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Practical_Injection_Molding/0824705297/files/00001___81021f2e5e12fc1247be40df80d5963d.pdfPRACTICAL INJECTION MOLDING
Practical_Injection_Molding/0824705297/files/00002___b3a91441849f4244794321df05eeb82f.pdfPLASTICS 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, Robert Bums 3. Carbon Black-Polymer Composites: The Physics of Electrically Conducting
4. The Strength and Stiffness of Polymers, edited by Anagnostis E. Zachariades
5. Selecting Thermoplastics for Engineering Applications, Charles P. Mac-
6. Engineering with Rigid PVC: Processability and Applications, edited by l . Luis
7. Computer-Aided Design of Polymers and Composites, D. H. Kaelble 8. Engineering Thermoplastics: Properties and Applications, edited by James
9. Structural Foam: A Purchasing and Design Guide, Bruce C. Wendle
Composites, edited by Enid Keil Sichel
and Roger S. Porter
Demott
Gomez
M. Margolis
IO. Plastics in Architecture: A Guide to Acrylic and Polycarbonate, Ralph 11. Metal-Filled Polymers: Properties and Applications, edited by Swapan K. 12. Plastics Technology Handbook, Manas Chanda and Salil K. Roy 13. Reaction Injection Molding Machinery and Processes, F. Melvin Sweeney 14. Practical Thermoforming: Principles and Applications, John Florian 15. Injection and Compression Molding Fundamentals, edited by Avraam l . 16. Polymer Mixing and Extrusion Technology, Nicholas P. Chefernisinoff 17. High Modulus Polymers: Approaches to Design and Development, edited by
18. Corrosion-Resistant Plastic Composites in Chemical Plant Design, John H. 19. Handbook of Elastomers: New Developments and Technology, edited by Ani/
20. Rubber Compounding: Principles, Materials, and Techniques, Fred W. 21. Thermoplastic Polymer Additives: Theory and Practice, edited by John T.
22. Emulsion Polymer Technology, Robert D. Athey, Jr. 23. Mixing in Polymer Processing, edited by Chris Rauwendaal
Montella
Bhattachatya
lsayev
Anagnostis E. Zachariades and Roger S. Porter
Mallinson
K. Bhowrnick and Howard L. Stephens
Barlow
Lutz, Jr.
Practical_Injection_Molding/0824705297/files/00003___cb53d1b7e0d097b697d8cd425f9fc995.pdf24. Handbook of Polymer Synthesis, Parts A and B, edited by Hans R.
25. Computational Modeling of Polymers, edited by Jozef Bicerano 26. Plastics Technology Handbook: Second Edition, Revised and Expanded,
27. Prediction of Polymer Properties, Jozef Bicerano 28. Ferroelectric Polymers: Chemistry, Physics, and Applications, edited by Hari
Singh Nalwa 29. Degradable Polymers, Recycling, and Plastics Waste Management, 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 Paul N. 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
35. Cationic Polymerizations: Mechanisms, Synthesis, and Applications, edited
36. Polyimides: Fundamentals and Applications, edited by Malay K. Ghosh and
37. Thermoplastic Melt Rheology and Processing, A. V. Shenoy and D. R. Saini 38. Prediction of Polymer Properties: Second Edition, Revised and Expanded,
39. Practical Thermoforming: Principles and Applications, Second Edition,
40. Macromolecular Design of Polymeric Materials, edited by Koichi Hatada,
41. Handbook of Thermoplastics, edited by Olagoke Olabisi 42. Selecting Thermoplastics for Engineering Applications: Second Edition,
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 and Yusuf 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
Kricheldorf
Manas Chanda and Salil K. Roy
and Roderic P. Quirk
by Krzysztof Matyjaszewski
K. L. Mittal
Jozef Bicerano
Revised and Expanded, John Florian
Tatsuki Kitayama, and Otto Vogl
50. Handbook of Polymer Testing: Physical Methods, edited by Roger Brown 51. Handbook of Polypropylene and Polypropylene Composites, edited by Har-
utun G. Karian
Practical_Injection_Molding/0824705297/files/00004___fb3cff34e0bd66e4059a3a9de2af4b14.pdf52. Polymer Blends and Alloys, edited by Gabriel 0. Shonaike and George P.
53. Star and Hyperbranched Polymers, edited by Munmaya K. Mishra and Shi-
54. Practical Extrusion Blow Molding, edited by Samuel L. Belcher 55. Polymer Viscoelasticity: Stress and Strain in Practice, Evaristo Riande,
Ricardo Diaz-Calleja, Margarita G. Prolongo, Rosa M. Masegosa, and Cat- alina 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
Simon
ro Kobayashi
Additional Volumes in Preparation
Practical_Injection_Molding/0824705297/files/00005___eaa86cb421fb39bcfb8a224c5d1807f7.pdfPRACTICAL INJECTION MOLDING
Bernie A. Olmsted Consultant Springfield, Massachusetts
Martin E. Dauis Consultant Prescott, Arizona
m M A R C E L MARCEL DEKKER, INC. D E K K E R
NEW YORK - BASEL
Practical_Injection_Molding/0824705297/files/00006___302e081f172fb6f739265babc60b11bc.pdfISBN: 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
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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 0 2001 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, elec- tronlc or mechanlcal, including photocopying, microfilming, and recording, or by any mformatlon storage and retrieval system, wlthout permlsslon In writmg from the publisher.
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Practical_Injection_Molding/0824705297/files/00007___eb384336047b39126ddbcad983f873c9.pdfForeword
The Society of Plastics Engineers is pleased to sponsor Practical Injection Molding by Bernie A. Olmsted and Martin E. Davis.
Practical Injection Molding provides a fundamental, 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 recruitment of authors to peer review and approval and publication of new books.
Technical competence pervades all SPE 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
111 ...
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Practical_Injection_Molding/0824705297/files/00009___86025aeb9bc46342d56c83872327b8aa.pdfWhy 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 better job of molding plastic parts and, equally important, enjoying the success of their improved accomplishments.
The mechanics, operators and set-up personnel in molding 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 their job. Most have learned fiom their predecessors who probably had even fewer educational opportunities. Product quality and output may improve dramatically through the education of this group of personnel and . . . . . there may even be increased enlightenment on the part of their supervisors and managers.
In addition to those people involved actively in the injection molding process, there are others who could also benefit fiom the book. Sales personnel responsible for providing the plastics materials, molding machinery and auxiliary equipment may find this book beneficial in rounding out their knowledge of 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 addition to 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 plastic is melted and the role played by the screw, the barrel and the machines temperature controls. The mold, with cavities and cores that form the shape of the finished product, is a critical element in the
V
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molding of a part that is free from defects and achieves dimensional integrity. Most injection molding machines manufactured today utilize sophisticated electronic controls that enable the successful molding of a product with a minimum of manual intervention. The operators 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 some of the chapters. We want this book to be 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
Practical_Injection_Molding/0824705297/files/00011___9422a8c84cb2fa4703f91ea219558834.pdfAcknowledgments
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 to Martin Davis of Westland Corporation for his interest, encouragement and for his willingness to co-author the book with me.
Bernie A. Olmsted
I have known Bernie for a number of years during which he has educated me in the practical application of technical knowledge to the fundamentals of injection molding. His depth of experience and broad knowledge of the subject have been helpful to me personally and to the company I founded, Westland Corporation. I am pleased to contribute to 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 basics of 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 at Westland Corporation, especially Wayne Hook, helped with the illustrations, for which we are most grateful. A special thanks to John W. Bozzelli of Injection Molding Solutions for his detailed review. His efforts have contributed significantly to the technical validity of the book. Most important, I must thank my wife, Vicki, who has patiently helped me participate in this effort.
Martin E. Davis
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Practical_Injection_Molding/0824705297/files/00013___ca5b46ab42883a82e228ffcad6c6afe2.pdfContents
1 Introduction ........................................................................................... 1 1.1 Elements of the Injection Molding Process ............................ 1 1.2 Summarizing the Elements ..................................................... 6
2.1 Thermoplastics ........................................................................ 8
2.3 Crystalline vs . Amorphous Materials .................................... 10 2.4 How Plastics Affect the Molding Process ............................. 11
2.4.1 Melting Characteristics .......................................... 11 2.4.2 Thermal Conductivity ............................................ 12 2.4.3 Shear Sensitivity .................................................... 13 2.4.4 Viscosity (Melt Index) ........................................... 14
3 Additives ............................................................................................. 16 3.1 Fillers and Reinforcements ................................................... 16 3.2 Plasticizers ............................................................................ 17 3.3 Stabilizers .............................................................................. 17 3.4 Flame Retardants .................................................................. 18 3.5 Colorants ............................................................................... 1 8 3.6 Adding the Additives ............................................................ 18
4 Loaders and Dryers ............................................................................. 21 4.1 Hopper Loaders and Conveying Systems ............................. 21 4.2 Dryers .................................................................................... 22
2 Plastics .................................................................................................. 8
2.2 Thermosets .............................................................................. 9
4.2.1 Hot Air Dryers ....................................................... 22 4.2.2 Dessicant Dryers .................................................... 23
5 hjection Unit ...................................................................................... 26 5.1 The Barrel ............................................................................. 29
5.3 Heater Bands ......................................................................... 33 5.4 Non-Return Valve ................................................................. 34 5.5 Screw ..................................................................................... 38
5.5. I Length-to-Diameter Ratio ...................................... 39
5.2 End Cap and Nozzle .............................................................. 30
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5.5.2 Screw Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.5.3 Compression Ratio . . . . . . . . . . . . . . . . . 40
5.5.5 Drive Design . . . . . . . . . . . . . . . . . . . . . . 41 5.6 Injection Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.1 Hydraulic Clamp System . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.3 Clamp Unit Specifications . . . . . . . . . . . . . . . . . . . . . . . . . 50
7 Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5 S.4 Helix Angle . . . . . . . . . . . . . . . . . . . . . . . 41
6 Clamp Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.2 Hydro-Mechanical System . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.4 Ejector System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
7.1 Mold Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
9 Robotics and Granulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 9.1 Robotics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 9.2 Granulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
10.1 Mold Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 10.2 Process Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 10.3 Process Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 10.4 Mold Start-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 10.5 Process Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . 95
11 An Overview - The Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 11 . l The Cycle - Defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 11.2 The Importance of Cycle Time . . . . . . . . . . . . . . . . . . . . 98
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Getting Started 86
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l 1.3 The Greater Importance of Good Production . . . . . . . 100
12 The Ten Keys to Successful Molding . . . . . . . . . . . . . . . . . . . . . . 102 12.1 Adequate Mold Venting . . . . . . . . . . . . . . . . . . . . . . . . . 102 12.2 Proper Mold Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 12.3 Using the Right Screw . . . . . . . . . . . . . . . . . . . . . . . . . . 106 12.4 Selecting the Appropriate Valve . . . . . . . . . . . . . . . . . . 111 12.5 Controlling the Heat Profile . . . . . . . . . . . . . . . . . . . . . . 113 12.6 Using Back Pressure Wisely . . . . . . . . . . . . . . . . . . . . . 119 12.7 Controlling the Injection Rate . . . . . . . . . . . . . . . . . . . . 120 12.8 Managing Screw RPM and Residence Time . . . . . . . . . 124 12.9 Performance Measurement . . . . . . . . . . . . . . . . . . . . . . 128 12.10 Preventive Maintenance Program . . . . . . . . . . . . . . . . 130
13 Thermoset Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 13.1 Thermoset Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 13.2 Machine Modifications . . . . . . . . . . . . . . . . . . . . . . . . . 134 13.3 Processing Modifications . . . . . . . . . . . . . . . . . . . . . . . . 135
14Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 14.1 Safety Requirements of ANSI B 15 1.1 . . . . . . . . . . . . . . 137 14.2 Safety Rules to Follow . . . . . . . . . . . . . . . . . . . . . . . . . . 140
15 Recognizing Molding Problems . . . . . . . . . . . . . . . . . . . . . . . . . . 143 15.1 Process Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
15.1 .l Brittleness of Parts . . . . . . . . . . . . . . . . . . . . . 143 15.1.2 Bubbles and Voids . . . . . . . . . . . . . . . . . . . . . 144 15.1.3 Burned Material . . . . . . . . . . . . . . . . . . . . . . . 145 15.1.4 Cloudy or Hazy Parts . . . . . . . . . . . . . . . . . . . 146 15.1.5 Drool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 15.1.6 Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 15.1.7 Flow Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 15.1.8 Gate Blush . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 15.1.9 Inadequate Color Mixing . . . . . . . . . . . . . . . . 150 15.1.10 Jetting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 15.1.1 1 Knit Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 15.1.12 Part Deformation . . . . . . . . . . . . . . . . . . . . . . 153 15.1.13 Poor Screw Recovery . . . . . . . . . . . . . . . . . . 154 15.1 . 14 Short Shots . . . . . . . . . . . . . . . . . . . . . . . . . . 155
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15.1.15 Sink Marks . . . . . . . . . . . . . . . . . . . . . . . . . . 156 15.1.16 Splay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
15.1.17 Warped Parts . . . . . . . . . . . . . . . . . . . . . . . . . 158 15.2 Component Wear Problems . . . . . . . . . . . . . . . . . . . . . . 159
16Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 17 Other Molding Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
17.1 Special Molding Processes . . . . . . . . . . . . . . . . . . . . . . 186 17.1.1 Two Color Molding . . . . . . . . . . . . . . . . . . . . 186
17.1.3 Gas Assist Molding . . . . . . . . . . . . . . . . . . . . 189 17.1.4 Powder Injection Molding . . . . . . . . . . . . . . . 189
17.1.2 Turret Molding . . . . . . . . . . . . . . . . . . . . . . . . 188
17.1.5 Intrusion Molding . . . . . . . . . . . . . . . . . . . . . . 191 17.1.6 Other Molding Processes . . . . . . . . . . . . . . . . 191
17.2 Molding Operation Items . . . . . . . . . . . . . . . . . . . . . . . . 192
Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 A Injection Molding Materials . . . . . . . . . . . . . . . . . . . . . . . 194 B Properties of Common Plastics . . . . . . . . . . . . . . . . . . . . . 196 C Recommended Plastic Drying Data . . . . . . . . . . . . . . . . . . 199 D Useful Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
Abbreviations and Symbols . . . . . . . . . . . . . . . . . . . . 200 Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
E Velocity Control on Injection Molding Machines . . . . . . . 203
Nozzle Adaptors (End Caps) . . . . . . . . . . . . . . . . . . 209 F Procedure for Application of Bolt Torque on
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
Practical_Injection_Molding/0824705297/files/00017___c1e70dccb66f27c4617d105f9a69c62c.pdf1 Introduction This book is intended to provide the hands on injection molding
personnel, the machine operators, technicians and mechanics, with an improved understanding of the basics of injection molding. Although injection molding has been in use in the United States since the 1930s, the operating personnel have generally learned the process from their supervisors, who either learned from their predecessors or gained the knowledge by trial and error. There have been few seminars, workshops or books that have offered educational opportunities to the operator with no college degree or formal training in plastics technology. This book will explore the elements of the molding process at the most basic level in the hope that it will contribute to the productivity and job satisfaction 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 involved in the injection molding process are illustrated in Figure 1.
- - Clamp Unit Mold
Injection Unit Plastic i I -
Controls Unlt
Temp Controller Grlnder
Figure l Injection molding elements.
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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 basics of how each element of this seemingly complex unit works and the role each plays in the successful molding of a part.
With the exception of robotics and additive feeders, 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 but is not essential to the successful molding of a part. Additive feeders are used to proportion the addition of colorants or other additives to the plastic pellets. While the feeder itself does not affect the process (if functioning properly), the quantity and type of additives may change some important processing parameters.
The injection molding machine itself consists of 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,
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HPM and various other U.S. and foreign manufacturers. The machine is illustrated in Figure 2. The diagram shows the clamp unit closed without a mold.
The injection unit heats, melts, pumps and injects the plastic into the mold when the mold is closed. The control unit monitors and, as the name implies, controls the functioning of the injection unit and the 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-numerically- controlled (CNC) milling machines that help automate the manufacture of complex molds. Usually these companies are small, employing less than 100 persons and require 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 fits, the area between the core 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 formation of 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
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In order to solidify the plastic part in the mold so that it can be removed (ejected), it is usually necessary to cool the mold. The cooling is 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 part of a temperature controller system that may serve several molds in several injection molding machines. Temperature controllers may take the form of chillers, as described, or in other cases, heating units (for thermosets), heat exchangers, and various types of water and oil temperature controlling devices.
The molders raw material, plastic, is usually purchased in pellet form. Each pellet is about the 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 will be discussed later, are bought or used in powder form today. Plastic is typically purchased from the manufacturer or a distributor and is delivered in 55 lb. bags (25 kg), in drums (about 200 lbs. or 100 kg), in 1,000 lb. or about 450 kg gaylords (big boxes) or in rail cars. Many types of plustic are available to use in the production of plastic products. Each type offers different mechanical properties in the molded form. Perhaps more important to 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 press is the function of the loader. A loader may be as simple as a vacuum powered unit with 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 that are nearby. A loader unit may be as complicated as a pneumatic material handling system that connects a large number of injection molding machines to storage silos (large, enclosed metal bins) and
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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, most plastics are hygroscopic (meaning the plastics are able to absorb and retain moisture). If the moisture is 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 types of dryers available today, but all involve the circulation of either hot air or dehumidified hot air through the pellets before they are allowed to enter the injection unit. Some dryers are quite small and fit on top of the press as a part of the hopper assembly and others are large enough to dry 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 dropped onto a conveyor (or removed by robot) for further 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 can be mixed 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 travel to 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.
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1.2 Summarizing the Elements
This introductory discussion of injection molding elements 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 basics of each element are pursued further. The table on the next page summarizes these elements for the readers review.
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I I I Element I Function I
I I Plastic
Transports plastic from storage to machine hopper Loader
Raw material used to mold parts
Removes moisture from plastic before entering the injection Dryer unit
I l Additives Colorants, lubricants or other ingredients added to the plastic pellets I ~
Additive Adds specified quantity of additive (colorant or other additive) Feeder
Part of the injection molding machlne that heats, melts and Injection Unit
to the plastic being fed to injection unit
~~~~
injects the plastic into the mold
Part of the injection molding machine that contains the mold and holds it closed during injection and opens it 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 mold to speed the solidification of the plastic part
Conveyor Transports finished plastic parts to areas for other operations or packaging and conveys the runners to a point where they may 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
Practical_Injection_Molding/0824705297/files/00024___576ef9aedada30bfb9e37bbff89aa3b4.pdf2 Plastics Although a technical discussion of 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 injection molding. Most injection molding is performed using thermoplastic material. However, some injection molding uses thermoset material. There is considerable difference between the two types.
2.1 Thermoplastics
The majority of 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 mixing it with virgin (unprocessed) material [l]. Depending upon how many times and under what 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 resulting parts are not critical.
Thermoplasticstypically have long proper names that relate to their basic chemistry type. The chemistry ofthermoplastics is rather complex and may be studied firther by reference to some excellent books [2]. Persons who are not acquainted with the chemistry of plastics or how plastics are manufactured can refer to them by their short name. All plastics have been given an alphabetic symbol that is a short name for each plastics longer technical name. Some of the more common plastics and their symbols include the following.
Practical_Injection_Molding/0824705297/files/00025___57db0787173fe08dce7d94c5251dc8b2.pdfLong Name Sym bo1
Polyethylene Polypropylene Polystyrene Acrylonitrile Butadiene Styrene Polyamide (nylon) Polycarbonate Polymethylmethacrylate (acrylic) Polyoxymethylene (acetal) Polyvinylchloride Styrene Acrylonitrile
PE PP PS
ABS PA PC
PMMA POM PVC S A N
Many of the long names begin with poly, which simply means many or multiple. The smallest repeating unit in the chemical structure of plastics (which is a molecule) is called a monomer. When several monomers are combined or joined, the resulting chemical structure is called apolymer. So it becomes clear that polyethylene, polypropylene and the others beginning with the letters poly are plastics consisting of a combination of monomers. A fiuther understanding of the molecular structure ofplastics is not required, but is available in several references [3].
A listing of 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 cannot be reprocessed or welded.
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The chemical change that occurs in thermosets is often referred to as curing or cross-linking [4]. Cured thermoset polymers cannot 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 (a coal 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 they are made and, as will be illustrated, in how they are processed.
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 long- chain molecules that are referred to as polymers. In the solid state, some of the polymer molecules are arranged in a very orderly, repetitive pattern and are called crystaffine. Others are structured in a very random arrangement without any order or regularly repeating pattern, and they are 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].
Crystalline
I
Amorphous
Figure 4 Illustrative drawing of polymer molecules
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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 plastics that are semicrystalline as being crystalline (as opposed to amorphous) materials.
The important thing to remember is that the physical 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 product. Although this book is not intended to include part design, we believe the molder should have some idea of the properties of the more common plastic materials, which are shown at the conclusion of the book as Appendix B. 2.4 How Plastics Affect the Molding Process
More important to the molder, however, is the fact that crystalline and amorphous materials react quite differently during the molding process. There are at least three vital differences in the way the two types of material respond to the melting and molding process.
2.4.1 Melting Characteristics
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 will 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 will see, this difference in the way the materials melt is an important factor in how the materials are molded.
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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 the plastic 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 S A N
I Y J E 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 influences the speed with which plastics can be heated, melted and molded. The second 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 heat also increases.
Stated another way, you cannot add more heat to amorphous materials and expect them to melt any faster! In fact, iftoo much heat is applied to amorphous materials, they will burn and degrade. Although not intended to represent any scale of values, Figure 5 illustrates the difference in the way the two types of material can absorb heat.
Practical_Injection_Molding/0824705297/files/00029___42914dbcd77fe6ae93a39d712194d953.pdf/ Crystalline
/ 0
0 I 0 morphous
Figure 5 Heat absorption characteristics of cytall ine 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 more sensitive to shear. Shear occurs when plastic pellets are compressed, rubbed together causing fiction or are signilicantly agitated during the molding process. High shear results in rapidly increasing the temperature ofthe material while being 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 them from a solid to a melt. Excessive melt temperatures in some 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 loss of mechanical properties (such as impact strength) cannot be determined until the part is subjected to impact tests or fails when performing in its intended use. In later chapters, you will learn how these three factors (melting characteristics, thermal conductivity and shear sensitivity) are controlled in the molding process. We must remember that all materials
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have a maximum limiting shear rate, beyond which they will degrade [ 7 ] .
2.4.4 Viscosity (Melt Index)
Another property of both crystalline and amorphous materials that affects the molding process is viscosity. Viscosity may be defined as the resistance of a fluid to flow. In other words, if a melted plastic is considered viscous, it is thick (like molasses) and will not flow easily. The viscosity of a melted plastic can be measured and given a rating called a Melt Index (MI). A high melt index means that the melted plastic is thin and watery (and has 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 index of plastics range from a fractional MI, meaning that it is less than one (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 MI or very low viscosity are more difficult to push or inject and, in some cases, more difficult to mold. Incidentally, it is 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 material from 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
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material into their bulk storage tank (silo). After processing with material from that silo for a few days, the quality of the melt changed and the same colors they had been achieving could not be 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 able to take a pellet of an amorphous material (such as acrylic) and a pellet of a crystalline material (such as nylon) and put them on a skillet that could be heated sufficiently to 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 and burn rather than changing more rapidly to a fluid state. It is likely that the nylon could survive the excessive heat (unless unreasonably high) and reach the molten state more quickly.
The lesson is: Amorphous materials are more likely than crystalline materials to degrade and burn if overheated during the "melting" process Avoid excessive heat when processing amorphous materials, regardless of the heat source.
Practical_Injection_Molding/0824705297/files/00032___59ba1e07d34fa9c09f1946fe88d86b3e.pdf3 Additives In addition to the raw plastic materials discussed in the last chapter,
there are a number of other ingredients that may be added to the plastic to mod% its properties. These other ingredients are referred to as additives and include plasticizem, fillers, reinforcements, stabilizem, flame retardants, colorants, lubricants and many others. 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 in the following pages.
3.1 Fillers and Reinforcements
Fillers and reinforcements are added to the plastic primarily to increase the stifkess of the 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 referred to as reinforcements [4]. Some of the more commonfillers include calcium carbonate (which is basically powdered limestone), talc (another powdered mineral that has a 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 and silica, in some forms, might be considered as reinforcements and can be quite abrasive to the metal surfaces in the injection unit and the mold. As one might guess, the addition of such substances to the plastic increases the melt viscosity of the material and, as we will learn, can sigmficantly affect the molding process and shrinkage.
Most of the reinforcements added to plastics take the form of small fibers, powders or flakes. The most common reinforcement is a glass 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 added to the plastic when it is formulated. Glass fiber reinforced nylon and polyester materials are quite common and are
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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 into contact. As ifthese are not severe enough, fibers are also made fiom 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 fiber forms, 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 used to reduce the stifbess of the plastic part, making it more flexible. In achieving the increased flexibility, plasticizers may also reduce the viscosity of the melt and hc t ion as a lubricant. Plasticizers are 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 will occur and may be associated with the release of hydrochloric acid. This may result in a loss of 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 with the plastic to help prevent degradation fiom excessive heat in processing. We will later learn the importance of the proper control of heat during the molding of all plastics, including PVC. It is also important that the stabilizers be compatible with the resins to which they are added to avoid chemical or viscosity problems in molding. Uvstabifizers are also important because they increase the molecular stability of plastics that are exposed to light. UV stabilizers help increase the weatherability ofplastics exposed to sunlight in outdoor environments.
Practical_Injection_Molding/0824705297/files/00034___cef1ccd6a9f52f8b25015e0555d1cb71.pdf3.4 Flame Retardants
Because so many of the products and equipment that people use in their everyday lives are now made of plastic, the degree to which each is flammable is very important. Major portions ofthe home, automobile, boats and airplanes are now constructed with plastics and these plastics must be as resistant to burning and smoke generation as possible. As a result,jlame retardants and smoke suppressants are added to many plastics to control the undesirable effects that can result fkom combustion. Flame retardants are added directly to the material when formulated allowing the molder to purchase flame retardant grades of material. Unfortunately, most of these additives are corrosive to the metal surfaces to which they are exposed, requiring special protective coatings to be used in the 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 pellet of colorant or as a liquid that is added to the plastic after it enters the injection unit. 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 and brilliance at minimum cost [8]. The technology supporting the coloring of plastic is somewhat complex and may be 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 additive feeders or blenders. There are different
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types of feeders and blenders used in the molding process. Some can mix and add only two different types of solid material while others 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 injection molding machine, in addition to the standard hopper. The more complex units are floor-mounted and use pneumatics to take the mixed material to the machine. It is not important for the reader to understand how these blenders and feeders 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 material to one reinforced with fiberglass fibers required that his screws needed to be manufactured fiom a more wear resistant material, have deeper flights and less compression. 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 black to the HDPE he was processing required a screw that more aggressively mixed and sheared the material or the resulting melt had windows or unmixed areas that would adversely affect the weatherability of the resulting product.
Yet another molder was adding titanium dioxide to his material to alter the color of his parts. TiOZ is a commonly used additive to achieve a very white coloration of the parts produced. However, in a very short time, the molder discovered severe wear in his barrel and screw. Research revealed that TiOZ can be very abrasive and requires that the injection unit
Practical_Injection_Molding/0824705297/files/00036___9aea95abfbd2720582947f3acae64fbb.pdfcomponents (screw, barrel, valve and end cap), and sometimes the runners and gates in the mold, be made from more wear resistant materials. In this case, the components were replaced with different, wear resistant materials and the wear previously experienced was no longer a problem.
Another molder was using silicone as an additive in his process. Silicone is known for its chemical and physiological inertness and has good physical and electrical properties that dont change significantly from very low to very high temperatures. Silicone is also water-repellant and anti- adhesive 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 embedded in the parts. After considerable research and experimentation, it was learned that silicone erodes certain flight hard- surfacing materials used on the flights of screws. The screws were rebuilt and chrome-plated completely over 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 and the 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 the moldingprocess 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!
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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. This is accomplished by one or more pieces of equipment categorized as dryers and loaders.
4.1 Hopper Loaders and Conveying Systems
As we learned in the 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 and cost effective solutions to transporting the plastic to the machine, whether in pellet, flake, granular or powder form. A vacuum conveying system consists of a power unit, a 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 upon the power unit to accomplish the task The pick-up devices are designed to allow a simple adjustment of the aidmaterial ratio at the pick-up point. 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 system has 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 construction are optional features available.
Practical_Injection_Molding/0824705297/files/00038___bd30006b4b1a849f9fcc39c88582913b.pdfAnother option is a portable combination dryedloader system that allows the drying of the material off-line and is moved into position when the dried material is 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 removed from 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 molecular bond with the material. Included among 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 corrosives at processing temperatures. The result is a premature corrosive wear on the surfaces of the injection unit components. This type of wear will be discussed in a later chapter.
Non-hygroscopic plastic materials do not absorb moisture, but the moisture in the air adheres to the surface of the pellets or flakes and can cause some of the same types of processing problems observed with 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 types of dryers used in 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 perform the drymg hct ion.
Practical_Injection_Molding/0824705297/files/00039___90326c4e9b3f91ea7a1f4fdf77d98eb6.pdfThe hot air dryer basically consists of heaters and an air blower and are typically mounted on top of the injection unit as a dryer hopper, replacing the standard hopper. Ambient air (air in the 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 moisture in the plastic and then moves it out of the hopper back into the room air [ 1 l].
4.2.2 Dessicant Dryers
The dessicant dryer utilizes small beads (referred to as dessicant) that can absorb a lot of moisture without undergoing any significant structural change. The dessicant dryers operate much like the hot air dryer, pulling the moist air fiom the plastic pellets in the drying hopper into the dryer through a filter, then through a layer of the dessicant beads (called the dessicant bed), which absorb the moisture, and finally to a heating unit where the dry air is brought up to a specified temperature. This dry air is then circulated through the plastic pellets in the drying hopper. The dry air becomes more moist as it leaves the plastic and the closed-loop process is repeated until the proper level of moisture allowed for the plastic is 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 (rid of moisture) and is ready for use again. A table designated as 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 four options 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 drying system [ 121. These units offer various controls, numbers of dessicant beds and other options that we will not address here. However, these features are important and should be carefblly studied when making a dryer selection.
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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 not all of similar size and weight, the vacuum will pull more of the lighter weight material ahead of the heavier. Whether the lighter weight material is an additive, regrind or simply fines, there will be an undesirable effect on the parts being molded. The same problem can happen in a hopper where the force is gravity. Clearly the heavier material that is closer to the center of the hopper will drop first, leaving dissimilar material around the 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 will 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 shearing of the material which generates steam as 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, there are an equal or greater number that have tried venting and have reverted back to drying the material before it is processed. The problems that can result with venting, if not carehlly monitored, include a partially or totally clogged vent port which allows undried melted plastic to proceed into the mold producing faulty parts. The question is, how much can the vent port be blocked before parts problems occur? Even if the vent port is cleaned religiously, it will partially block, fiom time to time, and the resulting parts are not made fiom the same degree of moisture-fiee plastic.
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When drying material, be sure to observe the suppliers range of the recommended percentage ofmoisture that should be allowed in the material. It is often forgotten, but material can be over-dried which can cause subsequent degradation and the inability of the material to flow properly in the melted condition.
The lesson is: If moisture contamination in the Pam must be avoided, drying of the plastic before molding is essential. Always dry to the suppliers recommendation and dont forget that any regrind must be added before the drying process is applied
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5 Injection Unit The injection unit of an injection molding machine (I") consists
of the elements shown in the schematic drawing below:
Barrel (also known as a cylinder) Screw Non-return Valve Nozzle End Cap Heater Bands Hydraulic or Electric Screw Drive Hopper
Non-retum Heater Vahre
I \ I I / I \ l I
Nonle End Cap Barrel Screw 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 molding of quality plastic parts will not occur. Not only does the injection unit have primary responsibility for the molding of good plastic parts, 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 contribute to this process.
The hopper holds the plastic pellets which are gravity-fed through the feed hole in the barrel. The screw has helical (spiral) flights which, when the screw is rotated, cause the plastic pellets to move forward in the
Practical_Injection_Molding/0824705297/files/00043___b6f43b69844e96bd22a6bf6eea211aed.pdf27 barrel. The barrel, which houses the screw, has heater bands surrounding it which heat the barrel and the 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 will call upon the controls to supply more heat. The temperatures called for and the actual temperatures, as measured by the thermocouples, may be seen on the machine control panel.
The screw also provides some of the heat to melt the plastic pellets, by squeezing and shearing the pellets against the screw flights and the barrel wall as the pellets move forward. This vital h c t i o n of the screw and how it is designed to perform this h c t i o n is covered in Section 5.5 of this chapter.
The screw drive is typically a hydraulic motor drive, although electric drive units are becoming quite common. The construction of the screw drive unit is beyond the scope of this book, however, it is simply a h c t i o n of converting hydraulic (or electric) power to mechanical power to 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 valve and the end cap. For reasons we will 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 refer to the screw as the "reciprocating screw."
The control on the injection molding machine can be set to allow the screw to move backward only a specified distance. The distance is referred to as stroke and is measured in inches or millimeters. (Ivote: All injection molding machines have a muximum stroke that is approximately equal to four (4) times the diameter of the bore of the barreL)
Practical_Injection_Molding/0824705297/files/00044___f72719231e097f9da42e78e8302b2d11.pdf28 When the specified distance is achieved, a signal is given and the
screw drive, injection cylinder and non-return valve perform their additional hctions. The screw drive stops 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 movement of the melt back into the 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 move forward 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 into the mold. The shot size is the quantity of melt injected into the mold, measured in ounces or grams.
The screw hydraulic system holds the 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 begins to rotate and the entire process is repeated. Thisprocess, beginning with the screw in the forward position, then reciprocating backward, pausing for the opening of the mold and ejection, and moving forward to inject the molten plastic of the next shot, is referred to as a cycle. The time required to complete one cycle is appropriately called cycle time. Cycle times can vary fiom a few seconds to several minutes, depending upon a variety of 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, causing a greater mixing and shearing motion inside the barrel. This forward pressure is curiously called backpressure and is fi-equently used to help melt the plastic and to increase the mixing action of the screw. Some back pressure is good; a lot of back pressure can be bad.
Practical_Injection_Molding/0824705297/files/00045___aeafa6b75e3f871fc6d00385c9fcca52.pdf5.1 The Barrel
The barrel, also referred to as the cylinder, is the cylindrical housing in which the screw rotates. It consists of a shell or backing which is the thick outer wall to provide strength. The shell may be lined with a variety of different materials to provide wear resistance. Some of the linings are cast inside the shell and form a metallurgical bond of the lining to the shell. In other cases, the shell may 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 secured in place by one of three methods. The barrel may have aflange 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 and through the casting and is secured by a large nut on the interior of the casting.
Figure 7 Three common types of mounting e n d of a barrel
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Another method used more recently involves sliding the barrel downward into the casting and securing it with a flange-like piece that fits down on top of the barrel. Most machine manuhcturers use a version of one of these methods and it is only important to the reader to be able to recognize the basic design of each.
All injection machine barrels have a feed hole 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 thermocouples are placed to sense the temperature of the barrel.
Although most barrel shells are made &om an alloy steel in the 4000 series (typically 4140 or 41 50) AIS1 designation, there are several alternative linings available which resist wear. The alternatives may be grouped into three types that relate to how the linings are manufactured. They include: nitrided barrels, cast bimetallic barrels and tool steel-lined barrels [ 5 ] . An understanding of the metallurgical considerations involved in each lining type is not within the 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 be repaired.
Because the screw fits very snugly into the bore of the barrel, it is vital that both the screw and the barrel be quite straight. A lack of straightness in either of the components can cause them to wear prematurely.
5.2 End Cap and Nozzle
It is easier to understand the function of the end cap and nozzle by referring to the illustration at the top of the next page (see Figure 8).
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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 not displayed in detail, the non-return valve is also shown. The construction of the valve and how it functions will be discussed later in this chapter.
Although Figure 8 illustrates a bolt-on end cap, many of the barrels and related end caps are threaded, allowing the end cap to be 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 referred to as a barrel adaptor, 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 through the 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 1 13 to 155 N/mm2 (16,000 to
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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 is usually specified by the machine manufacturer that is best achieved using a torque wrench. Over-tightening is as damaging as not tightening enough. Damage to 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 tip or 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 referred to as a sprue bushing. It is very important that the fit between the nozzle tip and the sprue bushing is correct (See Figure 9). The incorrect fit could allow the nozzle to back away during the high pressure of injection allowing plastic to leak.
Sprue Bushing
N d e Tip I
(Nonle Tip not sealed off)
l
conedm Imrrect Fit
Figure 9 Illustration of the fit of a nozzle t@ to the sprue bushing
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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 temperature of the plastic at the nozzle.
The nozzle tip is used to enable the matching of the orifice in the nozzle to the opening in 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 least three types of nozzles used in injection molding. One type has an open channel like the nozzle in Figure 9. A second type of nozzle involves a valve that closes the nozzle
