the mold itself and a Mold Temperature Controller. The Mold Temperature Controller is an external source that is connected to the mold water lines and also to either a chiller or water source. It controls the temperature and velocity of the water that flows through the mold.

72 Themolddesigner is responsiblefor the proper sizingand positioning of the water channels in the mold and how those channels are used to cool the parts. It is nearly impossibleto have too much cooling built into the mold because the degree of cooling can be regulated by the temperature and flowrate of the fluid. One ofthe considerations inthe selection of the type of material to use in the manufactureofmold cores and cavitiesis the thermal conductivity of the material itself. It is interesting to note that the tool steels that are typically used to make core and cavity inserts have twice the thermal conductivity of stainless steels. More significant, berylLium copper (which is often used where rapid cooling is essential) has more than twice the thermal conductivityof mold tool steels.

Cooling channels are holes drilledthrough the cores and cavities and they should be as close as possible to the surface of the plastic, considering the strength of the material used. Typical channels are at least 1/4”(6.35 mm) in diameter and3/8” (9.53 mm) is preferable. The channels intersect to allow for the flow through of the cooling fluid andare plugged at the end withsoft brass plugs. In some tallcores, such as a large drinking cup,a bubbler isused to providean agitation of thefluid,thereby improving the cooling capability. The best way to describe a bubbler that is it h c t i o n s like a vertically mounted drinking fountain. is the Another methodof directing the cooling fluid more effectively use of baffles. A baffle is a flatstrip of brass, stainlesssteel or nylon (to prevent corrosion) inserted in the core or cavity to divide the cooling channelanddirect the flow of thefluidinto two channelscausinga circulating or up-and-down action.

A well designedmold should have several cooling channels that surround the cavities and intrude into the cores to provide uniform and effective cooling. The “in” and “out” ends of the channels are threaded to allow for fittings to be inserted so that “quick connect” hose connections can be made. Incidentally, when the hoses are hooked up, care should be taken to minimize the number of loops in the system.

73

Eipgure 28 Illustration ofthe incorrectlooping of mold cooltng h e s

An incorrect designofthe mold cooling lines is illustrated in Figure

28. Multiple loops offof the same inlet source decreasethe cooling effect of the water because of its repeated exposure to warm cavities without a new cool water source. It is important that the temperature of the water coming out of the mold be no more than 4" F (2" C) greater than the temperature of the water coming in.

Although the Mold Temperature Controller is hooked to a chiller or water source, itis a closed loop system with one or more circuits through the mold. Water that is too warm or too cold may be dumped fiom the system and replaced with water of the temperature desired. The cooling systems use water, water plus glycol, pure glycol, oil or, in rare cases, air. Although the heat exchange fluidis usually pumped through the mold, in some cases, it is vacuum pumped allowingany leaks to leak intothe system

74

rather than out ofthe system and into the mold. It is importantto remember that the flow rate of the cooling fluid is probably more importantthan its temperature. Also it should be noted thatthe cooling liquidmay not be cool nor water. The coolant circulating through the mold is controlled at a temperaturerequired to insure the proper formationof the parts, whichmay be more than 100°F(38°C). When the desired temperature is above 150°F (66"C), liquids other than water (such as oil) are considered for use. A coolant that is too cold will potentially causeparts tostick inthe mold and not eject properly. Alternatively, a coolant that runs too hot can cause warpage in the parts and allow ejector pins to penetrate the'part causing ejector marks that are not cosmetically desired.

prsUra 29 Ilhrctration of p h d configuration of mold coolinglinen

Figure 29 illustrates a preferred method of providing mold cooling. There are other considerations in the design of mold cooling, however, the principles are the same as those discussed.

75

CASE STUDY NO. 10: Maintaining the Mold

Too many times we have observed molds that had not been properly maintained, both on and off the press. Rusty components, hobbed vents and parting linesthat are nicked or otherwise damaged prevent the successful continued useof a mold. The observance of simple maintenancetasks help keep a mold in good condition (in addition to routine mold makerwork), including cleaning the parting lines and spraying the mold with a rust inhibitor before removing fiom the press. In addition, ifthe mold is stopped during production,do not letthe coolant continueto runthrough the mold. Doing so not only can cause rust forming condensation but, if flowing through one halfonly, can affectthe fit when restarted, allowing potential damage to the mold. The lesson is: Take good careof the molds being used Many of them may have cost more than $100,000 and should betreated like a valuableprecision instrument, notjust a large blockof steel.

76

Control Most of the functions of the injection molding machine have been discussed in prior chapters, such as the clamp unit, the mold and the injection unit. In order to cause those functions to perform properly with any degree of automation, a control system on the machine is required. Becausetherearemoretypes of controlsthantherearemachine manufacturers (independent control manufacturers also make them),this chapter will discuss the types of control elements that are present on most machines. The information presented will also differentiate between older types of machines where the controls are more mechanical and rudimentary and the newer machines where microcomputers are used to automate many of the control functions. Because controlling and monitoring are both essential elementsof the control system, each will be discussed separately.

8.1 ProcessingFunctions There are several processing functions that mustbe performed on all injection molding machines regardless of the manufacturer or the of age the machine. These functions include:

Clamp Unit Control - this function includes control over the speed with which the moveable platen (and the mold) opens and closes without jerking, how far the platen opens and the amount of clamp tonnage pressure that is exerted on the mold to hold it closed during the cycle. It also controls some vital safety settings that prevent the mold from closing under abnormal conditions (i.e., a plastic part or other obstruction is present that would damage the mold or a worker). Extruder (Screw) Control - the speedof the screw rotation (RPM or % of maximum speed) and the amountof back pressure are determined with this control. Also categorized under this function grouping is the decompression of the melt at the nozzle, caused by the screw retracting or “sucking back” a small distance from its full inject position.

77

Temperature Control - the temperature settings of the heater bands on the barrel and the nozzle are controlled in this function. Each heater band zone and the nozzle may be independently set at the specific temperature desired.In some machines,an automatic heatstart-up or “presoak” of the barrel may be set to reach a certain temperature prior to starting the molding operation. m Injection Control- this fbnction includes the control of the shot size (usually stated in inches that the screw injects forward),the fill speed or velocity of the screw as it comes forward during injection, the transfer point wherethe screw speed reduces to commence the holding of the screw in place until the sprue fieezes off, and the hold pressure settings that are required while the part(s) are cooled and solidifl. m Core Pull - the selection of the core pull sequence and the positions requiredare determined by this function. Refer back to Chapter 6 to review the use of core pulls in the forming of the part.

Ejector Control - the speed, position and forward limits of ejection are determined in this function. Multipleor pulsing ejection is also controlled as is the retraction speed and limits of ejectors performing multiple ejection. m Purge Control - in addition to the control of various molding functions, as described above, some machines have a control over the purging of the barrel of unwanted plastic, whether to shut down, change color or other purposes. The screw rotate speed, back pressure limitation, purgetimeandnumber of purgecyclesthatarenormallycontrolled manually, may be controlled automatically.

8.2 ControlFunctions All of the injection molding machines manufactured today include a controller that is essentially a microcomputer. The control settings are usually accomplishedby pushing buttons on a computer screen rather than

78 turning a knob on a valve or flippinga switch. It is important to note that there is a difference between sensors (such as a pressure transducer or thermocouple) that measure or sense a condition, controls (buttons, knobs, valves or switches) that set limits on operating parameters (like speeds, distances, pressures) anductuutors that activatea machine function. In an automatedprocess,the controller (microprocessor-basedcomputer) is connected to the sensors and actuators and, based on program logic, cause the actuators to perform their function. Whether the machine is newenough to use a microcomputer, the first control decision is the operational mode. The choices are:

Manual m d e - where the limitsmay be set to desired parameters,but the machine will not perform any functions unless an activate buttonis engaged. This mode is typically used in starting a mold up or in shutting down and purging. Semi-Automatic m o d e - where all the machine limitsare preset but the machinestops atthe endof the cycle andwill not commencea new cycle until the machine safety gate is opened and closed. This mode is commonly used where ejection must be done manuallyor with manual assist (by theoperator) or when a mold is just starting up.

Automatic mode - where all machine limits are preset and the machine cycles repeatedly unless stopped by a safety conditionor by the operator. The following table illustrates some of the control functions and the type of control setting usedin both newer and older machines. the In older machines,much of the control wasachievedwithelectro-mechanical devices. Those devices were proneto wear and their dependability tended to diminish with age. In addition, changes usingthose devices were more difficult to achieve than the programmable logic controllers that are now used.

79 Table of Control Functions Control Function Older Machines Newer Machines ~~~

~~

Control settingon Controller

Wapen"

Limit switch

Clamp pressure

Mdd heQm or pressure relief VahR

Control setting(tons) on CS

zoneheeterband ssttings

Manual control

Control setting(degrees F/C) on CS

Fill speed Fill pressure (Stage 1 or

I

I

Manualflow control mI\R Manualhydraulic relief VahR

screen (CS)

I

I

Control setting (in oncs

or mmlsec)

Controlsetting (%, psi/bar) on CS

Hold prassure( S t a g e 2)

Manual hydraulic relid VahR

control setting (%, psibar) on CS

Hold time (Stage 2)

Manual timer(sec)

Control setting(sec) on CS

relief mlve

Back pressure

Control setting (%, psilbar) on CS Manual

scrpMlspeed(ratide RPM)

ManualW contrd valve

Control setting(% or RPM) on CS

Stroke length

Limit switch

Control setting (inlmm)on CS

Cushion

Se! by stroke length

Se! by stroke length

Ejectorstroke

Limit switch

contrd setting (idmm) on CS

core pull sequence

Switch

Control settingon CS

Tmferposition

Manualtimer basedon fill (sec)

c m settings on CS:

Back Pressure OdOff

Switch

Control settingon CS (offwhen purging)

screwsuckback

sec) Manual timer (10th~

Control setting (position- idrnm) on CS

Mold p r M l o n system

Limit Switch,Mold apen 8 dose slow (*)

Control setting(position - in or pressure %) on CS

Mold OpenlCltxespeed

Flow control mhe

Control setting (% speed) on CS (Sanem be profiled on CS)

Screw position (idmm), or Hydraulic pressure(psilbar). or cavity pressure (psibar)

-

80

In addition to the control settings illustrated in the table, there are certain actuators (buttons, knobs or switches) that are common to all machines. They are bkly selfexplanatoryby their name andare used when the machine is in the manual mode. They include:

> Controller OnlOff - thisactivatesthe

control screen (and microcomputer) allowing all of the other hctions on the machine to be activated. In essence, this isthe o d o f fswitch forthe machine itself.

> Start (Hydraulic Pumps) - this actuator starts the hydraulic pumps on the machine.All machines haveat least two pumps (eventhough they may be in the same housing), one with large capacity and one with small capacity. It is necessary to activate this button in order for the hydraulics of the machine to hction.

> Stop (Hydraulic Pumps)- this simplyturns off the hydraulic Pumps.

> Mold Open - when the machine is in the manual mode, this actuator opens the mold. It is used when mounting a mold,at start up and to the at other times whenthe mold needs to be opened for incidental work Gees of the mold halves.

> Mold Close - this actuator closes the mold for use during setup, shut down or other times whenthe machine is inthe manual mode.

> Heats OnlOff - this actuator turns the heater band heatcontrols on at whatever settings are provided on the controller. To activate the heaters for the oil in the hydraulic system, a separate setting must be activated on the controller.

> Carriage Foward/Back - this switch movesthe carriage (the sled which holdsthe injection unit) back and forward. This is used at start up, shut down and during purge. The carriage may be retracted for other reasons, suchas changing the valve, the screw and/or the barrel.

81

> Screw Rotate - during manual mode, it may be necessary to rotate the screw for alignmentto: (1) remove the screw collar nuts so the screw may be removed and(2) facilitate purging.

> Screw Pull Back - the screw must be retracted in a manual mode in order to ready the machine to commence a cycle. It is also used during screw removal and sometimes during purging.

> Inject Forward - also used during purging and at shut down, after purging is complete, the screw is left in the forward position. Eject Forward - allows the forward movement of the ejector stroke to manually eject parts and in checking the machine in preparation for processing.

> Eject Retract - used in the same hctions as Eject Forward and to ready the machine to commence operation on cycle.

> Core Pull In/Out - this activates the core pull mechanism that allows cores to be retracted or pulled in during setup.

8.3 Process Control With a basic understandingof the machine control functions, it is easy to understandwhy many of the process control functions are automated, using the capabilities of a microcomputer. This automation simplifies processcontrol in one sense, yet it also demands a great dealof expertise fiom the molder. It is clearly not within the scope of this book to provide any level of instruction relative to automated process control. However,webelievethatanyoneinvolved inmolding should have a conceptual understanding ofwhat process control is all about. Keep in mind that you must understand howto mold before you cantake full advantage of process control.

82

CASE STUDY NO. 11: If You Don’t Know, Ask! Most accidents and damage to machines, machine components and operators occur because of a lack of understanding of how the machine and its controls function. Some problems that have occurred in the field include: Setting the ejection pressureso high that the backhalfof the mold was pushedoff of the movable platen Setting the mold close speed too high causing the mold to hit hard upon closing, causing damageto the mold face Setting the eject forward stroke too long causing theejector pins to hob intothe mold These are but a fewoftheproblemsthat can occur when care and understanding are not involved in using the controls on the machine. The lesson is: Study carefully, go slowly and ifyou don’t know ask someone who the result ofa change in oneor more of the controls, knows.

83

9 Robotics and Granulators Materialhandlingsystemsthatmovetheplastic fiom storage facilities to the injection molding machine were discussed in an earlier chapter. It is equally important to consider the handling of the molded parts. Molders are now accustomed to automation andare seeking better ways to: (1) speed cycle times;(2) eliminate moreof the labor cost fiom the molding process; (3) avoid damage to parts that can be caused by normal ejection methods; and(4) perform secondaryoperations that are required in order for the molded partto be a completed product.

Robotics are providing some effective methods to accomplish some of these objectives. In addition, the trendtoward greater automationhas generatedimprovementsin the operation of granulators. Robotand conveyor-fed granulators are a more commonplace, cost saving option used by many molders.

9.1 Robotics In Chapter6, the use of sprue pickerswas described as a method of retrieving the runner system and sprue fiom the mold to facilitate part separation and movement. Quite often the sprue picker drops the runner system and sprue intoa nearby granulator while theparts can proceedto the next station on a conveying system. A form of robotics, the sprue picker is typically mounted to the stationary platen of the machine and, usingpneumatically operated cylinders to providethepower,utilizes extension arms and a gripping device that enters the open mold and“grabs” the runner system, removesthe runner out of the platen area, pivots and drops the runner in a specified place. Another form of robotics is the mechanical roboticarm. These are usually driven by the opening and closing motion of the moveable platen and, by linkage to the stationary platen, moves in and out of the open mold area. A typical linkage systemis a spherical camshaft system. The robotic arms are very fast, quite durable andoperate smoothly. This

84 device also removes the sprue and runner system and has become a wellaccepted method of removing delicate parts (such as molded compact discs) from the mold quickly, quietly and safely. A more sophisticated device that is commonly used in injection molding is referred to as the “pickandplace robot.” These robots, armed with microprocessor controls,servo motors and more elaborate gripping devices, can remove and place runner systems andor parts in precise positions, cut the parts separate fkom the runners, and also place inserts into the mold for the next shot. They are capable of total loads of 200 pounds (90.7 kg) and more and can perform a variety of secondary operations, including weighing, hot stamping, stacking, palletizing, box loading, tray filling and ultrasonic welding. Best of all, today’s robots can be “taught” a sequence for a particular mold set-up, store it and retrieve it the next timethat mold is used.

Because of the varied and complex nature of robotics, theyare only mentioned in this book. Theyare, however, becoming more and more used by progressive molders and savethe price of their purchase in relatively short payout periods.

9.2 Granulators Consistent withthe growth of robotics is the increasing importance of granulators (or grinders), pulverizers and other size-reduction equipment that is compatible with automated production. With growth the ofrobotic-feed granulators, greater automation inthe removal ofwaste and the recycling of materials now enables granulatorsthat offer throughputof 100 to 1,000 pounds (454 kg) per hour to be connected to automated production lines,machinemountedvertical drops (such as the sprue pickers discussed earlier) and conveyor systems. The newer granulators are more efficient, offer higher throughput, operate more quietly and produce a granulate quality that is more easily recycledas regrind. Regrind can be mixed with virgin material in quantities to 50% up and more, depending upon the material andpart requirements, allowingthe

85

processor to recoversomevalue systems or defective parts.

from otherwisediscardablerunner

Granulators are now available with adjustable speed drives to meet specific requirements and provide energy saving options such as automatic shut-down. The developmentof improved materials for granulator blades improves the quality of the regrind, providing more consistently sized granules and minimizing the quantity of “fines” (dust size particles) that make reprocessingof the material moredBicult. Granulators thatare operated manually are still a mainstayof most moldingoperations.Unfortunately,they are also the source of most injuries to molding personnel. The blades are very sharp and, despite careful operation, can become jammed or overloaded.

CASESTUDYNO. 12: Follow the Granulator Operator Manual There have been numerous cases, someknown to the authors, in which personnel have lost fingers and hands in granulators.occurred Most while trying to fiee up a jammed condition. Accidental activationof the start button by whatever means, can cause serious injury to the operator. The lesson is: When entering a granulatorto correct ajammed conditwn or to clean the machine, turn the power OFF! Read and reread the Operator Manual for the machine and follow the instructions to the letter.

86

10 Getting Started Getting started in the injection molding process includes Mold Setup, ProcessSetup, Mold Start-up and Process Documentation. Each of these steps is important, including the documentation of the process after it has been optimized. Although these steps should be performed by an experienced Process Engineer, the information presented below should give you a good guideline toward understanding what is involved in “Getting Started” in molding.

10.1 Mold Setup The objectiveof injection molding isto fill a mold with hot molten plastic and then cool the plastic to solidrfjr to the desired shapeofthe part(s) being produced. As we have learned, the mold contains a space between the cavitiesand cores that forms the shape of the product,plus the necessary runners and gates to direct the flow of the molten plastic into this space during injection. The size of the mold depends on the size and shapeof the parts to be produced. However,the size of a mold that w lifit into a given machine depends uponthe space betweenthe tie barsof the machine. For example, a machine with a rated 200 tons of clamp forcemay have a platen size of 28 inches by 28 inches (71 1 mm), but the space between the tiebars may be only 20 inches by 20 inches (508 mm), allowing a maximum mold size of 19%by 28 inches (495 by 71 1 mm). In this instance, the number of mold cavities will depend on the size of the mold, the projected area, and the weight of the parts and runner system. Total weight cannot exceed the maximum shot size of molten plastic capable of being produced by the injection unitof the machine. In many plants, especially custommolding operations, the molds are changed frequently,as production requirementsofvarious products are met. When a mold is removed and a different mold is put in its place, cooling lines must be removed from the outgoing mold andthen be reconfigured for

87 ~~~

~

the replacement mold. The ejector bars that connect theejector plate of the mold to the machine ejector plate mustalso be changed. This process can be time consuming. Although down time is costly, it is not as costly as expensive mold repairsifthe changing processis not done properly. There are many automated mold changing systems being offered today that can pay for themselves in saved production time. The key steps involved inMold Setup for a new mold (rather than changing a mold that has been run previously) may be summarized as follows: 1. Make sure that the mold size will jii the machine to be used. Nothing is more embarrassing than to have the mold lifted above the machine and find that it will not fit! 2. Examine the Set Up Sheet(a preliminary set up sheet provided by the Process Engineer based on his guidelines for the initial setup) to determine therepiredplastic material. Locate the material and, depending upon the material handling system, arrange loading for the material into the hopper of the machine. Clean the hopper, the magnets and any loading equipment and begin the loading of the material. Start thedryer, if drying is required, in sufficient timeto have material ready for processing.

3. Procure the proper ejector bars and checkto determine that they are of e q w f length. Ejector barsof unequal lengthwill cause uneven mold ejection, hang-ups of molded parts, and unnecessary wear of the ejector plate bushings and guide rods. 4. Clean the sur$aces of the clamp plates of the mold and faces

steel wool of the platens of any foreign material, wiping them down with pads and a degreaser. Insert an eye bolt in a hole on the mold that w li allow the most level handling and attach safety strapsand hoist hook. Be certain the straps or chains used with the eye bolt are adequate and ingood condition. A fiayed strap should never beused. In addition, insist that people stand clearas these operations are being carried out.

88 5. Using the hoist, lower the mold into the open platen space

(avoiding hitting the tie bars). If the mold is horizontal and cannot be loweredthrough the tiebarspacingfromthetop,itmustbemoved horizontally. In this case, a fork truck or crane will support the mold as it is moved to engage the locating ring. Molds suspended in the air tend to swing as they are being moved. Always maintain control of the mold and do not allow itto sway! 6. Check the locating ring on the stationary side of the mold to insure that it matches the hole in the stationary platen. This ring helps to hold the mold in place. It lines up the mold sprue to the nozzle of the barrel. 7 . Position the moldon the stationaryplatenwith the locating ring properly engaged and slowly close the mold so that it can be adjusted rotationally but not tip between the platens. Level the mold and attach clamps totheplatens. Because of the varietyof mold sizes, the numberof clamps needed will vary. However, it is better to have too many mold clamps rather than too few. They are needed not only to hold the mold against the platens, but to support the weight of the mold on the moveable platen. The proper tightening of the clamp bolts is also very important. The over-tightening of the bolts, which can strip the threads in the platens, can be prevented by using a torque wrench. The bolts must also be long enough to accomplish their purpose.

After the mold has been securely clamped to the platens, the safety straps maybe removed and the hoist maybe unhooked. 8. Open the mold to the desired daylight and set the mold open and closed controls,making sure that the settings will not permit a banging of the mold. Continue to fine tune these controls until the proper settings have been achieved.

89 Important Note: The Low Pressure Mold Protectionis a

system built into the molding machine to protect the mold faces, cbities and cores against damageif something is caught between them on m l d closing. The system is NOT for the protection of the operator or setup person. Theforce applied atthe pointof sensingthe obstruction would seriously damage a person’s hands orfingers!

9. Secure the ejector bars to provide necessary ejection capability. The ejector bars may be threaded into the mold and the machine ejector plate (for “tie down” ejection), secured to the mold only or allowed to “float.” Set the ejector stroke controls, repeating the adjustments until proper and not excessive ejection action can be assured. 10. Connect all required hydraulic, electrical and pneumatic power systems and make sure that all are hctional. Do not allow testing of theheatfunctions to becomeexcessivebeforethewaterlines are connected. Double check to see that all heaters, thermocouples, transducers and other sensors are connected.

1 1. Connect the water lines and loops, using as few loops as possible. Make surethe lines will not interfere withor be in the way of the mold movement norbe allowed to rest (or “ride”) on the tie bars. Recheck the fittings for tightness andproper flow direction and turn the water on. All water lines mustbe checked and any leaks must be fixed.

10.2 Process Methods Before proceeding with a detailed description of the steps in Process Setup, it is important to distinguish between two methods of processing that are used inmolding operations today. One method, referred to as the traditionalor conventional method, usesvelocity control (fill rate or pressure) as a means of he-tuning themoldingprocess.Because a

90 significant number of molders use thisapproach and you w l i be more apt to be exposed to it, we will describe it in detail under the Process Setup section of this chapter. The other method, referred to as the velocity control method, maintains a constant velocity (constant fill rate or pressure) and adjusts other variables to fine tune the molding process. This method, a more recently introduced and sophisticated approach, is based on a very valid principle, that the viscosity of the plastic melt changes with flowrate. As explaiied by J. W. Bozzelli, in Injection Molding Solutions, “Plastics do not flow like oilor water. Water and oil do not change viscosity as a b c t i o n of flow rate, plastics do. Plastics change viscosity as injection rate changes [17].” The setting of the fill rate (fill time or first stage pressure) is discussed in more detail in Appendix E. Obviously the use of velocity control in this manner affects modifications that may have to be made in troubleshooting a molding problem. Inthe troubleshooting chapterof this book, however, we haveassumedthat the conventionalmethod of processing is being used.

10.3 Process Setup The Process Setup includessetting the barrelheatprofile, establishing the injection stroke, speed and pressure, and selecting the transfer point. In addition, the controls over mold cooling must be set. 1. The heater band heat settings should be based on the percentage of the shot size of the machine thatis being used. Determine the shot size (including sprue and runner system, unless the setup is for a hot runner mold). The required shot size is easily determined by weighingthe part with its runner system and sprue attached. Then calculate the percent of the machine’s shot capacity used forthe material being processed,as follows: ‘h = Shot Size (oz) + [Machine Shot Capacity (02) + 1.05 X Sg*]

*For spec@ gravity (at room temperature) of materials, refer to Appendix A

91

Machine capacitiesare rated for processing polystyrene (PS) which has a specific gravity at room temperature of 1.05 grandcubic centimeter. The percent of shot size must be calculated based on the material that is beingprocessed.Bydividing the machine shot capacity by 1.05 and multiplying the result by the specific gravityofthe material being processed, the maximum shot capacity for that material will result. For example: Assume a machine shot capacity of 14 ounces in PS and the material being processed is HDPE at a specific gravity of.95 gdcc. Also, assume a shot size in HDPE of 4.7 oz. 14 + 1.05 = 13.333 x .95 = 12.67 oz (capacity of

the machine processing HDPE)

Then: 4.7 (Shot size in oz) + 12.67 oz = 37.1% of shot capacity Incidentally, if you want to know how many inches of stroke is required to fill the mold, simply multiplythe stroke of the machineby the percentage calculated above. Example: If the machine stroke is 8", then:

8" x .371= 2.97" of stroke is required 2. Set heater bandsettings in accordance with the following table. Set Heater Band Zone Settings as indicated, dependingupon the % of shotcapacitybeingused

e 30 %

30 - 50%

> 50%

Reer=AimMelt Temp

Rea=Aim+2CPF

Rear=Aim+WF

Middle=Aim+ l 0 T

Middle=Aim+ 1OOF

Mile = Aim +15OF

Front =Aim

Front = Aim

Front = Aim

Nozzle =Aim

N o d e =Aim

Nozzle =Aim

Aim Melt Temperature is the m o d ln igtemperature recommended by the resin

supplier

l

92

Example: Using the calculations and table above, assume an "Aim" melt temperature recommended by the resin supplier for the HDPE of 425 "F (2 18 "C) and the % of shot capacity of 37.1%, the beginning heat profile setting would be: Rear zone temperature= 445 "F (229 "C) Center zone temperature= 435 "F (224 "C) Front zone temperature= 425 "F (21 8 "C) Nozzle temperature = 425 "F (2 18 "C) The temperature settings suggested above represent the beginning point of the heat profile. It w li beoptimized by procedures that are described later in the book.

Also, many molders have not availed themselves of the advantages of a reverne heat profile. Those advantages will be set forth in detail in Chapter 12. 3. Using the inches of necessary stroke calculated above, set the stroke so that a full shot canbe delivered to the mold. 4. Set thetramferpoint,fillpressure andJill rate controls so that the mold will fill slowly and completely but not flash.Set holding pressure the same as the fill pressure andset back pressureat zero.

5 . Setthecooling time based on data provided by the mold designer. This time can be calculated, however, the calculation is rather complex andis not presented here.If the calculation is not available,there are some tables that can be consulted for approximate values. A sample table is shownon the following page[ 1l].

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10.4 Mold Start-up After allof the preliminary processsetup steps have been completed and double-checked andthe mold heats are up to temperature, molding is ready to begin. The majorsteps that should be included in getting the mold to run on cycle at optimum settingsare set forth on the following pages. You will note that there are some steps that are taken even beforeactual moldingbegins.These steps helpinsurethat the meltis of a proper viscosity and appearance and that the heater band controls are cycling properly with the screw hctioning normally. Remember, however, that for the machine to cycle, the screw must beretracted, the mold must be open, ejection must be retracted and anypulls coremust be in the “out” positwn. In addition, the purgeguard (the metalshield around the nozzle) must be closed. In summary, all safeties must be on and in order! 1. Before beginning to mold, a few air shots should be taken to assure thatthe melt has a uniform appearance and viscosity. Use automatic purge control or manuallypurge (with the carriage retracted and no back forward, duplicating to the extent pressure) using screw rotate and inject possible the timing of an actual cycle.

As the screw comes forward with an air shot, examine the melt, looking closely for any signs of unmelt. Observe how the screw moves

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backward during rotation (steadily without pauses) and review the heater band controls for proper cycling with no heat overrides. If no problems are apparent, you are ready to cycle. 2. Bring the carriage forward with the n o d e seating againstthe sprue bushing and, with the machine in semiautomatic m& and the back pressure on, start the cyclingto begin makingparts. Examine the parts for completeness and proper appearance with themold hctioning properly.

3. After a few cycles with good parts and no apparent problems, increase thefill rate andfill pressure gradually. Observe the cycles and part quality. If flash or burning occur, back off the fill pressure and/or fill speed until goodparts are again produced. (Note:This would obviously be different if processing using the velocitycontrol approach.) 4. Adjust the cushion and decompression to the desired levels and

set the screw RPM so that it recovers just prior to mold opening butdoes not allow the recovery timeto increase the cycle time. 5 . Adjust shot size and transferpoint so that the fill stage (first stage) achieves nearly allof the fill. You can tell thisby watching for any forward movement ofthe screw during packing.If any movement occurs, the first stage is not filling the mold. Continue to adjust until shot size, transfer point, fill rate and fill pressure are at desired levels. Fill pressure should be raised to 100 psi above the actual pressure required to fill the mold. At that pressure,any resistance to flow that mightoccur wil not call for pressures that could damage the mold. 6. Adjust the holding pressure until the part is of the quality desired and cosmetically satishctory. Increasing the pressure too much w li cause themold to flash and maycause the partto stick in the mold. 7. After the mold is cycling as desired withminimal back pressure (100 psi or less), check the screw rotatepressure (SW) of the hydraulic system. It should be between 800 and 1,200 psi (on a 2,000 psi rated

95

machine). Following step 8, it may be necessary to adjust the SW. This requires some experience and should be the function of a supervisor. Such changes may be made by referring to Chapter 12.5.

8. Next retract the carriage and take an air shot into a pan in order to check the melt temperature. Having heated the probe tipof a hand-held pyrometer (to atemperature 30-50°F hotter than the expected melt temperature) just before the airshot, quickly insert the probe intothe shot and determine if the melt temperatureis at the desiredleveL 9. If the melt temperatureis not at the desired level, adjust the heater band controlsto achieve the proper melt temperature and alsothe proper balance of the sources of heat energy (shear and heater band heat), returning the S W to the 800-1,200 psi range. Refer to Chapter 12.5 to make these changes.

10. After the heat controls are stabilized, the proper melt temperature is achieved and the SRP indicates a proper balance of heat energy source, change to the automatic mode and review the molding performance for several cycles. I f there are no problems, the cycle is optimized andthe processing setup should be documented as described in the next sectionof this chapter.

All of the procedures described in the Process Setup and Mold Startup sections abovemay be modified by the process engineer basedon his expertise and experience. Knowledge of these procedures and how and why theyare accomplished in the manner prescribed by the engineer is key to your understanding of the process. With experience, you may learn to perform the procedures yourself with limited guidance.

10.5 Process Documentation One of the most importantsteps in injection moldingis to carefblly document the optimized processing parameters on a Set Up Sheet. This document includes a description of the mold, the parts, plastic used, all controls settings andthe screw recovery and cycle times achieved after the

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desired cycle has been established. The next time the mold is run,the Set Up Sheet may be referred to for the proper settings so that a comparable cycle may be achieved. This information is'alsothe basis for a "Benchmark" against which fkture processing parametersof the mold may be compared. Performance measurement is discussedin a later chapter.

CASESTUDYNO. 13: Dealing with Set Up Problems When setting up the machine to process a mold that has beenrun previously on a particular machine, the new set upmay deviate fiom the set up on the Set Up Sheet thatwas developed forthe previous run. This is an indication that something about the material, machine, mold or machine components has changed. This has happened fiequentlyin the field andthe answers canlie in anyof the following: a new lot of material that is not the same as the previous lot;there are some machineh c t i o n s that may require maintenance; the barrel, screw or valve are not the same or are worn, requiring repairor replacement; the mold needs repair, perhaps vents have been hobbed, cooling lines partially blockedor corroded, and many other possibilities; andso on. The lesson is: Carefully document the process deviations and submit them to a supervisor whois in aposition to recommend or take remedial action.A majorpurpose of the Set UpSheet is toprevent a substandard processing of a mold which will result in lost efficiency and reduced profits. Also note thattwo machines, runningthe same mold design and the same plastic material may not result in the same processing parametersor part quality. Differences may result fiom the conditions described above. Although each machine should have aSet Up Sheet for each application, theyshould all be based on the sameplasticvariables.Deviations in performance need to be corrected by maintenance to the machine and its components.

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11 An Overview = The Cycle The injection molding machine produces a finished plastic part (or parts) in each completed cycle. The cyclecan typically be as short as four seconds or less and as long as several minutes. Only one part may be produced in the cycle (if it ais largepart) or more thana hundred parts may be produced (ifthe part is very small). For example, a fender or body panel for an automobile or a 55 gallon trash dumpster may be a single part produced fiom the mold. By contrast, perhaps 128 small components for a medical syringe might be producedin a single cycle. In some cases, the plastic is reinforced with calcium carbonate (powdered limestone) or small fiberglass fibers. A steering gear housing foran automobile or the rollers for roller blade skates may be the end product. Very small, thin-walled parts may require only a few seconds for their cycle, whereas a large thick part, such as an optical lens, might require several minutes.

-

11.lThe Cycle Defined The injectwn molding cycle is usually expressed as the time (in to seconds or minutes) thatis required by the injection molding machine produce a plastic part or parts from one filling of the mold. The cycle must include the time required to close and clamp the mold, inject the molten plastic into the mold and apply holding pressure, cool the plastic part, openthe mold and ejectthe part. The mold again closes, signalingthe start of a new molding cycle. During the time that the part is cooling, the screw begins to rotate and, as plastic melts and moves forward through the non-return valve,i i h g the chamber in front of the screw, the resulting pressure pushes the rotating screw backward until enough melted plastic residesin fiont of the screw (between the end of the valve andthe end cap) to make the next part. This overlap of machine functions, as well as the entire molding cycle, is best understood by reviewing a molding cycle diagram. See Figure 30 on the next page.

In today's molding environment,the machine desirablyoperates in the automatic mode, allowing one cycle to follow another withthe parts being ejectedonto a conveying system that takes them to another stationfor subsequent operations, inspecting andor packaging. In some cases,a robot picks the part(s) out ofthe mold, also allowing automatic cycling. During automatic cycling, the operatoronly intervenes when there is a problem. It operator tothe machine is commonto have an alarm system which calls the where the problem exists. If only one cycle is to be performed, or an operator is required to open thesafety gate, retrievethe molded parts and close the gate tostart the next cycle, we have learned that the machine is in its semiautomatic mode.Now is a good time to refresh your memory by referring to Chapter 8 for a review of thethree types operating modes.

11.2 The Importanceof Cycle Time It is apparentthat the more rapidlythe machine can complete cycles, thereby creating greater part production, the more profitablethe molding operation will be. There are instances where even as little as one second reduction in cycle timecan represent a signiscant percentage of profit.

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Example: Using a 300 ton, 30 ounce shot capacity machine. The mold is a four-cavity mold with a hot runner system. Parts weigh 71 grams with total shot size of 284 grams or 10 ounces. Production: 15 second cycle,operating in automatic mode,24 hourdday for 26 daydmonth. A 5% reject rate is standard and the machine operates 90% of the available time (the balance being reservedfor normal interruptions and maintenance). This standard or benchmark production rate is also used for latercomparisons to actual performance.

The importanceof cycle time is illustrated in the table below where (or a 14 we assume animprovement of one second on the 15 second cycle second cycle), a sale price of $.l5 per part. Materials cost $.40/lb and machine operatingcost is $45/hr. Operating expensesare 17% of budgeted sales. Benchmark profit is budgeted at 5% of total sales. Nde: Numbers are rounded.

I

Production for One Month 1secondcyclelmpravement

Tdal Plant Hours Totd Machine Hours Parts per hour Less Rejecls Good Parts per hour Tdd Good Parts per month

-

sales value

624 560 960 (481 912 510,720

I

$76,610

$82,070

33,610 25,200 14,000

35.990

T U Costs:

Materids MachineOperation n g E w = = Tdal casts Profit for the month ~~

~

1.028 (511 977 547.120

$3,800

I

25,200 14,000

75,190 $6,880

~~

As you can see fiom the illustration, the profitfiom operating the machine forone month witha one second improvement in cycle time can nearly double the profitability. The importance of cycle time cannot be overstated!!

~~

11.3 The Greater Importanceof “Good Production” We haveseen that the “multiplier” effectof producing more parts in the same number of machine hours through reduced cycle time is significant to profitability because most of the costs (such as depreciation, maintenance and overhead) are “fixed,” thatis, donot vary directly with volume. Of even greater importance is the need to produce ‘kood”parts or, in other words, avoid rejects. The productionof rejected parts not only carries allof the fixed costs but also the variable costs (material, labor and power) of operating the machine without any revenue. Producing rejected parts is worsethanhaving the machine“down.” At least,when the machine is idle, no variable costs are being expended. Using the same benchmark data, the following example illustrates the profit improvement realized byreducing rejects from the “standard” rate of 5% to an improved rate of 1%. Note: Numbers are rounded.

Production for One Month ”Benchmark Rate

Rejects from 5%to 1%

624 560 960

624 560 960

0

0

Total Plant Hours Total Machine Hours Parts per hour Less - Rejects Good Parts per hour Total Good Parts per month

912 510,720

950 532,000

Sales Value Total Costs Profit for the month

$76,610 72.810 $3,800

79,800 72.810 6,990

Profit Improvement

NA

$3,190or84%

The profit improvement from reducing bad part production is even greater than improving the cycleby one second. There are measures that can be taken to optimize cycle times and reduce the number of rejected parts that are produced. Some of these measures will be discussed in the following chapters.

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CASE STUDY NO. 14: Reducing the Rejects Consider the molder usinga new 57400 ton injection molding machine to produce coloredparts and operatingat 43.3% of the machine’s shot capacity. The time for the screw to recover was 16.0 seconds at a screw RPM of 75 and a screw drive motor pressure of 1,900 psi.In this set up, the molderwasexperiencing a rejectedpart rate, mostlydue to improper color mixing, of 5% to 6%. The operation was studied and althoughthe machine was new and was using new components, itwas believed thatthe screw was inadequate the to the color mixing task. A new screw was designed and installed with following results: Screw recovery time dropped from 16.0 seconds to 15.2 seconds which translated into a cycle time reduction of .8 seconds The screw RPM was reduced fiom 75 RPM to 50 RPM and the screw drive motor pressure required also was reduced itom 1,900 psito 800 psi, which reduced energy requirements The percentage of rejected parts dropped fiom 5% to 6% to .l% to .5% In thismolder’s case, the resultingimprovementinprofitabilitywas significant. Not only wasthe number ofrejected part reduced, but the cycle time and energy requirements were also improved. The lesson is: Do not underestimate the importance of cycle time of avoiding the production badparts. of and the even greater importance The solutwn may be as simple as using a different screw!

12 The Ten Keys to Successful Molding

The elements of injection molding have been presented and you have learnedhow to get started in the moldingprocess.Thecycleandits importance have been reviewedso you are now readyto begin assisting in the fascinating businessof injection molding. Through years of experience in working with molders, the authors have learned that there are a few considerations that are absolutely essential to successful molding. We would like to share these thoughts with you so that you wil not have to learn themby trial anderror.

12.1 Adequate Mold Venting Although the venting of the mold is a common and necessarystep in its manufacture, the importance of venting is often misunderstood by injection molders. To fill the mold cavities, the air in the cavity must be pushed out by the melted plastic flowing in. The more easily athe ir can be pushed out, the less likelihoodthere is of experiencing burnsor voids in the parts. In addition, if the air is more easily pushedout of the mold, a more rapid injectionrate can be used, which could result in ashorter cycle time. We have learned about the benefits of reducing rejected parts and lower cycle times. There are some guidelinesthat can be followed which should help in achievingthe optimum ventingof the mold. Consider the following: 1. VenkF should be located oppositegates to help the melt flow

push the air out of the mold as the cavities are filled. Pockets or areas of the mold that do not get properly vented can result in flow lines, weld lines and burns. One authority suggeststhat 30% of the perimeter of the mold should be vented. 2. Vents should be as deep and as short as possible without allowing any flashto develop. Flash is the ragged, thin edgeof plastic that can result from seepage out of the mold cavities atthe parting line.

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3 . The mold should berelieved. Relieving or removing the metal from the face of the mold base surrounding the cavities reduces the length of the vents and aidsthe venting efficiency. The resulting ventgrooves are illustrated in Chapter 7 (Figure 27). Theclamppressuremustbe concentrated aroundthe cavity to prevent flashing. 4. The runner system should also be vented unless the mold

utilizes a hot runner system. Air must also be driven out of the runner system and, preferably, not intothe mold cavities. We havelearnedthat the injectionmoldingmachine has many control elements that are used to develop amelt of uniform viscosity and moldable temperature.It also hascontrols to govern the pressure and speed of movement used to push this melt intothe mold. Resisting the melt flow into the mold are: > The viscosityof the melt > The nozzle size and temperature > The size and lengthof the runner system > The size of the gate(s) > The part thickness andthe length of flow in the cavities > The location and size of the vents > The temperature of the mold steel As you can see, the determination of why a mold is not filling properly, or has sinks, weld lines, voids and burnsis not a simple problem. A common @&flawed) solutikn is to raise the melt temperature which reduces the melt viscosity enabhg the cavities to fill more easily. The increase in melttemperature will cause cooling time (and the cycle time)to be lengthened. To avoid that, the mold coolant temperature is lowered in an effort to maintain the originalcycletime. As aresult, the injection pressure andor injection speed must be increasedin order to fill the mold before the plastic melt solidifies andstops flowing. Using this technique, it ispossible to produce parts of potentially unacceptable quality!

1 04

A better solutionis to check the vents to make sure that they are not filled with dirt and debris or, over time, have not become hobbed (made more shallow by constant clamping of the mold), decreasing their efficiency. Clogged or hobbed vents can easily cause the burns, voids and weld lines thatmay be occurring. Cleaning out vents and makingsure they are the properdepth should be a routine mold maintenance activity. On most molds the ventsare probably as deep as possible without allowing plasticto escape into them. Sometimes, however, they may be too long or aredirected into the space between the moldplates on clamp up. In either case, theair cannot get out at the required rate. A vent is an opening similar to a mold gate. An opening w i l cause

a pressure drop across its length as fluid flowsthrough it. Air is considered a fluid and experiences this pressure drop depending on its cross sectional area and its length. For example, consider a small diameter drinking iL. Now take the rest straw. Cut off apiece 1/2 inch long. Blow through of thestraw and blowthrough it. You willexperienceadecided difference inthe required effort (pressure) between thetwo sections of the straw. This is fkequently the problem with mold vents. Assume the vents in the mold are 1/4 inch wide and.008 inch deep. If the lengthof this vent is 2 inches to the edgeof the mold,there would be a large pressuredrop and it would take higher injection pressureto overcome this resistance. If the vent length were shortened to 1/2 inch, there would be less resistance, allowing the use of a more rapid injection rate. Improved part quality (fewer burns or voids) and increased production should result. Please refer back to Chapter 7 on Mold Venting fora fhther discussion of the subject. Remember,molds run millions of cycles. A certain amount of hobbing (compressing) of the mold steel takes place over the life of the mold. Even ifproper venting is providedin the beginning, a long running, trouble-fkee mold will require periodic inspection to determine that the venting is still adequate. It is one of the keys to successful molding!

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12.2 Proper Mold Cooling A large partof the success of molding a good part in a short cycle time is due to the cooling of the mold. In Chapter 7, we discussed the importance of moldcoolingandpresented the basics of how that is accomplished. The design of the mold must take into accountthe need to cool the plastic efficiently and uniformly to insure quality parts. Each machine cycleputs a load of heat intothe mold steel. The coolant medium, usually water, is circulated through passages in the mold to remove this heat. The hotter the melt, the higher the heat load. To compensate, the molder mayuse colder water. This procedure might make sense until you readize that it may ultimately be se&-defeating. Here 'S why:

As plastic flows into the mold, the material that touches the cold mold surface wil immediately start to solid^ as it slows andstops moving. The molten plastic continues to flow by and through the portion that has solidified. This process usually requires that even more heat be added to the mold or the melt to keep the material fluid long enoughto fill the mold cavities. It also requires a faster is completely shut injection speedto fill the cavities before the flow off by the cooledplastic.With colder mold surfaces,higher required which, inturn, may injection pressures and/or speed be will require increased clamp tonnage to prevent flashing.If the vents are not properly designed, the higher clamp pressure can reduce venting capacity, whichfirther aggravates the filling process. You cansee why decreasing the temperature of the coolant(water) may be selfdefeating!

As stated in Chapter 7, the answer lies in the placement of the cooling lines and the movement of the water rather than lowering the temperature of the water. Proper mold coolingallows the mold to be filled at the desired speed and assists in achieving quality parts in the lowest possiblecycletime. Proper moldcooling is another key to successful molding.

12.3 Using the Right Screw In Chapter 5, we learned that all screws are not alike. They may differ in the following ways(see Figure 31 below): wz)Ratio (ratio of the flighted length to the diameter)

Channel Depths (the depth of the channels in the three zones of the screw) Helix Angle (the angle of the flights of the screw relative to a plane perpendicularto the axis of the screw)

Number of Flights (number of primary screw flights, excluding secondary flights) Mixing Sections (short sections of the screw, near the meter end of the screw, that employ flutes, different helix angles, and barrier flightsthat provide a mixing action to the melted plastic)

I

Secondary Flight Figure 31 Injection screw elements, including secondary flights and mixing section

Theselection of theproperscrew for a givenprocessing environment should be based on the resin or resins to be processed [ 5 ] . Chapter 2 noted that the two broad groups of plastic, crystalline and amorphous, do not melt in the same manner nor do they conduct heatin the same way. The degree of sensitivity to shear also varies significantly between the two material types. Without getting too deeply involved in screw design technology,it is important to understand some fundamental differences in materials and howthose differences influence screw design. 1. Melting the plastic - one of the differences between the two types of materials is their resistance to deformation (convertingto melt) as their temperature increases. ExamineFigure 32 below.

M p I \

\ \

\

------

CnrsEelineMaterial

Amorp--

\

I

Figure 32 Melting characteristicsof amorphous vs. cytalline pobmers

Both materials soften somewhatat the glass transition temperature but the amorphous material, with no dehed melting point, continues to soften gradually until it reaches a fluid state. The highly crystalline materials remain ina relatively solidstate until the temperature reaches their melting point where they quickly change to melt.

108 2. Thermal Conductivitv - referring to Figure 5 in Chapter 2, a second difference between the two types of material is their relative ability to absorb heat (referredto as thermal conductivity). Amorphous materials simply do not conduct heat (i.e., absorb heat) nearlyas readily as do the more crystalline materials.Stated another way (asdiscussed in Chapter2), you cannot add more heat to amorphous materials and expect themto me& anyfaster? And . . .if too much heat is applied to amorphous resins, they burn andor degrade.

3. Shear Sensitivity - as a consequence ofthe first two differences, whichillustratetheheatsensitivity of amorphous materials, it can be understood why amorphous materials are also more shear sensitive. High shear rates result in rapidly increased resin temperatures which amorphous materials do not tolerate well. We can summarize these findingsin the table: Amorphous Materials m SaRen gradually with no melting point

Donoteasilyabsorbheat Are sensitii to shear

Crystalline Materials m Melt abruptly with a defined melt point

m Absorbheatmorereadily m Are nd particularlysensitii to shear

From these considerations, it may be concluded that amorphous Itl(Lferh!sshould be gradually (not abruptly) changed fiom a solid to a melt with as little shearas possible. Screws with longer transition zones, deeper channel depths and lower compression ratios help protect amorphous resins fiom burning or degradingandhelpensureoptimumphysical properties in the completed parts. In contrast, the more crystalline materials can be processed more effectively by screws with shorter transition zones, more shallow channel depths and highercomprmwn ratios. These principlesare illustrated in the following tableof Screw Design Guidelines.

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SCREWDESIGNGUIDELINES

Processes like an amorphous material

Bottle grade material

t .

(a)Dagreeof meterzonedepth. Example:Mediumfor2"(50mm)scre.v=.100-.125'. (b) Medium = 5 to 7 diameters. Short is less, long is greater. (c) Medium = 2.5 to 2.9to 1. LW is less, hgh is greater.

110

As the table illustrates, there is a large number of materials that require a medium channel depth, transition length and compression ratio. This is the predominant reason why machine manufacturers typically supply a “General Purpose” screw withtheirnewmachine.Althougheach manufacturer has a different definitionof a general purpose screw design, a typical design for a2” (50mm) diameter screw might beas follows: Channel Depths: Screw Profile:

Feed = .300” Meter = .120” 10-5-5 (a feed zone that is 10 diameters long, and 5 diameters lengthin transition and meter) CompressionRatio: 2.5 to 1

Thisgeneral purpose screwwouldprocessanumber of materials satisfactorily and, with some effort and a narrow “processing window,” would likely process nearly all of the materials listed. However, those materials requiringa long transition zone, low compression ratio and deep channel depths will not be processed withthe same efficiency, optimum melt quality andminimumrecovery timeas they wouldifprocessed with a screw designed for those particular materials. As stated above, not all machine manufacturers’ general purpose screws share a common design. There are some manufacturers that would consider a general purpose design to be as aggressive as the following: Channel Depths: Feed = .300 Meter = .loo” Screw Profile: 10-5-5 (asshown above) Compression Ratio: 3.0 to 1

This second design would not allow an optimum processingof amorphous materials. Amorphous materials processed with this screw could be burned and degraded due to the high shear that this screw would cause. This design wouldbe fine for nylon and many of the polyolefins, but should not used for ABS or acrylic!

111 There areother factorsthat affectscrew design, suchas the viscosity of the material andthe fillers and reinforcements included in the materials. viscosity) require ascrew that has Materials witha high melt index (i.e., low much more shallow channeldepths in order to melt the plastic efficiently. Reinforcements, suchas glass fibersor calcium carbonate, require a screw that is somewhat deeperin channel depths with a lower compression ratio than the unfilled versions.And. . .they also require that the screw be made out of a wear resistant material. The main point to be understoodis that if the very best molding of parts is to be expected, thenthe very best melt qualityis needed This is accomplished with screw a that is designedfor the material beingused This is true whether processing a variety of materials or processing the same materials overan extended periodof time. The use ofaproperly designed screw ranks very high among the "Ten Keys to Successful Molding!"

12.4 Selecting the Appropriate Valve Chapter 5 discussed the use of the non-return valve to prevent meltedplastic fiom flowingbackwardinto the screw channelsduring injection.When the screwcomesforward as aram, the valvecloses, preventing the backnow ofmaterial. When the screw rotates backward, the valves opensto allow melted plasticto flow through the valve intothe area in front of the screw (and valve). Because the melted plastic must flow Over a rear seat andthrough flutes or, in the case of a ball check valve,the flow musf go through small openings (ports), there is a great opportunity for a shearing of the material. Assuming that the screw and heater bands have done theirjob and allof the plastic is melted by the time it reaches the valve, the following rule couldbe stated:

Select a valve that achieves the required shut-offfor shot controlbud causes the least possible shearing of the plastic as itflows through the valve.

The efficiency of a valve can be quantified bythe ratio of injected volume to plasticated volume. Typically efficient valves range between 95% and 97%, meaning that only 3% to 5% of the melted plastic flows back into the screw channels as the valve closes [141. The valve efficiency is partly a function of closing time, i.e., the time required for the valve to close starting withthe forward motion of the screw. Failure of the valveto be efficient and close rapidlycan influence the completeness of the shot (and desired part weight). Due to the mechanics of design, ball check valves shut off with greater efficiency and speedthan ring valves. However, ball check valves also create greater shear of the melted plastic and a pressure drop as it moves through the valve. Moreover,if greater shearis caused by the valve, the melted plastic will havean elevated heatas it goes through the nozzle and enters the mold. Because most of the heat mustbe removed from the plastic in the mold in order that solid parts may result, a hotter melt w l i produce a longer required cooling time. . . and a longer cycle. This same undesirable shear can be caused by ring valves that are worn or are not designed to be as “free-flow” as possible, where the opening through the valve is restrictive.

Followingthesuggested rule, themoldermust select the for theprocessing situatwn, giving considerationto the appropriate valve following factors: 1. Avoid restrictive valves when processing amorphous or shear sensitive materials. 2. Use maximumfree-flowvalves,manufacturedfrom wear resistant materials, when processing reinforced plastics.

3. Plastics with low viscosity (high melt index) require valves with the most rapid and efficient closing capability. 4. Viscous (lowmelt index) plastics require valves with maximum

openings and fkee-flowing characteristics.

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12.5 Controlling the Heat Profile Setting the barrel heat profde is one of the least understood and mst importantfixtors in injection molding.The proper heat profile plays a major role in molding parts that possess their intended physical properties, are fiee fiom degradation, heat splay, andstreaks and insure trouble-fiee continuous operation. Improper heat profiles often result in rejected parts, low production rates, and premature wear of barrels, screws and valves. Proper control of the heat profile involves: (1) understanding the sources ofheat;(2) setting the heater band controk utilizing the best heat profile type for the molding situation; (3) andbalancing the heatsources. 1. Sources of Heat - the heat required to melt the plastic comes

fiom two sources: the heater bands and the shear caused by the screw working and conveyingthe plastic.The conductive heat fiom the heater bands canbe reasonably controlledthrough the settings of the temperature controls. Shear heat that is generated from the shear of the plastic against itself, the screw, the barrel wall, and inmixing devices, is much more difficult to control. There are no settings for shear heat.

Excessive heatfrom shear is usually indicatedby a heat override in abarrel zone (often the center zone where compression of the plastic begins), heater bands sewing that zone are not cycling and screw rotat shear is the entire heat source! pressure is high. Under these conditions, The heater bandsmay as well bedisconnected! Evenin faceof these facts, there is a natural inclinationto reduce the heaterband settingsor even direct a fan atthe barrel. This approach does work and increasesthe problem. To reduce shear heat, the screw RPM and/or back pressure can be decreased, however, thisis a solution of limited value because may it also extend the cycle time and result in a lower production rate. The best solution is a surprise to many, and that is to increase the heater band settings in the zones just behind (upstream) the zone experiencing the override. Yes, increasethe heater band heat. The melt temperature will

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not go up, but instead go down. Why? Because the uncontrollable shear heat is reduced to permit a balance of shear heat and controllable conductive heat. The softening of the plastic by the heater band heat in the zone preceding the overriding zone reduces the shear heat in the zone indicating to an override. The heater bandsin the override zone should again begin cycle. If this technique does not work, it indicates that: (1) the heater bands, whose settings were increased, do not have sufficient wattage to accomplish the objective, or (2) the screwdesignis too aggressive (transition zone too short and/or compression ratio to high), or (3) there may be excessive screw and barrel wear that is causing a level of shear heat too great to overcome. Anotherbenefit of this approach should be the corresponding decrease in hydraulic pressure required by the screw drive motor during screw rotate (recovery). Back pressure may even be reduced to further improvetherecoveryrate,productquality,reject rate andenergy consumption. 2. Setting the HeaterBandControls - theheater band heat controls should be set based on thematerialsbeingprocessed,the percentage of the shot capacity being utilized and whetherthe material is heavilyreinforced.Settingtheproperheaterband temperatures also requires that you know the rear zone temperature recommended by the material manufwturer and the desired temperature ofthe melt to be injected into the mold. When setting the heat profile, the following choices are recommended:

W Flatprofde - can be used where the shot is in a range of 20% to 40% of the machine’s shot capacity, the resin is not

reinforced or heavily filled and the residence time is adequate but not excessive.

115 W

An Ascending Proflle is acceptable whenthe shot size is

less than30% of machine shot capacity andthe calculated residence time is long, i.e., more than4 minutes. This profle should not be used when processing reinforced or heavily filled materials or with short-to-medium residence times. W A Hump Profile is a very good solution for processing most non-reinforced materials where the shot size is in a range of 25% to 50% of the machine’s shot capacity and residence timesare not extremely longor short, i.e., in the 2 to 4 minute range.

A Reveme Profde is exceUent for use with all semicrystalline and reinforced or heavily filled materials.It is also recommended where screw recovery and/or residence times are short and where the shot size is 50% or m r e of the IIlaxbnLlm.

L-TargdsdMeltTwnperaturs

Figure 33 Illtmation of heater band heat profile ranges

The heat profile guidelines described providea starting point and should be adjusted to produce the best melt qualityat the lowest possible moldable temperature withall heater band zonescycling. 3 . Balancing the Heat Sources- after setting the heater bands, the sources of the heat energy should be balanced. To do this involves an understanding of the hydraulic pressure requiredto rotate the screw. If the screw is doing no work, this pressure (referredto as Screw psi a 2,000 psi Rotate Pressure or SRP) will be very low, less than 800 on system. If the screw is meeting a lot of resistance to tunring, hence, doing a lot of work, theS W might beas high as 1,800 psi. The cause of the work required of the screw is the resistanceof unmelted plasticto the rotation of the screw. If the plastic is softened or partially melted, the resistance can be reduced. If the centeror front zoneof the barrelis showing a heat override, check theSW. It’s a good betthat it will be registering well above 1,200 psi, indicating that the screw is doing a lot of work. As the heater bands behind the overriding zones are increased in temperature, the S W will decrease, the plastic willmeltmoreeasilyand the screw will recover (retract) in a more rapid and consistent manner.It is not uncommonto see a screw recover erratically, pausing inits backward movement, when heat is overrides are detected. This simplyindicatesthatunmeltedplastic moving into the transition zoneof the screw with insufficient heatto melt uniformly andprogress up the screw. Experience in a number of field cases and under controlled laboratory conditions supports the conclusionthat heat developed from the two sources in approximately equal amounts produces the best molding results. This balance of heat energysources is achieved bydjusting the so that the SRP memures between 800 and 1,200psi heater band setting in a 2,000 psi system (Figure 34). These readings indicate that heat produced from shear energy is between 40% and 60% of the total, a desirable range.

117

l

Screw Rotate Pressure (SW in psi 1m 1000 m

3 l0 0

Pressures greater than 1,200 to 1,300 psi indicates that the screw is working hard, creating potentially excessive shear. Readings under 800 psi suggest that too little shear energyis being used indicating that the process may be experiencing long residence times, long cycles, or abnormally high melt temperatures. It is unfortunate that all injection molding machines do not have the ability to measure theSRP on a continuous basis. If your machines do not have this ability, it is very inexpensive to put a gauge on the hydraulic system to measure the S W .

Adjusting the heat projile to achieve the correct processing temperatures, appropriate SRP and desired melt temperature requires some experimentation and patience. It is difFcult to develop a simple decision chart or a step-by-step routine to accomplish the heat profile objective, however, there are some general guidelines that may help:

118 ~~

~~

~~

~~~

~~

~~~

~

~~

1. Set the heater band settings as suggested inthe following table: nor Shot

Qw=nY amore 25% or less

Average affront Brearmnes

Desiredmett temDeratUre

temmre

Desiredmett Desiredmel temperature

Desiredmett temperature

Desiredmelt temperature

Desiredmett temperature

H-

sugptedby resin mfr

miredmelt temp+30°F

Midmett temperature

Hunp

Suggestedby resin mfr

Desiredmett temp + 30 O F

Desiredmelt temperature

Rewse

Desiredmelt temp+40°F

Awagedfront

&rearzones

Desiredmett temperature

Desiredmett temp+40°F

Awageoffront &rearzones

Desiredmett temperature

Des r iedme tl temp+ 5o°F

Averageoffront 8 rearzones

Desiredmett temperature

I

bsthan2 minutes

Flat

2 minutes ormore

Flat

c%$

~essthan2 minutes 2minutes

amore

Lessthan2

% % $

minutes

More

2minutes

m&% amore ~

~

~

Lessthan2 minutes

Desired mett temoerature

resin mfr

2 minutes Ascending Suggested

Rewse

by

I

~~~

Reverse

2. Determine if the CenterZone is overriding. If it is,ruke the Rear Zone IO "Fand the Center Zone 5 "l? Allow the heats to stabilize, indicated by a proper cycling of the heater bands. If the Center Zoneis still overriding,repeat the adjustment, adding ten more degreesto theRear Zone and five more degrees to the Center Zone and stabilize. If this does not cause the Center Zone to cease overridwand cycle properly, the screw may be too aggressive for the material, the valve may be too restrictive andor the screw and barrel may have excessive wear. Determine the cause and start over fiom step 1.

3. After the Center Zone is cycling properly andnot overriding, check the SRP. The Screw Rotate Pressure should be in the 700 psi to 1,300 psi range, depending upon shot size, recovery rate and other variables. If the SRP is too high, raise the Rear Zone 5 O F and check.If still too high, raise the Center Zone5 O F and check. Repeatthese steps until the

119

SRP is within the desired range.If the S W is too low, reverse the process, lowering the zones, one at a time, by 5 OF and check. 4. After the Center Zone and SW are responding as desired, check the Front Zone.If it iscyclingproperly,check the melt temperature with a pyrometer. If it is too high, lower the Front Zone 5 “F. If too low, increase the Front Zone5 “F.

5. If the Front Zone is overriding, raise the Center Zone 5 OF and check.If still overriding, repeat the five degree adjustment also and add 5 OF to the Front Zone. If these adjustments do not solve the problem, check the valve for proper design and/or excessive wear and check screw design for restrictive mixers at or near the endof the screw. The guidelines suggested above relativeto setting heat profiles for injection moldingare not universally accepted, but they work1 Monitoring the SW on a system tells the moldera great deal about what is happening in the barrel and provides a valuable assist in solving molding problems. Setting and controlling the heat profile is one of the most important “Keysto Successful Molding.”

12.6 Using Back Pressure Wisely Back pressure results from hydraulically restricting the backward movement of the screw during the screw rotate portion of the molding cycle. This pressure helpsforce any air in the melt back andout through the feed throat of the barrel. In addition, back pressure causes the screw to work the resin harder and increases the melt temperature of the plastic through the increased shear. There are both advantages and disadvantages to the use of back pressure. It should also be noted that the machine itself will generate about 50 psi when thecontrol is set at zero pressure [ 5 ] . Advantages - as previously discussed, shear heat creates a more d o r m melt temperature and viscosity. In aproper amount, back pressure

helps create a better melt quality with improved flow characteristics and potentially better part quality. Back pressure can also enhancethe mixing of color which can improve part quality. Fine tuning of the plasticating process is possible through the prudent use of back pressure where the proper screw design is not available. Disadvantages - backpressureincreasesmelttemperature, restricts the recovery of the screw and can lengthen both recovery and cycle times.Because the screw works harder,moreenergy isconsumed increasing the cost of production. Excessive back pressure w liresult in increased wear of barrels, screws and valves and, if used with glassreinforced resins, may cause the breakageof the glass fibers, reducing the mechanical properties of the parts.

There are no universal rulesfor the use of back pressure, however, (50 to 100 psi) helps the molding process by stabilizing a little back pressure shot size, recovery rate and feeding without sacrifice in part quality or production cost. It is important that in a continuing processing environment, back pressure should not be used as a substitute for a proper heatprofileor a correct screwdesign. Back pressureis clearly one of the “Keysto SuccesshlMolding” and mustbe used wisely.

12.7 Controlling the Injection Rate The maximumshot size(or shot capacity)of an injection molding machine is listed bythe manufacturer as the number of ounces of genera2 purpose polystyrene (GPPS) that the size of the barrel and stroke of the each shot. This shot size can machine will allow to be injected into the mold be calculated in two steps, as follows: Shot Displacement (in3)= Barrel DiameteP x .7854 x screw stroke

Shot Capacity (Ounces GPPS) = in3 (above) x SgPS x .5778

For example:Assume a machine witha barrel bore diameter of 90 mm (3.54 inches) and screw stroke of 15.75 inches (400 mm) and a specific gravity (Sg) of polystyrene (atmelt, not solid &m@) of .97 grams/cm3. Note that the .5778converts grams/cm3to ounces/in3.

Displacement (in3)= 3.542x .7854 x 15.75 = 155 cubic inches Shot Capacity (ounces) = 155 x .97 x S778 = 86.8 ounces Note: The area of a circle may be calculated: a. Pi (3.1416) x radiusz, or b. .7854 x diameter2. Because d2is 4 times as great as rZ, Pi must be divided by four in the second calculation (3.1416 + 4 = .7854) In this instance, the machine manufacturer listed the shot capacity at 85 ounces. Keepin mindthat various manufacturers use different factors in the final determinationof the rated shot capacity, however, these calculations should approximate their listings. The injection rate ofan injectionmoldingmachine is also a calculatedfigurebased on themachine’shydraulicsystem(usingthe maximum hydraulic oil in gallons per minute and the area of the injection cylinder). The rate indicates how fast a specified number of cubic inches of meltedplasticcan be pushed out of the barrel(into the air without resistance) in one second. Machinery manufacturers list this injection rate in their literature as: cubic inches per second (inches3 per second) or cm3 per second. For e m d e : The machine above witha shot capacityof 85 oz. has a listed injectionrate of 28 in3 persecond. All of this theoretical information is very important to the molderof thin wall, longflow parts, such as drinking cups, food containers and similar items. The cavities ofa thin walled part must be filled as fast as possible before the thin wall of melted plastic sets up and prevents total filling.

122

Unfortunately, the molder is often unable to achieve the filling speeds specified in the machine literature, resulting in rejected parts andor slower cycles. When a molder does not know why a machine is not successful in achieving the desired rate, the first solution selected may be to raise the melt temperature to lower the viscosity of the melt. With the resulting hotter melt, the molder may then attempt to use a colder coolant in the mold to save cycle time. This, too, as we have learned, is not successful. Meltedplastic is a viscousfluidand does notfloweasily. addition, there are restrictions to the flow of the melt caused by: 1. 2. 3. 4.

In

The size of the nozzle orifice The size and shape of the runner system The size and placement ofthe mold gates The effectiveness of the venting in the mold

All of these factors must be considered in controlling the actual injection rate. Current machines have five or more stagesfor which an injection speed (in inchedsecond) may be set and the point at which the speedis increased or lowered may also be set.

For example: In setting the injection velocity profile for a new mold, the shot size (in inches of stroke)and cushion are entered into the control. (Cushion is the quantity of melted plastic that is left in the barrel after injection takes place. It is set in fiactions of an inch.) Thesesettings w l i cause the screw to begin its forward movementat a point equalto the shot size (expressed in inchesof stroke) + the cushion (alsoin inches) away fiom the fully forward position. Stage l (the first part ofthe injection stroke) may then be set relative to injection speed (in inchedsecond) and % ofthe injection stroke distance. Hence, the screw mightbe set to move 2.00 idsec until the screw reaches a point where 80% of thestroke remains. Then Stage 2 may be set, at a higher speed until60% of the stroke remains, and so on. Stage 5 would typically drop in speed for the last

123

portion (say 20%) of the stroke. At that point, the transfer position is reached, which changes the screw actionfrom injection to pack (or hold). Part ofa control screen for setting the Injection Velocity Profile for one injection molding machineis illustrated in Figure 35.

I

INJECTION VELOCITY Cushion Pm]

mow

Rpm 35 Typical control screen showingfhe injectionvelocityproAls

When starting up a new mold,it is best to start slowly using modest injection speeds, whichmay create short shots (assuming the creation of short shots doesn’t create a problem in their removal). As the mold begins to fll more hlly with increases in speed, the parts may be examined to determine ifthere are burns (indicating injection speeds that may be too high) or voids or weld lines (indicating speeds that may be too slow).

Controlling the injection rateis a critical element insuccessful injection molding, however, the manipulation of injection speeds comes fiomexperience gained in molding varioustypes of parts and involvesother factors, such as injection pressure and heat profiles. Do not overZook the importance of the restrictionstoflow discussed previously which should be investigated if quality parts cannot be produced inthe desired times.

124

As previously discussed,there is another schoolof thought for the calculation and control of injection velocity. This differing approach is presented in Appendix C.

12.8 Managing Screw RPM and Residence Time An investigationof many moldingapplications would likely disclose

a screw RPMset at a high rate ofspeed in order to recover as rapidly as possible. This is importantinrunning a thin walledpart or in other is limiting to the overall cycle time. applications where screw recovery time However, no purpose is served if the screw recovers, stops and waits for several seconds beforethe mold can be opened and parts ejected. Rapid screw recovery is possible when the melt ishotter, often associated with the high heat profile. In such circumstances, the temperature of the melt may be higher than required andthe cycle timeis adversely affected.In addition, the quality of the melt w libe poor. As previously stated, the use of a proper heat profile anda screw designed for the material being processedare keys to successllmolding. SuccessfuC molding is &o aided by rotating the screw as slowly as the cycle will allow, leaving one or two seconds before the mold opens. Melting the plastic efficiently requires heat fiom the heater bands and shear heat fiom the screw. Shear heat fi-omthe screw only occurs when the screw is rotating or injecting, not when isit in a klly retracted position waitingto inject. In addition to controlling RPM, the use of torque control is also important. Torque is the amount of energy necessary to turn the screw. Some materials, suchas polystyrene, polypropylene and polyethylene have low viscosities and require the least amount of torque. In contrast, acrylic and polycarbonate have more than 50% greater viscosity. As the viscosity of the material being processed increases, is there also a need to increase a high and low the available torqueto turn the screw. Most machines have torque range setting. The low torque setting enablesthe faster screw RPM which can be used for many of the materials being processed.A switch to

125

the high torque setting reduces the screw speed by about 30 to35% but increases the available torqueby 50 to 55%.

One of the reasons that molding machine injection units (usually 22:l L/D and less) are shorter than extrusion plasticating units (typically 24:1 and longer) is residence time.Residence time is the amount oftime that apellet of plastic resides in the heated barrel before being injected into the mold ( k ,from the time it enters the barreluntil it leavesthe time barrel in the form of melt). Because there is very little residencein an extrusionbarrelduringwhichtheplasticisbeingheatedandmelted, extrusion barrelsare often 30:1 to 36: 1 L B . Even with the added length, extrusion grade resins contain heat stabilizers which enable more shear he to be used to melt the plastic without degradation. For injection molding, residencetime is an importantfactor to be considered in judgingthe adequacy of a given molding application. To determine residence time, it is necessary to calculate the inventory of plastic thatis in the barrel whenthe screw is in the back position. There are some complicated formulas used to make this calculation, buta short-cut formulaworks reasonably wellin most cases. Screw Inventory = (RSC+ 1.05) x F x (L/D + 20) x Sg

Where: RSC = Rated shot capacity of the machine F = Factor (see table) Sg = Specific gravityof the plastic @ room temerature in gramdcc Thefactor referred to makes allowance for the size of the injection unit and the fact that the useh1 L/D is actually somewhat less than the L/D that is commonly used (see L/D definition in the Terminology section of the book). The factoris based on the bore diameterof the barrel, as follows:

126 Bore Diameter Factor Bore Diameter Factor 30mm & l e s s 31 mm- 49mm 50mm - 69mm 70mm -79mm

1.80 l.65 1.30

1.25 80mm - 89 mm 1.20 90mm - 104mm 105mm - 109mm 1.45 1.15 IlOmm &greater 1.10

Residence timemay then be calculated using the following formula: RT(minutes) = Screw Inventory + Shot Size x (Cycle timeh0)

Where:

RT is the Residence Time in minutes Screw Inventory isas calculated Shot Size is the actual size of the shot used Cycle Time is actual cycle time in seconds

A sample set of calculations assuming the following processing parameters is presented below foran 18:1 L/D machine:

Rated shot capacity = 54 ounces Bore diameter = 80 mm Actual shot size = 43 ounces Cycle time = 55 sec. Specific gravity of PC at room temperature = 1.20 g/cc (54 + 1.05) x 1.25 x (18 + 20) x 1.20 = 69.43 ounces (say69 oz) 69 + 43

x

55/60 = 1.47 minutes

Under these conditions,withapproximately80% of theshot capacity of the machine being used aand residence timeof a little morethan a minute, it would be possible to find some m e l t e d pellets or unmixed color in the resulting parts. The molder would most certainly needto use an aggressive reverse heat profile and bring the screw backas slowly as the total cycle would permit, usinga screw properly designed for the material being processed.

127

If theseprocessingparameters do notresult in a satisfactorily molded part, the molder might consider lengthening the injection unitfiom 18:l to 22: 1 or more. A 24: 1 L/D would increase the residence time to about 2 minutes which, with a proper processing set up, might very well accomplish his objective. that times Experience in the field and laboratory indicates residence should ideallybe greater than2 minutes andless than 5 minutes. When residence times are less than 2 minutes, it is not uncommon to see unmelted pellets in the parts andor added color not being adequately mixed. Screw rotate pressure is usually higher under these conditions. When residence times in excess of five minutesare used, resin damage in the form of surface defects (burn marks) andor the loss of mechanical properties (such as impact strength) become common. It is apparent thatthe calculation and management of residence time is an important key to successful tmlding. This becomes even more clear when the residence time tooislong. Under these conditions, materials sensitive to excessive periods of high temperature (including PC, ABS, PVC, acetals, cellulosics and others containing flameretardants) may exhibit evidence of burninganddegradation.Thecorrectivemeasures to counteract high residence times include:(1) lowering the screwRPM, (2) reducing back pressure, and (3) reducing barrel heat in the feed zone. In this case, an ascending heat profile would appropriate. be If these remedies are not effective, the only remainingcure is to reduce the shot capacityof the machine, hence, downsizing the injection unit. Downsizing the injection unit involvesa new barrel witha smaller bore andshorter length (to allow fora 20: 1 L/D), a new screw, end cap and valve. Downsizing should be done only with the assistanceof the original machinemanufacturer or a competent barrel and screw manufacturer. Because theshot capacity is less and the hydraulics are the same, some very high injectionpressurescan result. These pressuresare sufficientto result in damageor injury to both equipment and personnel.

128

12.9 PerformanceMeasurement All plastics processors should maintain some type of production record which enables a comparison of actual production performance against a production standard or benchmark. In those operations where such a record is not maintained in written form,it is frequently kept in the mind of one or more production supervisors. Unfortunately, unwritten records are not capable of being accessed, summarized or analyzed [5]. Performance measurement begins witha simple Set Upor Process Sheet.

are The Process Sheet contains all ofthe processing parameters that carellly developed when a particular mold is first set up to run. The processsheet sets forth the drying time, barrelheat profile, mold temperatures,clamptonnage,injectionrate, filling andholding pressures, back pressure and all of the times that are included in the overall determinationof cycle time. Included are the times forfill,pack, holri, cooling, screw recovery and total cycle time. These times,together with the other set points in the process, constitute the plan that has been made to effectively and profitably produceparts from the mold.

The measurement of the actual results compared with eachof these processing parameters is essential to the determination of the success of the molding operation. This evaluation of production performance is also critical to the determination of undesirable trends in productivity of a particular machine, shift andor plant. Moreover, the record isvital to the immediate correctionof processing problems antUor to component wearlong beforethe problems become seriously damaging equipment or processing profitability. There are a number of real-time production monitoring systems in use today and many of them offer scheduling, production floor status reporting, machinestatusreporting andother desirable evaluative measures. Such systems are u s e l l in assisting production management in problem identification and solving. These systems generate reports and computer screen displays similarto those shown:

129

This report not only indicatesthe number and percentageof parts that are rejectedbutalsoshows the number that wereactuallyproduced as compared with the number that should have been produced.

M8Cbk No.

This report (Figure 37) can be the basii for a varietyof corrective action. If the cycle time is slower than the original setup, it may bedue to excessive recovery, SU, or cooling times or other causes. A deteriorationin recovery time couldbe the first indicator that a valve,screw or barrel is worn. It should benoted that plastic materialsdo vary somewhatfrom lot to lot and processing conditions may require some change each time a particular mold is used. However, if such changesare always inthe same direction (raisiiheats, increasingscrew RPM, changing back pressure), a problem is likely developing that needs attention. Pe@ormance measurement is an essential toolfor successful molding.

12.10 Preventive Maintenance Program

Preventive maintenance is the process of “putting out fires” as theyoccur. A PreventiveMaintenanceProgramshouldenable the identification of all machines and components, permil the scheduling and recording of maintenance activities andprovide reporting that give assurance that the maintenance activities are being conducted properly and ona timely basis. Additional reports may be developed that allow the evaluation of the performanceof components (screws, barrels, valves) and the materialseom which theyare made relativeto the typesofplastic resins they are processing. All machines and machine components have an approximate wear life and by identifLingthat life and making maintenance changes before the life expires,production can continue on a scheduled basis.It is much better to schedule the replacement of machine parts and components at a time that fs convenient to production rather than be forced to make such repairs or replacements on an emergency basis. There are a number of computerized maintenance programs available that list all of the normal maintenance activities and their fiequency, either based on a number of days of service or machine hours of use. Based on such fiequencies,a maintenance schedule can be developed to perform the activities routinely. Problemsor trends in undesirable performance or wear life can be documented for later follow-up. A well-conceived Preventive Maintenance Program is the foundationof efficient production and easily qualifiesas one ofthe “Keys to SuccessfulMolding.’,

The “Keys to Successful Molding” are essential to the profitable operation of an injection molding facility. Because this was a long chapter, we thought you might liketo see allof the “keys” listedin one place. You may reviewthese in the tableon the next page.

131 The “Ten Keys to Successful Moldingm

1

Make sure that the mold and the runnersystem are adequately vented.

2

Determine that theplacement of cooling lines and the movement of water in the mold will provide the required mold cooling.

3

Use the right screw for the materialbeing processed.

4

Select the appropriatenon-valve for the molding process.

5

Set the optimum barrel heat profile for the process and balance the heat sources.

6

Use the least amount of back pressurenecessary and do not use it as a substitutefor the proper screw.

7

Control the injection rate to achieve the proper fill rate without burns,voids or weld lines in the parts.

8

Determine that theprocess has adequate (but not excessive) residence time and managethe screw RPM to rotate asslowly as thecycle will allow.

9

Measure the continuing performance of the process against benchmarks that represent the desired production standards.

10

Adopt a Preventive Maintenance Programthat provides assurance thatall maintenance activities are being conducted properly and on a timely basis.

132

13 Thermoset Molding Injection molding thermoset materials is quite similar to molding thermoplastic materials. The changes required to mold thermoset materials are caused by the differences in the materials themselves. Thermplastic materials can be melted, molded and solidified into parts and those parts can be ground up, remelted and remolded intodserent parts. There is no chemical change that takes place in molding thermoplastics. In contrast, thermoset materialsundergo achemicaZ change or polymerization during as itsolidljiws. This curing which the molten materialcures (or cross-links) process occurs within a specifiedtemperaturerangewhichshould be achieved just after the melt enters the mold. Premature curing of the material can leaveit solidified inthe barrel and its removal is a very difficult process. Once cured, thermoset material will not melt again. If exposed to intense heat, the cured material may burn, scorch or disintegrate, but never melt. Although there are some thermoset materials that are activated by catalysts or by mixing two liquid chemicals together, with neither process requiring heat,the most common thermosets are temperature-activated. The temperature-activated materials are those discussed in this chapter and include the most typical resins that are formaldehyde-based and include phenoplasts and aminoplasts [4]. Because of the complexities of these materials, the discussion ofthermoset molding presentedin this chapterwill be limited and principally involve a presentation of the basic differences between processing thermosetsas compared with thermoplastics.

13.1 Thermoset Materials There are five major familiesof thermoset molding materials. They include:Phenolics,Polyesters, Aminos, Epoxies,andAllyls.Phenolics account forthe greatest volume of thermoset material used[15].

Phenolics are a reaction product of phenol and formaldehyde. These materialsare available intypes that offer strength, heat and electrical

133 resistance andare often glass reinforced. In the molded condition, phenolics typically turn to a brownor black color over time. These materials, before being molded, have a limited shelf of lifeapproximately six (6) months.

Polyesters offer most of the same molded characteristics but are perhaps easierto process. The curing time of polyesters canbe much faster than a phenolic. In contrast with phenolics, polyesters will maintain their original molded color. Aminos are very hard and have abrasion resistant surfaces. They can also be processed to have a wide range of colors that are retained. Aminos were originally used in the manufacture of dinnerware. These materials are most often compression rather than injection molded. Epoxies are noted for excellent dimensional stability, resistance to most chemicals and the ability to flow and cure under very low pressures. As aresult,thesematerials are oftenusedfor the encapsulation of semiconductors, coils and similar products. Allyls, in theirmolded form, arecharacterized bydimensional stability, chemical resistance, mechanical strength and heat resistance. Most importantly, they retain their electrical properties under high temperatures and humidity. The most common allylis diallyl phthalateor D M . A fiuther study of thermoset materials is not within the scope of this book, however, itis important to understandwhyandunderwhat conditions thermosetsare desirable. If the product is used in sustained temperatures in acess of 80°C (176°F) or is stored at temperatures (248"F), thermosets area consideration. In addition, greater than120°C thermosets have good dimensional stability little withwarpage and good chemical and electrical resistance. Although some thermoset materials are molded by compression, injection-compression,high-temperatureinjectionandtransfermolding processes, requiring differentmold designs, the discussion in this chapter will discuss onlythe injection molding process.

134

13.2 Machine Modifications Thermoset injection molding machines are available ina wide range of sizes. In addition,moststandardthermoplasticinjectionmolding machines canbe converted to process thermoset materials. There are two major differences in processing thermoset materials that influencethe modification of the thermoplastic molding machines. 1. Thermoset materials will cure rapidly ifsubjected to heat above a spcljied range. Consequently, the heat source for processing thermosets must be carefblly controlled. The temperatures for used thermoset materials are typically much lower than those for thermoplastics (commonly less than 300°F or 149OC). Thermoset materials also have a higher viscosity than mostthermoplasticmaterials. As a result of thesefactors,machine modifications are required, as follows: The barrels havewaterjackets that surround the mainbarrel with three (or more) zones wherethe circulating water temperaturemay be controlled to gradually increase the temperatureof the resin so that it is melted just prior to being injected. This is the principal heat sourcefor melting theplastic. M T h e m s e t screws have zero compressiolL whichvirtually eliminates shear as a heat source. Typical screws have a 1:1 (or less) compression ratio and simply convey the resin through the barrel so that the heat to melt the plastic is conducted ftom the water jackets through the barrel wall into the material.

Smear tips are commonly used insteadof non-return valves. Because of the potential for shearing of the material goingthrough a valve and because the melt is quite viscous, a conical tip at the end of the screw(or smear tip)is used in placeof any typeof valve.

N o d e s at the end of the barrel are typically water-cooledor temperature-controlled to avoidanyunwantedshear of the material. This type of nozzle also helps maintain a proper balance between a hot mold (350-500°F or 177-260°C) where the material cures and a relatively cool barrel (1 50-200°F or 66-93°C ) [161. 8

8 The barrelis typically shorter, in the range of 12 to 16:l U D . The shorter barrel helps avoid excessive residence times andany shear that may be a by-product of conveying the material up the screw.

2. In addawn tobeing heat sensitive and viscous, most thermoset material is very abrasive. Because many thermoset materials are rather brittle in the curedandsolidcondition,fillersandreinforcements are common additives. The abrasive area in the injection unit is the point in the barrel whereit becomes melt.This area is the lasttwo to three flights ofthe screw andthe conical smear tip. Withina short time, these areas can wear to the point where the flights are gone andthe ability ofthe screw and smear tip to “hold”duringcuringtimeis severely lessened.Component modifications required are: 8 Barrels are typically lined awith highly wear resistant material, such as tool steels or premium cast bimetallic linings. 8 Screws and smear tips are mQdefrom wear resistant tool steels or aretungsten carbide coated to prevent excessive wear.

13.3 Processing Modifications Thermoset molding requires a curing time whichoccurs in the mold while the machineis in a “holding” position. Cycle timeis as important in thermoset molding as it is in thermoplastic molding. Therefore, the closer the melted material is to its curing temperature when it is injected, the shorter the time thatis required to hold the mold closed. Careh1control of

136

the heat inputin the barrel (from the water jacket) is essential. Because the runner system of a thermoset moldingis not capableof being saved, ground up and used as a regrind added to virgin material, the elimination of the runner system is desirable. This objective may result in a “cold runner system.” Nearly identical to its counterpart for thermoplastics (the “hot runner system”), the cold runner manifold simply maintains the temperatureof the melted (but uncured) thermoset material within a range acceptable for injecting but not so hot that curing couldbe initiated.Thetypicalrunner systems in thermosetmolding are quite extensive andthe ability to use a “cold runner system” resultsa significant in cost saving. A similar arrangement is referred to as “live sprue molding,” where the conventional sprue bushing is replaced awith water-cooled sprue bushing. This permits the sprue portion of the runner system to remain melted but uncured, readyto be molded in succeedingshots [151. For the reasons previously presented, it is desirable to avoid high screw RPM andexcessivebackpressure whenmoldingthermoset materials. Both of those conditions can giverise to unwantedshear resulting in higher than desired melt temperatures. In addition, thefiff rate must be controlled so that too much fiictional heat does not develop. Control to avoid excessive melt temperatures in thermoset molding cannot be overemphasized.

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14 Safety As stated in the booklet Omrator’s Handbook for Plastic Iniection Molding, producedby the SPI Molders Division and published by the S.P.I., “Injection molding machines are lightning fast, tremendously powerful, blisteringly hot and run by electricity.” The pressures, temperatures and electrical currents are more than suflicient to severely injure an operator or helper. These hazards have resulted in crushed or amputated fingers, hands, arms, legs and have, in some cases, terminated human life. Sincethe creation andacceptance of the AmericanNational Standard ANSI B 15 1.1, now in a recently updated version,all horizontal injection molding machines manufacturedandor sold in the UnitedStates B 151.1 must comply witha set of well-defined safety standards. The ANSI document is available fiom the American NationalStandards Institute, 1 1 West 42nd. Street, New York, NY 10036 fora nominal fee.

14.1 Safety Requirements ofANSI B151.l Rather than repeatall of the sectionsof this publishedstandard in this book, only the major elements of the standard are condensed and presented below. This standard refers to the Horizontal Injection Molding Machine (HI“). There is another standard for vertical machines. In this reference, the machinewill be referred to as IMM (injectionmolding machine): 1. It is the responsibility of the manufacturer of the IMM to fUrnish instructions relativeto the care of the machine. Your Company should have this document available for review.It is the responsibility of your Company to ensure that maintenance and setup personnel are competentand trainedincaring for, settingup,inspectingand maintaining theZMM. In turn, these personnel should make certain that any persons assisting in these h c t i o n s are also properly trained. (Once trained, theoperatorsand helpersalso have a responsibilityto follow certain safety rules thatwill be covered later in this chapter.)

138 2. The IMM has an operator’sgate as the primary safety device which servesas a barrier, blocking off access to moving parts,the mold and splattering of hot melt while the IMM is operating. The gate is mounted on the operator’s side of the machine and has a window, made of a shatter proof material, allowing the operator to view, but not access, the area of the movement of the mold. The operator’s gate must be closed before the machine can be operated If the gate is power-operated, it must have a pressure-sensitiveswitchthat also preventsthemachine fiom being operated if the gate is open.

3. Every IMM has three safety interlockswhich prevent specifled machine operations, as follows: a. ElectricalInterlock - which prevents any of the following motions as long as the operator’sgate is open: platen closing injectionforward screw rotation

core action ejectormovement

b. Mechanical Device - which prevents the platen from closing if the operator’s gate is open. It also has a monitoring device that causes an alarm if the mechanical device is not functioning. c. Hydraulic or PneumaticInterlock - which also prevents the platen from closing if the operator’s gate is open. Thisinterlockalso has amonitoringdevice that verifies that it is functional. 4. A rear guard covers the same spaceon the back of the machine

(opposite the operator) as does the operator’s gate on the fiont. This is not an operating gate and is equipped with a mechanical latch that has two interlocks that separately interrupt the control circuit andthe power circuit, preventing all IMM movements fi the guardis open or removed.

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5 . A top guard is also installed on smaller machines where an

operator could reachover the top of the operator’s gate. If the top guard if the top is moveable, it must also have an interlock preventing all motion guard is open. Note: Keep in mind that the top guard is installed only on small machines. Larger machines may permit personnel to climb up and access hazardous areasfrom the top of the machine. Such access must be prevented by other safeguards and Company safety rules.

6. There are also guards that prevent access to hazardous areas (such as the mold area) through the parts discharge opening and, if a nozzle shutoff device is used, prevent screw rotation with the operator’s gate open if the nozzleshutoff device is in the open position. 7. Guards that relate to areas other than the mold (platen) area are also required. Theareas covered by these guards include: feed openings purging protection electricalsystems injectionbarrel covers injection unit swivel interlocks

safety signs vent covers

Of particularnote is the safeguarding duringpurging. A guard is provided to protect thefront, rear, andtop of the purging area over the barrel nozzle and behind the stationary platen. This guard is interlocked to prevent screw rotation, screw forward and injection carriage forward motion whenthe guard is not in position. Purgingis also prevented with the operator’s gate open. In larger machines where the operator can stand between the operator’s gate and the mold area, an emergency stop oremergency reverse button is provided that is readily accessible to the operator. Additional devices prevent the unintentional return of the gate to a closed position and allow thestart of a cycle onlyafter complying with a prescribed sequence of pushing two safety buttons and closing theoperator’s gate. The latter system is referredto as a double acknowledgment system, which is fixther protected by a monitoring circuitor presence-sensing devices.

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8. Operators, setup and supervisory personnel be must trained in on an the use of all safety devices and procedures before starting work injection molding machine. The standard suggests that a checklist be prepared to ensure that operators are keptaware of procedures.The checklistshouldbekeptin the instructionmanual of the Ih4M and periodically the employer should havethe operators check off the list. The employer should provide a clean and adequately spaced work If the parts discharge area requires modificationto area around the I”. allow for conveyors or chutes, the employer should provide appropriate safety devicesto protect personnel fiom any attendant hazards. Safety signs to a specified color, size, format and are to be provided and should conform content. Additional provisions for guards and safety, particularly where very large machinesare used, are presented in the standard. This standard should be available for review by all molding personnel.

14.2 Safety Rules to Follow Although not intended to be an all-inclusive listing of safety do’s and don’t’s, safety rules to be followed when working around injection molding operations should, at a minimum, include the following: I . Perform the Machine Safety Check. Machinery manufacturers provide a protocol or procedural routine for checking the various safetydevices on the machine,especially those relating to the operator’s gate. These are often found printed on an instruction plate at the operator’s station or a decal nearby. These typically state the procedures to assure that allsafetyinterlocks are functioning at the start of each shift. The shiftforeman or other supervisory person should perform this check to ensure that all safeties are hctioning properly.

2. Don’t be careless! Treat the machine and auxiliary equipment with respect and think carehlly before initiating any operation.

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3. Wear theproper clothing.Follow the company’s standards for protective wear, includingshoes,gloves,eyeglassesand ear protectors. 4. Understand all the machine’s safety devices, where they are

and how they work. Make sure all are ‘hctional before work begins. Also determine that operator’s gate windows are not broken, cracked or removed. 5. Do not remove or defeat the guards for the operator’s gate.

These guards insure that personnel cannot reach into danger area.

the main

6. Do not climbor crawl up onto the machine while it isrunning. A slip can mean a fall and on larger machines this could result in very serious injury. 7. Do not fw or perform any maintenance functions while the machine is running. Shut off the power before makingany adjustments. 8. Do not reach overor under closed guards,the hopper orfeed throat while machineis in operation or while motors are running. 9. lfthe mold should be stopped during closing,the hit machine stop button. Do not open the operator’s gate and attempt to quickly fix the problem or remove any plasticor other obstacle.

10. Stand clear of the machine when a maintenance man is working onit. Do not attempt to help unless heasks you to doso. 11. Keep the workingareaclean! It is veryeasy to slip on hydraulic oil, spilled plastic pellets, parts and scrap. Sweep it up. And remember, molten plastic is very hot even long after it has been purged. Usegloves or tools to movepurgedplastic or newly molded parts.

142 There are other concernsrelative to theworking area around injectionmoldingmachines.Electrical heaters andwiringbothcarry relativelyhigh voltage andcancauseseriousinjury.Be carell when working around anywiring. Remember that thewiring around the nozzle is more exposedso look out for it. Report any exposed or fiayed electrical wiring or electricalboxeswithoutproperlyfunctioning doors. Scrap grinders are a great potential for injury. Shut the power completely off before workingon oraround the grinder. Safety has to be establishedby management as a necessary part of any operation. The practice of safety is wisely called risk management, which meanscontrollug both human and mechanical factors that can cause harm. Everyone must be a "risk manager." Everyone involved must do their partto act in a safe manner andto insure thata safe work environment is maintained.

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15 Recognizing Molding Problems As experience is gainedinworking in an injectionmolding operation, it wil become increasingly importantto understand the typesof molding problems that can occur. Molding problems cause defective parts and producing defectiveparts is the most undesirable condition that can occur in molding. A defective part includes the variable costs of labor, material and power and although the part may be captured, ground up and the material reused,the cost of grindingis added as well. These costs can never be recovered. By contrast, if the machine were down, that is, not producing anyparts, there are no variable costs, only the loss ofproduction time. Molding problems may be placed in two categories: (1) process problems, and (2) component wear problems.

15.1 Process Problems Most molding problems can be identified by either examining the parts or by observing the performanceof the injection molding machine. In either case, it is important to understand the nature of the problem, the possible causes of the problem and some potential solutions to the problem. Presented in the followingparagraphs are some of themost commonly experienced typesof problems, their potential causes and some typical solutions. As more experiencein molding is gained, this listingcan be expanded to include other situations requiringmodifications to the molding parameters thatare necessary to produce the desiredparts.

15.1.lBrittleness of Parts Brittleness in parts can be recognized by physically examining them, but moreoften, parts that are presumed to have impact and tensile strength are subjected to laboratory tests which will confirm the presence or absence of the required properties.

144 Causes:

Brittleness is commonly caused by a lossin molecular weight in the part resultingfiom excessive heat and/or shear occurring during the molding processor improper drying. Excessive moistureparts in produced fiom polycarbonate and nylon (among others), may also cause a reactionthat can produce brittleness. Brittleness may also be caused by excessive residence time. The residence timemay be calculated (see Chapter 12.8) and should range from 1.5 minutes to no more than5.0 minutes, depending uponthe type of resin being processed.

Potential Solutwm: Determine that the proper drying procedures are being used, especially with hygroscopic resins. Determine thatthe screw has the proper design anddoes not excessively shearthe resin. Correct the heat profileto gain more heat from the heater bands and lessfiom shear. Check melt temperature with a pyrometer to determine thatthe desired temperatureis being achieved. Make sure that all heater bands are operational. m Check the residence time andif it is too long, reduce the rear zone temperature and use an ascending profile. If this does not solve the problem, consider alteringthe shot size of the injection unit.

15.1.2 Bubbles and Voids Bubbles are ordinarily the result of trapping air inside the molded part. Voids may appear similar to bubbles but representthe absence of air or a vacuumin the part. Voids typicallyoccur after themelt is injected into the mold.

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Causes:

Inadequate venting in the mold or runner system, preventing the air in the cavities from escaping during injection. Excessive suckback or melt decompression after injection may cause a void in the part.A determination should firstbe made whether the problem is a bubbleof gas or a void, then the solutions applied. Potential Solutions:

Clean the vents in the mold and runners, increase the venting,orresurface the moldface,re-establishingthe vents. Reduce the injectionfill speed allowing more time for the air tobe expelled through the vents. Reduce the clamp tonnage pressure, taking care not to flashtheparts.Iftheventsarepartiallyhobbed,this solution may help. W

Decrease the melt decompression and reduce the nozzle temperature. Use a reverse taper nozzleor a shut-off nozzle.

15.1.3 Burned Material Burned material is evidenced by part discoloration or black, sooty marks in the parts. Similar to the previous problem, the surface of the part can be burned by dieseling. Dieseling results from the compression of air in the cavities resultingin very high heat which burns the carbon, hydrogen and oxygen in the plastic.

146

Causes: Dieselingorthebuming of theplasticcausedby the compressed, heated air in the cavities, results in black, charred surface bums in the part. Excessive melt temperatures causedby shear, long residence times, incorrect heat profile or malfunctioning heater bands.

Potential Solutions: Cleanand/oraddtheventsinthemoldandrunner system. Also clean the ejector pins. W Reduce the injection fill speed and/or reduce theclamp pressure, taking care notto flash the parts.

Determine thatthe screw has the proper designto avoid excessive shear heat. W

Make sure the valve being used is not too restrictive for the material being processed. W

W Determine that the proper heat profileis being used and that all heaterbands are operational.

If these solutions fail, gate size maybe increased to help avoid any shear taking place during injection. W

15.1.4 Cloudy or Hazy Parts Clear parts that have a hazy or cloudy appearance. Causes: Contamination of the resinor the useof too much regrind. Lack of proper or desired mold finish. Stressing of the material during the molding process.

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Potential Solutwm: Determine that previous processing ofcolored material or a different typeof resin is adequately purged. Check the materialsource (i.e., grinder or storage boxes) that might indicate thattoo high a percentage of regrind is being used. Check the cores and cavitiesto determine that the desired finish is present. Process with a melt temperature that is not too cold nor a fill rate that is too slow. Check for the proper heat profile andmelt temperature. Determine that the fill rate is appropriate for the material. Determine that the mold temperature is correct for the material being processed.

15.1.5 Drool Drool is the “oozing” of melt (failure of the melt to “fi-eeze off’ either at the nozzle or at the gate in a hot runner system). Drooling can cause soliditied materialto be introduced into the nexf part which causes surface defects. It may also interfere with theflow of the material and/or the mechanical properties of the parts. Causes:

Ineffective nozzle design, inadequate heat controls at the drool location and sometimes by inadequately dried hygroscopic material.

148 Potential Soluiions: Check for proper procedures for drying the material. If possible, measure the moisture content in the material. Use a reversetaper nozzle or a shut-off nozzle. Also, X a nozzle filter is used, make sure that the filter is clean, allowing effective melt decompression. Lower the temperature of the heat source at the drool location, i.e., the nozzle or hot runner probe. Increase melt decompression or suckback. Watch for evidence of splay. Reduce back pressure.

15.1.6 Flash Flash is the ragged edge of the part occurringat the parting line. Causes:

Assuming that the mold is properly constructed (including adequate sizing of the gates and good parting line match), flash can result fiom avariety of processingconsiderations,including injection pressure that is too high, an incorrect heat profile and improper settingof clamp tonnage.

Potential Solutions: Reduce injection pressure used for packing andor filling. Determine that the proper heat profileis developing the desired melt temperature. Check the melt temperature with a pyrometer.

149 Increase the clamp tonnage,if available. In rare instances, excessive clamp tonnage can cause flash. Make sure the proper clamp tonnage is being used.

15.1.7 Flow Lines Lines in the part that are shaped likethe letter J or U. Flow linesare more commonto polycarbonate, polyesters and acrylics. Flow lines should be distinguishedfiom knit lines. Knit linesare discussed at section 15.1.1 1. Causes:

Core or cavity surface defects,melt a temperature thatis not on target, a mold temperature that istoo cold and improper filling procedures can all cause flow lines.

Potential Solutions: Check the mold for scratches or surface blemishes and have them corrected. Increase the melt temperature,usingtheproperheat profile and confirmby checking with a pyrometer. Increase the mold temperature to ensure even cooling. Decreaseinjectionflow rate andincreaseinjection packing and/or hold pressureor hold time.

15.1.8 Gate Blush Gate blush is the hazy surfaceimperfectionfound location on the part.

at the gate

150 Causes:

Gates may be damaged, too small or the wrong type,or the coldwell in the sprue may be too small. Melt fiacture may be occurring as the molten plastic is forced throughthe gate(s). The melt temperature may be too low.

Potential Solutions: Change or repair thegate(s) andor increase thegate size. Checkcoldwell in thespruefortheproper correct, ifnecessary.

size and

Reduce the injectionfill rate (velocity, not pressure). Increase themelt temperature by raising thefiont zone or nozzle settings andor increase the mold temperature.

15.1.9 Inadequate Color Mixing The inadequatemixing of the colorant additives is usually apparent to the naked eye in examining the parts. However, in some cases, the use of a spectrophotometer may be required to determine that colors exactly match thecolors desired. Causes:

If the color concentrate carrier resin does not have a melt index comparableto the mainresin, theywill melt at a different rate, resulting in different melt viscosities and potentially poor color mixing. However, in most cases, the resinis not adequately melted to permit the proper mixing andor the screw is inadequate to the mixing task.

151

In some cases, restrictionsin the resin flow path, caused by a restrictive mixing section at the end of the screw, a non-return valve that causestoo much shear,an incorrect nozzle,or gates that are too small, can all cause too much shear heat resultingin color separation. Separation can alsooccur as a result of improper heat profiles or an inadequate screw design that causes excessive shear. But remember. materials will not mix unless they are in a jluid state.

..

Potential Solutions: Match the color concentrate carrier to the material being processed or closely enough to allow melting within the same heat range.

If a mixing device is used, be sure that it is a fiee flow, distributive typeof mixer that will not shear the material. Use fiee flowvalvesandlarge nozzles.

ID straight-through

Run a reverse or hump heat profile to help avoid too much shear fiom the screw. Reduce the injection speed, screw RPM and/or back pressure. Be sure that all mating surfaces in the fiont-end barrel assembly are perfectly matched, avoiding any hang-up areas. Process with a clean, polished screw, barrel, valve end cap and nozzle. Use a screw designed to adequately melt the base resin andthe concentrate and mix bothmeltedmaterials. Considertheuse of a well-designed,fiee-flowtype of mixing screw.

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If the material is not adequately melted and in a fluid state, increase the back pressure and screwRPM and adjust the heat profile, loweringthe barrel heat control setpoints. Usually, ifthe material is not adequately melted, a change in the heat profile and screw design are needed to correct.

15.1.l0 Jetting Jetting appearsas worm tracks or squiggly markson the wall of the finished part. The problem gets its name fiom the cause rather than the resulting condition or appearance. Causes:

Jetting results fiom a squirting of the melt directly on the mold surface (hence,the part surface) rather than flowing normally into the mold cavity. As a result, the thin films of squirted melt solidifies ahead of the rest of the melt leaving the marks described above. This condition commonly occurs when the gate is in the wrong locationor the melt fiont has nothing to deflect fiom.

Potential Solutions: Reduce the injection fill rate, profilingto fill more slowly. Increase the temperature of the meltand surfaces.

the mold

If the solutions abovedo not work,the size or location of the gates may have to be changed.

15.1.11 Knit Lines Knit linesare visible lines inthe part wheretwo or more melt flows a weak point in the part. join. Knit lines may be a cosmetic problem and

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Causes:

Heavy knit (also called weld lines) are typically caused by an inadequate mold design. If thetwo (or more) massesof molten material do not come together quickly enough to weld together while in a melt condition, a knit line will result. Potential Solutions:

Increase the melt temperature and the injection speed. It may help to also increasethe temperature of the mold. Increase the packing or hold pressures and time. Also adjust the transfer point and/or method for velocity and pressure control. If these solutions don’t work,the mold design shouldbe rechecked.

15.1.l 2 Part Deformation A part maybe deformed if it stickson the cavity-half of the mold. Deformation also occurs during ejection and may result from improper ejection and can include ejector markson the part. Causes:

Most part deformation occurs as a result of using improper ejectiontechniquesorinadequatemolddesigns.Ifthereare blemishes or hang-up spots on the cavity or mold undercuts that are poorly designed, ejection will be hindered and part deformation may result.

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Potential Solutions: Check for equal lengthknock out rods that activate the ejector plate. If the platedoes not move squarely, improper ejection canoccur. Also determine that the ejector pins are the proper size for the part. Small ejector pins w li not adequately ejecta large part, so an increase in the ejection area may be needed. Consider the use of air poppets to assist in ejection by releasing the vacuum that occurs beneath the part as it is being ejected. Slow the cycle if ejection is occurring while the part is still too warm. . . or improve the mold cooling. Determine thatthere are no defects on the core or cavity halves ofthe mold. Also check the mold undercuts for their adequacy to retain the part on the core halfof the mold but not impede ejection. Increase the cavity-half mold temperatureandor reduce the core-haK mold temperature to facilitate the ejection process.

15.1.l3 Poor Screw Recovery If the screw fails to recoverwithsufficientspeed to avoid lengtheningtheoverallcycletime,itshould be considered a molding problem. Causes:

The three most common causes of poor screw recovery are: (1) an improper heat profile; (2) a screw that is improperly designed to process the materialbeing used; and (3) a worn screw or barrel,

155

or both. A doubling of the screwharrel clearance can cause a 25% drop in the production rate, usually associated with slow screw recovery.

Potential Solutions: Use a heat profile that generates at least halfof the energy needed to melt the plastic.Thiscanbemeasured by determining that the screw rotate pressure(extrude pressure) is at least 40% (but not more than 60%) of the available hydraulic (or electric) energy available. For shot sizes of 50% or more, or that involve highly filled material, consider a reverse heat profile for better results. Determine that the design of the screw being used is appropriate to process the material being molded.If there is a question, consult one or two reliable screw manufacturing concerns. Measure the wear in the barrel,screwandvalve opportune times,butnotlessf?equentlythanevery months. Replace or repair worn components.

at 6

Minimizebackpressureandavoidusinghighback pressure as a substitute for good screw design.

15.1.14 Short Shots A part that is incomplete in anyrespect is considered a “short shot.” The problemis easily recognizedas being a partthat is not fully filled out.

Causes:

Short shots often result from starting up the IMM after a cycle interrupt. Sometimes they result when some ofthe processing

parameters are not fine-tuned.This might include the transfer point, melt or mold temperatures, packing pressure and other variables. Occasionally, short shots are caused by inadequate gating.

Potential Solutions: m Allow the process to stabilize after a cycle interrupt, discarding 3 to 5 shots to achieve proper part fill. m Adjust processing variables such as the transfer point, fill rate melt and mold temperatures, packing pressure, and

or fill speed. Check for a worn or faulty non-return valve that could result in inconsistent shot sizes. If gating is found to be the problem, increase the gate size and/or runner size.

15.1.15 Sink Marks Sink marks are a depression in the surfaceof a part, often associated with areas of increased wall thickness, such as ribs or bosses. The sink marks will typically appear on theofside the part opposite the area of heavy wall thickness.

Causes: Sink marks are usually the result of improper mold or part design, including gate and runner size, the solutions for which are involved with the complexitiesof mold and part design andare not presented here. However, process changes can be made to help minimize the problem.

157

Potential Solutions: Increase the pack and/or holding pressure and time. Reduce the melt temperature using a proper heat profile. Reduce the mold temperature, especially the side the with most visible sink and increase the temperatureof the other haE. This may permit a longer packing time while fieezing the outer surface. Also experiment with post mold cooling, including placingthe molded part immediatelyin water. Increase thesize of the gate(s).

15.1.l6 Splay Splay is the term applied to silver streaks in the part surface or through the part thickness in thin-walledparts.

Causes: There are two major causes of splay. One occurs as the result of processinghygroscopicresinswhich have notbeen adequately dried. The other isreferred to as heatsplay,which occurs as the result of entrapped gasesor excessive heat that burns off volatiles.

Potential Solutions: For splay caused by moisture, check the material for proper moisture content. If not within specification, check the dryer forproper operation and for proper drying time. Check the mold for small water leaks.

Splay resulting from excessive heat may be prevented by using the proper heat profile (assuming that all heater bands are functioning properly) and a screw that does not cause excessiveshearheat. Also reducenozzletemperature (assuming the use of a correct nozzle design) and determine that the non-return valve is not causing unwanted shear. Reduced back pressure and injection speed may also help. Splay causedby gas entrapmentmay be the result of too much meltdecompression (suckback).Also check the resin for any contaminants, including excessive “fines” (small particles or resin) which quickly absorb ambient moisture.

15.1.l7 Warped Parts Parts that havea warped shape usually result fiom an inconsistency in the coolingof the part for each half of the mold. Causes:

Warped parts most often result fiom an improper mold design and mold construction, which are not coveredby this book. However, some processing adjustments can be made to help avoid warpage.

Potential Solutions: Adjust the moldhalftemperatures, cooling the hothalfof the mold (typically the cavity half)rather than heating the up cold half. Check water flow in all circuits. Reduce injection forward time andor increase cooling time. Also, increase the fill speed. Having read the section on process problem that can be experienced in injection molding, we believed that it would & helpful to provide youa

159

more simplified guideto troubleshooting processing problemsin the form of a table. The table may be referred to quickly and if the more briefly wordedcausesandpotentialsolutions in thetablerequire further The explanation, youcan review that section in the more detailed narrative. table on troubleshootingisprinted at the end ofthis chapter.

15.2 Component Wear Problems Excessive wear of the screw, barrelor non-return valve can cause extensive molding problems. Undetected wear can result in many of the problems that wereidentified as processproblems.These can include excessive heat, splay, inability to hold a cushion andmany others. Not the least of the problemsis the reductionin the production rate. Two separate studies have resulted in the same conclusion that: Doubling the clearance betweenthe barrel andthe screw can result ain reduction inplasticating capacity of25% The abilityto hold a cushion or pack properly is greatly affected by the degree of wear on the outside diameter of the non-return valve ring or the wear on the rear seat. In addition, wear inthe stroke section of the barrel can allow the valve ring to -and and break under the pressure of injection. Causes:

The premature wearof the screw, barrelor valve can have several causes. First, thecomponents may not be madefrom the proper materials that are adequate to resist the wear characteristics of the resins being processed. Second, inadequate an screw design relative to the resin being processed can cause the premature wear of the screw. Third, an incorrect heatprofile andor heater band mahnction can resultinrapidwear of the screwandbarrel. Fourth, the inadequateremovalof moisture may allow water vapor to unite with acid-generating gases can thatresult in corrosive wear. Fah,processingparameters, such as excessive back pressure or improper shutdown or start up, can also result in component wear.

160 And finally,contamination ofthe material (failure to remove metal fragments and other items) can cause wear and breakage.

Potential Solutions: Special wearresistantmaterials,such as particle metallurgy tool steels or special alloys mustbe used when processing heavily reinforced resinsor resins that develop corrosive acids. For example, PVC can develop hydrochloric acid, acetals result in formic acid, fluoropolymers generate hydrofluoric acid andso on. Screws that are notproperlydesigned can causea pluggedflow,forcing the screwagainstthebarrelwall resulting in adhesive wear. In addition, a screw that is too aggressive (compression ratio too high andor transition zone too short) can result in abrasive wear inthe transition zone of the screw and the corresponding area in the barrel. Screws thatare too shallow for processing viscous materials or materials that contain a high percentage of reinforcement or fillers also contribute to premature wear. The use of excessivebackpressure to compensate for an improper screw design shouldbe avoided. Heater band settings that are too low in the feed and transition zones can cause excessive shear, also resulting in adhesive and abrasive wear. Use heater band settings to achieve 50% of the heat energy from conductive heat rather than shear. Consider “hump” and “reverse” heat profiles. Be sure to check all of the heater bands to determine that they are hctioning properly. Residencetimesthat are too longfortheprocessing conditions encourage corrosive wear. Calculate the residence time for each processing application and try to avoid times that exceed5 minutes.

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The proper procedures used during shut down of the molding machinehelp avoid both component breakage and corrosiveweur. It is veryimportant to shut downthe molding machine with the barrel properly purged and the screw in the forward position. If resin solidifies near the be fiont of the screw or the valve, the valve can easily snapped off fiom the torque generated at the subsequent start up. Moreover, if the machine is allowed to be shut down with material the in screw channels,that material can promote corrosion while cooling and sitting overnight. Shut down procedures are usually carefully enumerated by the injectionmoldingmachinemanufacturer.Follow those recommendations!

Be sure that proper screening and magnetic devices are included in the hopper to assure that the material is not contaminated or contains bolts, nuts, pencils, ball point pens, and so forth!

Troubleshooting Processing Problems Potential Solutions

Problem

Recheck drying procedures Confirm used p q x r screw design , Review heat profile 8 back ,CheckraSidencetilTl€!

Part Brittleness

BUMICE,

a voids

- Inadequate

ventingofmdd or runner system

-Excessiisuckbackormelt d e c a m p e rs s h

Burned Material

. Clean 8 increase venting . Reduce i n @ i fill speed . Reduce Clamp tonPreSSUre .Decrease melt decompression 8 reduce nozzletemperature .Use taper or shutoff nanle

- Excessive shear -LOngreSidencetime

.Clean 8 increase venting

-lncorredheatprofile Malfunctioning heeter bands

.Confirm use d praper screw design

-

- Resin contamination

- Using too muchregrind - Improper mdd finish - Stressing of material

.Reduce injectionfill speed .Increase gate size

- Check valw for too much restriction - Review heat profile 8 check heater bands - Determine proper purgingd prior mat&& -Checkfortoohigha%ofregrind -Checkcores&cavitiesforproperfinish Assure that melt temperature is correct - Determine thatfill rate is not too s l w check mokl for proper temperature

-

- Improper nanle design

- Check

-lnadeqwteheatcontrolat drod location Inadequatey l dried material

-LawertemperatureddrodSWrce

-

- lnjectian tW high - Incorrect heat prafile - Improper clamp tonnage

drying procedures -Useteversetaperorshutoffnanle

- Increasemelt decompression - Reduce back - Reduce injedion pressureused for packing and/or filling -checkheatprofileandcheckmelt

e tmperau trempyranee tr

- Increase clamp tonnage,if mailable - Check mold for blemishes

-coreorcavity" " ~ r e t W C d d

- Mold temperature

- Improperfill rate

toocold

-Increasemelttenperature&checkwith PF-w - Increase Inold temperature -Decressein~ficmrate&increase packing and/or hold pressureor time

- Repair gades and/or increasegate size -Checkspruecddwellforpmpersize

- Reduce injection fill rate (VelOCny)

-chedcmelttemperaturemwrandera increesefrontzone8nanleseitingif t w low

163 Causes .Meltindaxdresin&cdor concentratecarriernot compabble .Excessiishearheatinmelt

fcmrm

- Improperheat profile .Inadequatesccew design

Potential Solutions ~Impr~matChofCOl~CCflcentratecanier MI to resin being processed . Determine thatmixing device does not shear the material . Check VahR for possible restridion8 Shear

. Run revecse or hump heat profile

to help avoidtoomuchshearfrunscrew . Reduce injectionspeed, screw RPM and back pressure . Use a weldesignedmixing s c r w .If matetial isn d melted, alter the heat profile and change to propertydesigned SCreW

- Gate too small in

relationto wal thickness of the part

- Reduce injectionfill rate - Increase temperatureof melt 8 mold surfaces

- Increasesize of thegates - Inadequatemold desgn

- Increasemelt temperature 8 injection

speed

- Increasetemperature af mold

- Increase packingor hokl pressures8 time -Recheckthedesiindthehemdd

- Check knock out mds for equal

length

- Determinethatejector pins are of proper Size

- Consider air poppets to assist ejection - If part is stillwann when e j e c t e d , slow the cycleorimprovemoldcaoling thatthere are no defectsin cores or cavities 8 that mold undercuts areadequatetoretainthepartonthe core half -lncreasecavitymddtempeiatureand/or reducewtemperature

- Determine

- Improper heat prdk - ImprOperlydesigned screw -WOmSCf€Wandlorbarrel

- Processing par;ameters are m finely tuned - Inadequate gating

- Use a heat profile that generatesat least halfoftheenergyneededtomettthe plastic - Determine thatthe screw design isproper - Measure screwlbarrelW a r e p l a c e if worn - M i n i m i back pressure

164

Sink Marks

Potential Solutions

Causes

Problem

- Improper mold design, including gate8 runner Size

- Increasepack &/aholding pressure 8 time

- Reduce melt tempennure, using a proper heat profile -Red~~~~!rokltemperatuEUItheSidewith

the most visible sinkand incresse the temperature on the other half -Determineifchangesinthemolddesign are required

-C

M material for propar moisture content

-Checkm0ldforsmallwaterIe&s - Determine u s e d proper heat profile -checkforanypointsdshear,suchasthe n o z z l e , s c m . runner system or gat= - Check for sccessive melt decanpression - Determinethat resin does mt h a w excessive "fines"which absorb ambient

wk.

moisture

warped Parts

- Improper mold design or cons-

- Cod the hat half (typicallythe cavity half) dthemold. Checkwaterflwinall arcuits - Reduce inpction forward time &/or increase cooling time.Also increase fill speed.

165

16 Terminology The following pages include terminology that is commonly used in the plastics industry and more specifically, in injection molding. Reference ismade to glossariesalreadypublished,such as those in the Plastics Engineering Handbook, Fifth Edition; The Kayeness Practical Rheology for Handbook; and in the RecommendedGuidelineNomenclature Machinery Components by the Machinery Division of the S.P.I. In all cases, those definitions have been simplified to facilitate their usefulness. abrasive wear - wear caused by the continual contact, under pressure,of hard particlesin the resins against a barrel lining, screw surfaces and valves or othercomponents. The abrasiveparticles may be reinforcements, such as fiberglassfibers,calciumcarbonate, powdered metals and others. accumulator - an awclliarycylinderequippedwithaplungerthatis mounted on an injectionmoldingmachine (or blowmolding machine) to provide a fast delivery of melt. The accumulator is f l e d between shots with melt comingfiom the main barrel. It can aid in injection speed butdoes not add injection pressure. additive - a substance compounded into a resin to mod% its processing as stabilizers, plasticizers, characteristics or physical properties, such flame retardants, lubricants and others. adhesive wear - wear resultingfiom two metals rubbing against each other, such as the screw flights and valve rings coming into contact with the lining of the barrel during injection molding operation. air shot - injecting (into the a i r )a shot of molten plastic without the or examining the melt quality. carriage forward, i.e., in purging ambient temperature - temperature of the air or other substance surrounding an object.

166

amorphous - term applied to resinshavingnocrystallinemolecular structure. anneal

- heating an article to

a predetermjned temperature and slowly cooling it to relieve stresses. Usually relates to annealing steel or plastics.

atactic - a term used to identi@ a polymerchain that has a random arrangement of units. autoclave - a closed vessel used to conduct a chemical reactionor other operations under pressure and heat. An autoclave is used in making particlemetallurgy tool steel in a processcalledhotisostatic pressing (HIP). automatic mold - an injectionmold (or other type ofmold) that permits the entire molding cycleto be repeated, time after time (untilstopped) without human assistance. axis (of a screw or barrel) - the accurately straight centerline extending through the screwor barrel from which to measure concentricity. back pressure- the hydraulic restriction of the backward movementof an injection screw during the recovery portion of the molding cycle. ball check valve - a valve mounted at the discharge end of an injection screw usedto allow forward flow of plastic during screw recovery and no flow during the injection stroke. The valving is accomplished by the movement of a ball inside the valve housing. barrel - a cylindrical housing in which the screw rotates, including the inner surface lining material, or replaceable liner,ifused. Also commonly referred to as a cylinder. barrel shell - the outer thick wall of the barrel made froma strong metal backing material (suchas chrome-molybdenum alloy steel, 4 140 or

167 4340) to provide strength and hold the inner lining. barrier flight - a secondary screw flight of reduced outside diameter designed to separate melted plastic from the solid (unmelted) plastic by allowing the melt to pass over the barrier flight fiom the solids channel intoa melt channel. bayonet adapter - a cylindrically shaped device with holding pins that threads into a thermocouple hole (on a barrel, for example) and retains a spring-loaded thermocouple. bearing surface- the portion of the screw immediately behindthe flighted length which prevents the escape of material and provides a seal between the screw and the barrel. Sometimes referred to as a hub.

bell end - an enlarged portion of a barrel at its discharge end which provides addedstrength to withstand internal injection pressures. bimetallic - a term used to indicate that a barrel is composed oftwo (or more)metals,commonlyused to refer to barrels thathave a centrifbgally cast lining. blister ring - a raised portion of a section of a screw root of sufficient height and thicknessto effect a shearing actionon the polymeras it flows betweenthe blister ring andthe inside wall of the barrel. blow molding - a method of fabrication in which a warm plastic parison (hollow tube) is placed between the two halves of a blow mold cavity and forced to assume the shape of that cavity by the use of air pressure. Pressurizedair is introduced into the inside of the parison through a blow pin thereby forcingthe plastic parison againstthe surface of the mold that defines the shape of the product (suchas a bottle). bore - the inside diameterof a barrel which houses the screw.

168 bulk factor - the ratio of the volume of loose plastic feedstock (pellets or powder) to the volumeof the same weightof resin after molding. calcium carbonate - a filler or reinforcement usedin thermoplastics thatis typically groundor powdered limestone.It is an abrasive ingredient in the plastic that can cause premature abrasivein injection wear unit components. carbon black - a very black pigment usedas a filler because of its useful protection against ultraviolet light andenhances the life of plastic products intended for outside weathering applications. carbon fiber - a reinforcement additive to plastics used for lightweight, high strength and high stiffhess applications. The fibers are abrasive andcancausetheprematureabrasive wear of injection unit components, similarto calcium carbonate and fiberglass fibers. cavity - the part of the injection moldthat usually forms theouter surface of the molded part. Molds are fkequently identified by the number of cavities they contain (such as a sixteen cavity mold) and as a group (multi-cavity mold). channel - the space between the flights of a screw, bounded at the surface by the inner lining of the barrel (with the screw in the barrel), through which plastic is conveyed and melted. channel width - the distance across the screw channel in a direction perpendicular to the flights, measured at the periphery of the flight. check ring - the cylindrically shaped component that moves back forth and along the axial lengthof a non-return ring valve. During injection, the ring shuts off against the rear seatof the valve, preventing melt fkom flowing backwardtoward the meter endof the screw. During screw recovery, the ring rests against the fiont seat (or stud in a three-piece valve), allowing melt to flow forwardthrough the valve into the discharge end of the barrel.

169

chiller - a system comprised of a refrigeration unit and a coolant circulation mechanism (reservoir and pump) that maintains the desired heat balance in an injection moldby circulating temperature controlled cooling fluids through the mold. Chillersare typically self-contained units available from their manufacturers. chrome plating - a process that deposits a hard film of chrome onto wear surfaces involved in injection molding, such as screws, end caps and mold cores and cavities. Typically the plating thickness ranges from .001” (.0025cm) to .005” (.0127cm). clamp force - the pressure applied to an injection mold to keep it closed during the injection and hold portions of the molding cycle. This force is typically expressed in tons (kilograms). clearance (screw/barrel) - the difference betweenthe outside diameter of the screw and the inside diameter(bore) of the barrel. coefficient of thermal expansion- the small growth in the dimension of a material (metal, plasticor other) per a unit changein temperature. For example, a fraction of an inch (mm) in growth of an item of specified dimension per increase of a given number of degrees in its temperature. cold slug - the first material to enter an injection mold (in a mold other than a hot runner mold) which is typically cooled below the effective molding temperatureas it passes through the sprue orifice. The cold in a space provided directly opposite the sprue opening slug ends up called a cold slug well. color concentrate - a quantityof die or pigment combined with a quantity of resin (usually in pellet form) that may be added to a measured quantity of the bulk resinto achieve a desiredcolor in the finished plastic product.

170

compression ratio - the factor obtained by dividingthe volume inthe screw channel of one diameterofthe feed sectionof a screwby the volume in the screw channel in one diameter of the meter section of the screw. However, the term compression ratiois commonly usedto describe the channel depth ratio, which is simplythe ratio of the channel depth in the feed section ofthe screw divided bythe channel depth in the meter sectionof the screw. concentricity - a term used to describe the relationship of two cylindrical shapes to a commonaxis, such as the relationshipof the outside and inside diameters of a barrel to a line running straight through the center of the bore of the barrel fiom one endto the other. If both diameters are a constant distance fiom the center line throughout their length, theyare said to be concentric or have concentricity. A deviation fiom concentricity is referred to as runout. copolymer - a polymer thatis synthesized (madefiom) more than one type of monomer, whereas a polymer consists of only one type of monomer. core (in a mold) - that portion of an injection mold that forms the interior surface of the molded part. core (in a screw)

- an internal hole extending down the center line of a

portion of a screw used to circulate a coolant or heating mediumto better control the temperature of the screw. core pin - a pin that is included in an injection mold that is used to mold a hole in the molded part. corrosive wear - wear appearingon the surfaces of screws, barrels, valves or other injection unit components resulting fiom the attack of various acids. Acids come fiom the polymers themselves or from additives, such as h e retardants, foaming and coupling agents, and erode and pit the metal surfaces with which they come into contact.

171

counterbore - the recessed area in the discharge end of a barrel (or screw) which acts as a pilot to ensure the concentric fit and sealingof the end cap (or valve) to the barrel (or screw). cross linking- the establishmentof chemical bonds between the molecular chains in polymers. Cross linking is commonly accomplished by chemical reaction triggered by a combination of heat and pressure. Once material has cross linked, itis like cured thermoset material (i.e., it cannotbe remelted or reprocessed to make parts). crystalline - the term applied to some polymers whose molecularchains possess auniformityandanorderlystructure. The opposite structure in polymer molecularchains is referred to as amorphous. cycle - in injection molding, the complete, repeating sequence of operations in molding parts. Cycletime consists of the elapsed time between one point in the cycle and thesame point in the next. daylight (in molds) - the distance between the stationary and moving platens of a molding machine when they as arefar retracted (open) as possible, without an ejector box or any spacers. degradation - the term applied to polymers that have been damaged in their physical or chemical properties by excess exposure to heat, light or other cause. density (in plastics)- the weight per unit of volume of resins either in their solid state at room temperature (solid density) or at their melt temperature (melt density) as expressed gramskubic in centimeter or pounds/cubic foot.Also referred to as specific gravity. dessicant - a substance thatis used to dry other materials (such as plastic feed stocks) because of its ability to absorb moisture. Dessicant in the form of beads are usedin dessicant dryers.

172

dispersive mixing - the final melting and mixing of a solid (unmelted polymer or pigment) with a fluid (such as melted polymer). distributive mixing - the mixing of a fluid (such as a melted polymer) with anotherfluid(such as a liquid colorant).Distributivemixing is aimed at achieving thermal and color uniformity where no further solids meltingis required. draft - the angle or degree of taper used in a side wallor clearance in an injectionmoldcomponent (such as the side of a thin-walled container) to facilitate the removal of the parts during ejection. drooling - the leakage of melted plastic ffom a nozzle during the injection portion of the molding cycle. ejector pins - rods or pins that push a molded partoff of the core or out of a cavity during ejection. The pins are usually attached to an ejector bar or plate that can be actuated by the ejection systemof the machine. Ejector sleeves can be used instead of pins to eject parts (especially round parts). elastomer - a polymer that at room temperature stretches under a low stress to at least twice its length and snaps back to the original length upon releaseof the stress. end cap - the steel component that bolts to the discharge end of an injection barrel andalso adapts (holds) the injection nozzle. family mold- a multi-cavity mold where each of the cavities forms oneof the component parts of the assembled finished object. Theterm is also applied to molds whereeach of the cavities produce a different part in the interests of economic production. feed opening - the hole through the feed section of the barrel through which the unmelted plastic feedstock (such as pellets or powder) is introduced. It is also referred to as the feed hole or feed port.

173

feed section(or feed zone of the screw) - the part of the screw that extends under the feed opening andreceives the materialto be processed. The feed section (in injection molding) normally has a constant channel depth andconveys the material to the transition (or compression) sectionof the screw. filler - material addedto the plastic feedstock for the purpose of improving its physicalproperties (such as strength) or its process ability (such as a plasticizer or heat stabilizer), or to reduce the cost of the material. fines - very small particles (commonly under 200 mesh) that accompany largerparticles (or pellets)ofplasticfeed stock. Finesoccur especially in regrind materials.

fisheye - a pellet of plastic that has survived the molding process without being completely melted. flame retardant - a chemical that is compounded into a resin to make it (and the finished partfiom which it is made) fire-resistant. flange - a short, collar-like section of a barrel, with a larger diameter, through which bolt holes have been placed to assist in mounting and securing the barrelto the machine casting. flash - extra plastic attached to the molded part along the parting line which, under most conditions, is a defect and must be removed before the parts are acceptable. flight - the helical metal thread-like, raised portion of the screw which defines the screw channel. flow line - a line in a molded part caused by the joining of two melt flow fionts during injection. Also called a weld Zine or knit Zine.

1 74

foaming agents - chemicals addedto plastics that generate gases during the melting process causing the resin to form a cellularstructure. gas assist - the term used to refer to the introduction of a gas (such as nitrogen), under pressure, into the melted resin, eitherat the nozzle or the mold, to cause the molded part to have a hollow, gas-filled center thus requiring less plasticto mold the desired part. gate - the small, short and usually restricted section of a runner at the entrance to the cavity of an injection mold. The molten plastic enters the cavity through the gate during injection. Gates have various shapes and forms, depending upon the design of the partto be molded. glass transitiontemperature - the temperature at which amorphous polymers change fiom a hard, brittle (or glassy) condition to a viscous, softened, more elastic condition. guide pins - large pinsin a mold that maintainthe proper alignment of the core and cavity halves ofthe mold when it closes. Also called leader pins. heater bands - electrically powered heating elements that fit around the outside diameter of a barrelto control the conductive heat used to helpmelt the plastic.They are commonlymade fiom mica or ceramic and are connected to machine controls that permit the desired heat settings forthe various heating zonesof the barrel. helix angle - the angle of the flight of a screw relative to a plane that is perpendicular to the axis of the screw. The most common helix angle is 17.6568' which forms a square pitch.

homopolymers - a polymer thatis based on only one monomer. Typically such homopolymersare referred to as polymers.

175

hopper - the b e l shaped container that is mounted directly above the barrel (andthe feed opening) which contains a supply of feed stock (such as pellets) to be processed. hopper dryer - the combination of a hopper and a drying system where hot, dry air flows upward through the hopper containing the feed stock pellets, drying the material. hot-runner mold - aninjectionmold inwhich therunnersystemis insulated fiom the rest of the mold and remains hot so that the center of the runner nevercools during normal cycleoperation. hygroscopic - the term that refers to resins indicating that they absorb moisture fiom the air. injection pressure - the pressure exertedon the melted plasticby the screw and valve to cause the plastic to be injected into the mold. The pressure is expressed in psi. injection rate - is the calculated rate, expressed in cubic inches per second (cubiccentimeterspersecond), at whichthescrew(andvalve assembly)injectsmeltedplasticintothemoldattheinjection pressure specified. inlay - the hard surfaceportion of a flight that does not extend completely across the widthof the flight. insert - a part made of metal or other material that is molded into the plastic part (or pressed into the part) as the molding cycle is being completed. jetting - the turbulentflow of melt fiom an undersizedgate or thin section of the mold into a thicker section rather than uniform the radial flow of the melt fiom a gate to the ends of the cavity. Jetting can result in irregular marksin the final molded part that are unacceptable.

L/D ratio - the ratio of the working flighted length ofthe screw (that is, the distance fiom the fiont edge of the feed openingto the forward end of the screw flight when the screw is in the forward position)to its outside diameter. In practice, the ratio is simplGed to dividing the flighted length of the screw by its nominal diameter. lead - (of the screw)is the distance, measured along the axis of the screw, fiom one edge of the top ofa screw flightto the same edgeof the same flight after one completeturn. liner - the wear resistant, removable sleeve(s) that form the inside diameter of a barrel. lining - the wear resistant portion of an injection barrel that forms the inside diameter. The lining may be metallurgically bonded to the inside diameterof the barrel shell or may be removable linersthat are press or shrink fit into the barrel shell. locating ring- a metal ring located on the backof the stationary half of an injection mold that helps alignthe mold to the machine platen and the nozzle of the injection barrel with the entrance of the sprue bushing. melt - plastic materialin a molten condition. melt channel - the screw channel in a barrier screw designed to collect and convey forward the melted resin. melt index - a measure of the viscosity of various plastics in their melted condition. Melt index is determined by the number of grams of melted resin at374°F (190°C) thatcan be forced through a 0.0825" (.2 1 cm) oriiice during ten minute a period when subjected to a force of approximately 43 lbs/in2 (298kPa). melting point - the temperature at which a resin changes fiom a solidto a liquid. Usually associated with crystalline type of resins.

177 meter section - the portion of the screw at the forward (discharge) end which has screw channels of a constant depth and a length of at least one turn of the flight. mixing section- a sectionof the screw (usually less than 3 diameters long) that has special geometry designed to enhance distributive andor dispersive mixingof the melted resin. mold base - the assemblyof all parts of an injection mold, other thanthe cores, cavities and pins. Standard mold bases of various sizes may be purchased from manufacturers to expedite the mold making process for more simple molds. monomer - astartingmaterial,based on theelementcarbon,that is synthesized (made from) simple, oil-based raw materials. Monomers have typically low molecular weight and are put together by a process known as polymerization to form polymers (or copolymers). movable platen - the moving platen of an injection molding machine to which half of the injection moldis secured during operation. nitriding (gas) - the hardeningof the surface of certain alloy steels caused by heating the steel in an atmosphere of nitrogen (ammonia gas) at approximately925°F(496°C) to 950°F ( 5 10°C). The resulting surface is very hard (70 RC or higher) with wear resistant hardness (50 RC or higher) extending to a depthof .007" (.018cm) to .015" (.038cm). The processis commonly used to provide wear resistance to the surfaces of screws, valves and the inside diameter of barrels. nitriding (ion) - similar to gas nitriding, achieved by heating steel to approximately 600°F (3 16°C) in an atmosphere of hydrogen gas while addinganelectricalchargetothesteelandintroducing nitrogen gas. The positively charged steel is bombarded by the hydrogen and nitrogen gas ions resulting in a surface hardness similar to that achievedby gas nitriding. Ion nitriding produces a slightlydeeperandmoreuniformhardnesswithlesspotential

178

distortion and contaminationof the steel thandoes gas nitriding. non-return valve - a valve mountedon the discharge endof a screw that allows the flow of melted plastic in one direction only (forward) duringscrewrecovery.Duringinjection,thevalveallows no backward flow and enablesthe screw to perform like a plunger. nozzle - a device that threads into the end cap and adapts the discharge end of the nozzle (and nozzle tip)to the sprue bushing in the mold. nozzle tip - a device with an inner orifice of various designs andsizes that threads into the nozzle and adapts the nozzle (and nozzle tip) to the sprue bushing of the mold.

olefins - the term applied to a group of hydrocarbons which are the basis for certain polymers, most notably polyethylene and polypropylene. overall length(of a screw) - the total length of a screw which includesthe flighted length, theshank (drive end) butnot the non-return valve. part - the term that is applied to the product of the injection molding process. For example,a molded cup, container, toyor other object is referred to as a part. parting line - the term applied to the line on an injection mold (and as evidenced in the molded part) where the halvesof the mold met in closing. pilot (in screws)- an internal cylindrical surface at the discharge ofend the injection screw that is usedto accurately locatea non-return valve to the screw. pitch - the distance, measured parallelto the axis of the screw, fkom one edge of the top of a flight to the same edge of the next adjacent flight. In the case of a multi-flighted screw,the pitch is less than the lead.

179

plastics - the term applied to polymers. Also referred to as resins. plasticizing capacity - the maximumquantity of a specified plastic that can be elevated to a uniform and moldable temperaturein a given unit of time. This capacity is generally expressed in ounces (grams) per second or pounds (kilograms) per hour. This term is also referred to as the recovery rate of the injection screw. plasticizer - a material compounded into a thermoplastic to increase its workability and flexibility and which may lower the melt viscosity and the glass transition temperatureof the plastic. pocket (in screws) - the location where a screw flightis initiated. Most commonly, the feed pocket is located at the intersection of the bearing of the screw drive and the beginning of the first flightof the screw. polymer - a combinationof polymerized monomers that consistsof many identical repeat units.AU plastics are polymers (or copolymers) but not all polymers areplastics. Cellulose is a polymer but it cannot be processedlikeaplasticmaterialunless it is modified. See homopolymers, copolymers and monomers. press fit - an interference fit achievedby mechanically forcingthe smaller piece into the larger piece (with the larger piece having a slightly smaller inside diameter than the outside diameter of the smaller piece) by use of a press or similar machine. Press fits of .003" (.008cm) are common in barrel liners press fit into the prepared barrel shell. purging - the cleaning out of the injection unit (barrel, screw, valve, end cap and nozzle)of all plastic that might remain tiom the immediately precedingprocessing. If changingcolor or shuttingdowntlie machine, a differentplastic may be forced through the unit (sometimes with the nozzle withdrawn from the wold). Purging materials are also available. Purging sbu€dalways occur before

shutting down the machine. rear radius (of screw flights) - the radiusat the intersectionof the rear or trailing side of the screw flight and the screw root. Usually this radius is larger than the fiont radius and may change fi-om one portion of the screwto another. register (of a screw) - the cylindrical portion of aninjection screwat the discharge endof the screw that is accurately machinedto match the rear seat of the non-return valve. regrind - ground up scrap that may be added to the "virgin" plastic feed stock and remolded. See scrap. reinforcement - a strong, inert material compounded into a plastic to improve its strength, stifkess, andimpactresistance.Typical reinforcementsincludefiberglassfibers,fiberglassspheresand calcium carbonate. resin - solid or semi-solid organicproducts of natural or synthetic origin, generally of high molecular weight with no definite melting point. However, resinis commonly usedto designate any polymer thatis a basic material for plastics, i.e., the terms resins, polymers and plastics are used interchangeablyin the industry. Rockwell hardness - a common method of expressing the degree of hardness of a material by testing its resistance to indentation, under pressure, by a diamond or steel ball. Resultsmay be expressed on various scales, the most commonof which is referredto as the C scale and is identilied as RC.Thismeasurementpermits the comparison of the hardness of various materials thatmay be used for barrel linings, screw surfaces and valve components. root (in screws) - the surface of the screw betweenthe flights, usuallyof a cylindrical or conical shape thathas a diameter smaller than the outside diameterof the flights.

runner - the channel that connects the sprue with the gates in a mold. Also refers to the molded form connecting the sprue witha molded part. scrap - any productof the injection moldingoperation that is not partof the primaryproduct. Scrap may include flash, short shots, runner systems, rejected parts and sprues,all of which may be ground up and added to "virgin" material to be remolded. The ground scrap is referred to as regrind. screw

-

a helicallyflighted shaft which rotates within the barrel to mechanically work, convey and inject the plastic being processed.

screw diameter - the dimensionofthecross-section ofthe screw, including its flights. Althoughusually expressed as a nominal diameter, such as 2" (50mm), theactualdiameter is usuallysmaller than the nominal diameter, suchas 1.964" (4.989cm). screw speed - the speed with which the screw rotates as expressed in revolutions per minute(RPM). shank - the rear, non-flightedportion of the screw thatfits into the quillof the injection molding machine at the drive end. Also sometimes referred to as the screw drive. shelf life - the periodof time thata molding compoundor plastic feedstock can be stored withoutlosing any of its physical or molding properties. short shot - an incomplete molded part resulting fiom an incomplete filling of the mold cavity. shot - the yield fiom one complete injection molding cycle, including the parts, runner systems (unlessa runnerless mold) and flash.

shot capacity - the maximumvolume, expressed in ounces (or grams), that an injectionmolding machine can produce during one molding cycle

shrink fit - an interference fit, similarto the press fit, achievedby heating the larger receiving piece and cooling the smaller mating piece allowing the smaller pieceto enter. Once the pieces touch, the fit is established. In somecases,barrelliners are shrink fit into the prepared barrel shell. sink mark - a depressionor dimple on the d a c e of a molded part caused Often by local shrinkage of the plasticunderneaththemark. associated with unusually thick bosses or support ribbing on the underside of the part. smear tip - a conical device used in place of a non-return valve at the discharge endof the injection screw. It is typically used with high viscosity, heat sensitive polymers where a non-return valve could cause degradationof the material. solids channel - the continuation of the feed channel in a barrier screw designed to contain and convey forward the m e l t e d polymer. splay - linesappearing in thepartaftermolding,typicallycaused by inadequate removalof moisture fiom the plastic feed stock. Splay can also be caused by excessive shearing of the melt during the molding process. sprue - the primary openingin the mold that accepts the melted plastic for further distributionthrough a runner systemor hot runner system. square pitch - the term applied to a single flighted screw which has a pitch (or lead) equal to the nominal diameterof the screw. For example, a 2" (50mm)nominal diameter screw with a2" (5Omm) lead. stationary platen - the large plate of an injection machine to which the fiont plate of the mold is attached during operation. This platen does not move during the molding process.

183

stripper plate - a plate in the injection mold thatstrips the molded part(s) from the cores. talc - a powdery material (hydrous magnesium silicate) that is compounded into the plastic feedstock as a reinforcing filler. thermocouple - anelectricaldevice withtwo metallic conductors in contact that produce an electrical current whose magnitude is dependent upon the temperature at the contact point. Thermocouples are typicallyusedinbarrels to help monitor and control theheat required of the heater bands. thermoplastic - a plastic that w l i repeatedly soften when heated and harden when cooled. They are contrasted with thermosets (see below), which soften when heated only once andbecannot remelted again. Most polymers used today are thermoplastics and include styrenic polymers and copolymers, acrylics, cellulosics, polyethylene, polypropylene, vinyls, nylons and various fluoropolymers. thermosets - a material that softens as a result of a chemical reaction to heat, pressure and other factors,andharden (or cure) intoan insoluble and infusible condition. Thermosets cannot be resoftened like thermoplastics. Typical thermoset materials include phenolics, unsaturated polyesters, aminos (melamine and urea), alkyds and epoxies.

TIR - an abbreviation, meaning "total indicator reading," to describe the deviation in concentricity of a measured surface from a selected surface as shown on a dial indicator. transition section (zone) - the portion of a screw between the feed and meter sections where the flightdepth decreases from deep(at the end of the feed section) to shallow (at the beginning of the meter section). This area is also referred to as the compression section.

184 two-stage screw - a screw that is typically used in a vented application, consisting of a feed, transition and meter section (all of which are shortened) followed by a decompression section (nearly as deep as the feedsection). The decompression section is followed by a second, short transition sectionand a finai meter section. Usually two-stage screws are longer than standard screws with L/D ratios of 24:l to 32:l. ultrasonic welding - amethod bywhichmoldedplastic parts or components are joined together through the application of a vibrating mechanical pressure at ultrasonic frequencies (20 to 40

m). vent (in a mold) - shallow channels or small holes cut into the cavity to allow airto escape as the melted plastic is injected.

-

vented processing the use of a barrel with a vent port and a two-stage screw to accomplish the removal of volatiles (gases and water vapor) fromthe material being processed. vent bleed - the unplanned escapeof melt through the vent port (vent hole) during the operation of vented barrel processing. vent port - an opening through the barrel wall, usually located just forward of the center of the barrel, to permit the removal of air,gases, water vapor and volatilesfiom the material being processed. vent stack (or chute) - a device surrounding a portion of the vent port designed to preventanymeltedplasticthatmightescape fiom collecting on the barrel, heater bandsor wiring. virgin material- plastic feedstock that has not been processed previously in an injection molding operation.

185

window - a term, usually referred to as the “processing window,” that is used to describe the degree of latitude available to the molder in changingprocessingparameters and still producingacceptable molded parts.

186

17 Other Molding Notes What has been presentedin the first sixteen chapters has been fhirly basic information about injection molding and the elements of machine operation. There are someinterestingspecialmoldingprocessesthat should be mentioned so that you may be aware of their use. In addition there are some other items relatingto molding machineoperation that may prove helpfblas well.

17.1 Special Molding Processes In addition to all that has been presented in this book regarding injection molding,there are still other types of injection molding processes that shouldbementioned.Theseincludetwocolormolding, turret molding, gas assist molding, powder injection molding, intrusion molding and others. Most of theseprocessesareconsideredmoretechnical and, therefore, are not described in detail. However,a brief descriptionofthem may be helpfblif you should encounter them in your work.

17.1.1 Two Color Molding You have seen coffee mugs that have a white interior and colored exterior that have obviously been injection molded. Also, keyboards on phones andcomputersare in two colors, but it is clear that the numbers and letters have not been paintedon the keys. The answer is that two different colored plastics have been injected into the same cavity, at butthenot same time. There are several ways to achieve two color molding. The most commonly observed approach involves two injection units, each havinga different color feed stock. The mold, which might have two cavity inserts in the cavity plate, is equipped so that the entire cavity plate indexes180" allowing a second color fromthe second injection unit to inject over the first color. Let's illustrate:

1 87

The core and cavity sets for positions A and B form the white interior of the coffee mug, with that interior being retained the on core (and not ejected). The core plate of the mold indexes 180" and the cores with the molded interior now sit in slightly larger cavities B and A, allowing the second injection unit to inject the dark colored material the around smaller interior part that sits on the core. The molded interior actually becomes part of the core for the molded exterior. With thetwo color part now complete, the mold opens the andtwo colored partis ejected, andthe process begins again.It is obvious that the cavities for the darker exterior must shut off accurately to prevent the darker material from covering some portion of the interior may that not be desired. This illustration is quite simple but even the most complex two color molds allow the first colorto become either part of the core or form the cavity for the second injectionoperation. Step 2

Step 1 The interior color (white) IS injected into cavities A and B. The part stays on the cores as the plate indexes.

After rotation, the second darker color is injected over and around the light interior color, formingthe exterior of the cup.

1

After Step 1,the plate with core inserts indexes (rotates) 180 degrees so that the second color may be injected over the first.

Figure 38

188

7.1.2 Turret Molding Perhaps you will, atsome point in your molding career,be able to observe themoldingofpreformsusingtheturretmoldingprocess. Preforms look like a test tube with very thick walls. They are called preforms because they are later placed in a blow molding operation, heated and blown into a larger plastic bottle used to contain a variety of fluids, including your favorite cola or other soda drink.

Obviously, if more of the preforms canbe manufactured withina given time, a greater profit will accrue to themolder. One of thelargest manufacturers of injection molding systems developed one of the more fascinating applications of turret molding to mold the preforms. Instead of the core half of the mold being a plate with core inserts, it is a large four or six sided turret that hascores on each of itssides. After injection on one set of the cores, the turret indexes to the next position, leaving the prefor on the first set of cores to cool, and so on. The first set of preforms is ejected as the third index (assuming four sides) is completed and just before that set of cores is indexed into position to mold the next set of preforms. Each side of the four sided turret may have60 or more cores which fit into a set of cavities. Moving Platen

Block

Figure 39

189

17.1.3 Gas Assist Molding Gas assist molding is used where there is a need to reduce the amount of plastic material and related weight inthe part andor produce thick-walled parts without sink marks and air bubbles in an economical manner.Simply stated, a gas injection unit is attached to the molding machine which contains nitrogen.The nitrogen is injected at a pressureof approximately 5,800 psi (400 bar) through the machine nozzle, or a gas injection needle can be used to inject the gas directly intothe cavity [14].

If thenitrogen is introduced through the nozzle, the cavity is first partially filled with injected melt followed by the introductionof gas, which fills out the part against the cavity wall with the gas in the center of the completed part.

This process is expensive because of the added components and becomes complicated during part and mold design. However, in some cases, the savings in the overall molding process far outweighthe added costs of design and equipment.

17.1.4Powder

Injection Molding

Another fascinating developmentin injection molding involvesthe addition of a large quantity of ceramic powder or metal powder to the plastic to produce parts that otherwise would have to be made from metal or other material. Quite often these parts are complex in design, and if made fiom metal, would require several machining operations. Powder injection molding is essentially a three step process, partof which is not dissimilarto the metal sintering process that has been used for a number of years. First the metal or ceramic powder is added to the plastic, quite often a polyolefin such as high density polyethylene (HDPE), so that the plastic is 20-30% by volume and the powder 70-80%. Waxes are usually addedto the mixture as well. The HDPE is simply a carrieror

190 a binder and allows the flow of the combination of materials to be injected into a cavity. The design of the part may involve core pulls and slides to assist in the formation of parts with complex designs. M e r the injection molding process is complete,the parts are placed into an ovenat a temperature of approximately 915-95OoF (480-5 10OC). At this temperature,the plastic is removed from the part, leaving it in the approximate shape desired but in very fiagile condition. The third step is the sintering process where the parts are placed into an oven set at rather high temperatures, such as 2,200°F (1,200”C) for metals, where the part is completed and now in usable form. Because of the three steps, there is a significant shrinkage fiom the original injection molded partto the completed sinteredpart, often as much as 2530%. The shrinkage is simply calculated and allowances made the during design of the completed part andthe mold.

Step 1

Step 2 I

Injection mold the part with powdered material added to polyolefin

Place the partinto an oven at about 93OOF to plastic carrier

l

Step 3 Transfer the partto an oven at about 2,20OoFto complete remove the the sintering process

Powder Injection Molding Process

The authors have seen parts made from powdered metal that had perfectly squarecorners and various holes and ribs in the parts. Suchparts would have otherwise had to be machined fiom a metal block, requiring several steps and considerable time.We have also viewed ceramic parts to be used as insulators in electrical applications that were producedin the powder injection molding process. There is a drawback to the process, however, the molding ofthese materials simply“eats up” barrels and screws and other parts of the injection unit, plusthe mold cavities and cores. All must be made fiom special, wear resistant materials.

191

17.1.5IntrusionMolding There are cases where the molder desiresto make a part thathas a greater volume of plastic than the maximum capacity of the machine’s injection unit. In such instances, the moldera combination uses of injection and extrusion to completely fill the cavity with the desired volume of plastic. This process is called intrusion molding. Thefirst step involves the injection of melt into the cavity by causing the screw to rotate while in its screw back position. A sufficient amount of plastic is forced into the cavity in this manner, much like extrusion, so that a full shot would complete the necessary volumeto fill the cavity. The secondstep is the conventional injectionof a full shot into the cavity, adding to the material that is already there, resulting intotal the volume desired. Although ithas been described intwo steps, the sequence of these steps is relatively continuous, causing a steady filling of the cavity. can only be It should be noted, however, that this intrusion process used where therotate pressureof the screw is sufficientto cause melt to flow into the cavity. In addition, not all materials are of a viscosity that would permit the processto be successful. Although the process is not used extensively, it does afford the molder the opportunity to occasionally mold parts that would otherwise require a largerinjectionunitandperhaps a largerinjectionmolding machine.

17.1.6 Other Molding Processes There are other special molding processes thatare interesting but are onlybrieflymentionedhere.Theyinclude insert molding, where components, suchas metal pieces,are placed into the mold and are molded into the part. You have seen knives with plastic handles and other similar metdplastic combination parts that are molded in this fashion.

192

In the chapter on additives,no mention was made of foaming or blowing agents. This agent is usually added to the material by the raw material supplier and, during injection, causes gas bubbles to form causing the thermoplastic materialto become a foam Thisfoam molding process produces parts with thicker walls that are light in weight with greater stifhess. Although the surface of the parthas a smooth appearance,it may not be of the same qualityas a conventionally molded part. The materials that are commonly used in foam molding include polystyrene (PS) and polypropylene (PP). However, most other thermoplastic materials canbe used in this process. Some larger parts that are used in the automotive andelectronics industries are producedusing the reactwn injectionmolding (MW process. RIMinvolvesthemixing of two or morereactive liquid components and injecting the resulting mixture intoa closed mold at low pressure. The most commonly used plastic is polyurethane (PUR). Parts weighmg more than10 pounds can be produced by this process and can be thin-walled becauseof the lower processing viscosities or thick-walled due to the uniform curingof the mixed components throughout the part. This process is fairly complicated and depends on the molder’s knowledge of the mold design required by this process. chemistry of the components and the One final process that is being utilized extensively is decoration molding. This process combines injection molding and hot-film printing. During this process, film that has been placed on the parting line of the mold separates fiom its supporting material and is essentially molded or bonded to the part. This process is good for attaching labels or other printed materialto the molded partin a single operation.

17.2 Molding Operation Items The Molders Divisionof The Societyof the Plastics Industryhas produced anexcellent bookletthat is recommended for distributionto all personsinvolved inmolding operations. It is called the “Operator’s

193 Handbook for Plastic Injection Molding.” The booklet contains information about the moldingprocess,moldingsafety,plasticmaterials,molding machines and molds. It also includes some information on quality control. The booklet is available fiom SPI at a nominal cost. From timeto time, youmay be required to assist in the removal and replacement of a screwor valve, whichwil require the removal of the end cap (nozzle adaptor) fiom the end of the barrel. This is hot work and requires the use of long insulated gloves (assuming thatthe machine has been running priorto the change). One of the most important steps in this process isthe reattaching of the end cap to the barrel. This process,ifnot done correctly, can cause damage to the end of the barrel andor an improper fit of the end cap to the barrel, causing potential additional damage. The tightening of the bolts that attach the end cap to the barrel should be done with a torque wrench and follow the process specifically outlined in Appendix F of thisbook. Become acquaintedwiththis procedure and usethe guidelines presented.

194

APPENDIX A

INJECTION MOLDING MATERIALS THERMOPLASTICS GROUP

Density gm/&

Technical (Long) Name

Temp OF

Solid

I

Melt

AcrylCNlitl-ik3BLftadiityR3M3

ABS

Amorphous

228 Tg

Acetals: PolyoJcymethylene

POM

Crystalline

358 Tm

1.17 1.42

Acrylics: Polymethyl Methacrylate

PMMA

Amorphous

203 Tg

1.05 1.20

Cellulose Acetate

CA

Crystalline*

446Tm

1.14 1.22

Cellulose Acetate Butyrate

CAB

Crystalline'

284 Tm

1.08 1.15

CelluloseAcetate Propionate

CAP

crystalline'

374Tm

1.10 1.17

Peduoro Ethylene Propylene

FEP

Crystalline

527 Tm

1.49 2.12

PerfluoroAlkoxyAlkane

PFA

Crystalline

582Tm

Unk 2.15

IOnanerS

"_

Crystdline

205Tm

.Q3

.73

Ketones: P-etone

PEEK

Crystalline

633 Tm

1.30

Unk

I cellulosi:

.Q7 1.08

I

I

I

FlUWopdymers:

195 APPENDIX A THERMOPLASTICS GROUP Technical (Long)Name

. SYmd

Molecular Type

Critical

Densltygmlccs

Sdid

OF

Mdt

polyethylenes: Linear LW Denslty

LLDPE

Crystalline

250Tm

.70 .93

L W Density

LDPE

Crystalline

221 Tm

.76 .92

High Denslty

HDPE

Crystalline

278 Tm

.73 .95

PdyphenyleneOxide(m0dii)

PP0

Amorphous

235 Tg

Polyphenylene Sulphide

PPS

Crystalline

550 Tm

pdypropylene

PP

Crystalline

348Tm

HIPS

Amorphous

PSU

Amorphous

1.08

.90

Unk 1.07 .75

.90

polystyrenes: ~~

High Impact 1.04 210 Tg Polysulfone

~~

.96

1.16 1.24

374Tg

~~

Vinyls: Poryvlnyl Chloride (Rigid)

PVC-R

Arorphous

1.40 188 1.22 Tg

Polyvinyl Chloride(Flexible)

WC-F

Amorphous

1.30 194 Tg 1.20

Rigid ChlorinatedW C

CWC

Amorphous

230 Tg

1.54

Unk

SAN

Amorphous

300 Tg

1.07

1.00

Styrene Acrylonitrile

Processes like an amorphous material.

Also may be processed as an amorphousmaterial.

196 PROPERTIES OF COMMON PLASTICS PRINCIPAL PROPERTIES

MATERIAL

APPENDIX B COMMON USES

TYPE

I

opaque)?'ha\Rlow shrinbge dunng moldingand typ<y have high k pact&en& they have poor chemical reslstance and poor lubrlclty. ~

Hmwver,

~

Good impadstrengul,rigidity, heat stability, high gloss and resistance to aging.

Housings for appliances,electronic, and computer equipment. Autanotive grills, instrument clusters8 body panels. Also luggage.

Very rigid with good impact

Food packaging and consumer disposables. Appliances, electronic housings and toys

atkwtemperatures. canbe used with foods. ~~

~~~

Outdoor lighting, glazing,lenses tool 8 Very tough 8 dimensionally appliance housings and aircraft interiors. staMe OVBT a wide temperature range. ~ l e n t o p t ~ c l a r i t y 8Trim, bezeis, taillight lenses 8 lamp good electrical propertes. housings for automobiles. PE1

(Pdyetherimide)

High strength 8 rigidity at elevated temperatures. very

Commercial aircraft interiorcomponents. Gears, bearings 8 piston rings. Autcmc$iive, electric 8 electronic parts. O f f i i equipment components.

stable. Heat, flame 8 chemical resistant. Goodelectrical properties. Hard, hQh gloss,tigii 8 the bast transparency 8 optical ytili 8 properties. Good Wedher& canusewithfood.

Lenses, outdoor 8 indoor lighting, taillights, instrumentpanels, dials and signal lights. Bewage 8 food dispensers and false teeth.

PP0

Madifii PP0 is most common. Excellent mechanical 8 dielectric pqmtesdwidetemprange. water resistant.

Pumps, valve handles, shaverheads, filters, d c 8 appliance housings. Internal TV parts 8 switch housings.

PS

Good rigidity 8 strength with excellent cbnty 8 unlimited cola range. Good electrical pmperks8canbeusedwith food.

Food containers, packaging8 housewares. Disposable dishes 8 cups. used for tays,c o m b s , closures. tape cassettes 8 furniture. ~

Commonlyadruded but some parts are injection mdded due to its resistancetoweethering, chemicals 8 impact. Excellentweetherability,

eledrical

prqxrka 8 chemical and impact resistance. Good surface alQ=mxe.

Handles, seats and accessories that are expcsed to weather or chemical environments. Also used where flame retardanceis required. Pipefittings, housings 8 chemii containers. Handle grips, vacuum cleaner parts 8 beach shoes.

197 PROPERTIES OF COMMON PLASTICS MATERIAL TYPE

PRINCIPAL PROPERTIES

APPENDIX B COMMON USES

SAN

Similar pmperhes to PS with increased chemical resistance (esp food) 8 higher mechanical 8themlalplqXrks.

Used for cups, tumblers, trays, picnic ware, cosmetics and packagingitems. Also used formedical. automotive8 appliances.

PSU

Outstanding tensile strength at high temperatures. Good high temperature creep resistance 8 resists acids.

A u t parts, m p u t e r parts, househdd appliances, coffee rdcers, hgh intenslty lamps, shower heads 8 hot drink dimers.

Crystalline: Crystalline materials are typically opaque (rather than clear) and incur high shrinkagein the molding process. These materials exhibit good chemical resistance and lubricity but low ImDact strewth. Cellulosics am vwy tough & rarely bred(. Good color 8 transparency with high gloss. CAB is weather resistant.

CA 8 CAP are used foreyeglass frames, pen barrels, tod handles, toys 8 ornaments. CAB is used foroutdoor 8 electrical items.

HDPE

POl@hylsnewithdensiabove .Wprowding stiffness, toughness. stresscrack resistance 8 kiw temp properties.

Usedforalltypesdmokledparts, including c o n t a i n e r s , c o v e r s , housewares. toys, furniture, disposable medical items 8 closures.

LCP

Liquid crystalpolymer is very tough with excellent electrical properties, is chemically inert8 has high temp resistance.

Used for high temperature, long wear parts, especially in the electricaVelectronicfM and automotim

LDPE

LW denslty polyethylene hasIW temp toughws, good stresscrack & tear resistancewith good

Used for toys,lids, contaim and many dthesmetypesoTitemsmadefrom HDPE.

CA, CAB (L CAP

(Cellulosics)

stiness. ~

PA (Nylon)

PBT

Nylonisvwytcugh8ableto withstand repeated impacts. It has excellent abrasion resistance &goodlubricity. Nylonis Chemically resistantwith good high temperature resistance.

PBT has highstability, good lubficii 8 surface gloss. It also has good chemical resistance8 electrical-.

Gears. bearings, housings 8 batterycasesaremadefrannylon. cams,

Football face guards,gaskets and handles for high temp appliances are ouler applications. PBT is used in electricaVelectronic applications8 is m m o nto many automotive applications because of its chemical resistance.

198 PROPERTIES OF COMMON PLASTICS MATERIAL TYPE

PRINCIPAL PROPERTIES

APPENDIX B COMMON USES

PET

PET hasgreat strength and senriceebiliity athigher temp. PET has dimensional stability, chemicd 8 hea resismce.

Used in manyelectricd, autamoti\re, appliance and industrial applications. Also commonly used for bdtles 8 containers.

POM

Acetds haw?the highest fatigue resistance d any plastic with great impadresistance. Good temperature, chemical8 cmep

Gears, leuws, housings, bearings 8 other industrial uses. Seat belt buckles, dashboards 8 clips are among the autanative uses. Toys, pen & pencil housings, food & water contad are other applications.

1-(

resistance.

PP

PP has very high tensile strength Containers, housewares, hinged applications, &hospital usesare & good fatigue, abrasion & c h e m i i resistance. PP is a good eledrical insulator.

a m m o n , Used for closures, disposables

8 a u t a n d i i applications.

199 RECOMMENDED PLASTIC DRYING DATA

APPENDIX C

[l31

Drying Parameters Plastrc Symbol

H Y g ~ ~ P l c

Hours

TemperatureOF

ABS ."

YeS

24

190-200

CA

YeS

2-3

160-180

CAB

YeS

2-3

160-180

CAP

YeS

2-3

160-180

HDPE

No*

160-180 1 -2

IOnOITW

YeS

150-160 7-8

YeS

LCP ~~

3-4

300-310

~~~

160-180 1-2

LDPE

No*

PA6.6/6,6/12 (Nylon)

YeS

PBT

YeS

PC

YeS

3-4

25G270

PEEK

YeS

3-4

PE1

YeS

6-7

300320 300-310

PES

YeS

3-4

300-320

PET

YeS

PSU

YeS

4-5

250-260

PUR

YeS

2-3

180-200

160-180

5-6

250-270 2-3

2

250-270 2-4

PP

No*

1-2

17G190

PP0

YeS

2-4

m250

PPS

YeS

3-4

280-290 ~

~

~~

PS

No*

1-2

180-190

PVC (Mble)

No*

1 -2

160-180

PVC (Iigii)

No'

1 -2

160-180

SAN

YeS

3-4

180-190

* Although notclassifled a8 hygroscopic,these materialsdo require drying.

200

APPENDIX D

Useful Data Abbreviations andSymbok: ratio D = Nominal Diameter Dmin = Minimum diameter Ffd = Feed flight depth Ffl = Feed flight length H< = Helix angle hp = Horsepower hpmax = Maximum HP ID = Inside Diameter OD = Outside Diameter CR

= Compression

lengthof screw L/D = Length to Diameter Ratio Mfd = Meter flight depth Oz/sec = Ounces per second P/hr = Pounds per hour psi = Pounds per square inch RPM = Revolutions per minute RTF = Residence time factor T = Torque in foot pounds fl

= Flighted

LRPM = Lowest RPM screw can deliver at full HP RC = Rockwell hardnessC scale RSC = Rated shot capacity of an injection molding machine Sg = Specific gravity or solid density (g/cm3) Sgm = Specific gravityor melt density (g/cm3) SI = Screw inventory of Dlastic when hll -

cm = centimeter ft = foot g=gram

in =inch J = Joule kg = kilogram

A

kW = kilowatt k w h = kilowatt hour Ib =pound m = meter mm = millimeter Mpa = megapascal

Nm = newton-meter oz = ounce Pa = pascal W = watt W-S = watt second yd =yard

201 APPENDIX D

Calculations: L/D Ratio = fl of screw

+ OD

of screw

Compression Ratio = Ffd + Mfd Shot capacity of an injection molding machine in ounces = DZx .7854 x injection stroke in inches x Sgm x .5778 Stroke (in inches) requiredfor adesired shot size = [Shot weight in ounces + (.4538x DZx SW)] x 1.03 Screw Inventory (in oz) = SI = (RSC+ 1.05 X RTF) X (L/D + 20) X Sg Bore Diameter 30mm & less 3 1 m-4 9 m ~OXIXII-69mm 7Omm-79mm

m

Bore Diameter 1.80 80mm - 89mm 1.65 901"n - 1041nm 1.45 ~ O ~ X I X I1091nm I1.30 llOmm&greater

g"J

1.25 1.20 1.15 1.10

Residence Time (min)= SI + Shot size (oz) x Cycle time + 60

I Lead (of screw)

= (pi x

D)

x

Tan H