Handbook of Molded Part Shrinkage and Warpage pdl Jerry M Fischer Plastics Design Library Copyright © 2003, Plastics Design Library / William Andrew, Inc All rights reserved No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the Publisher Cover Art © 2003 by Brent Beckley / William Andrew, Inc Library of Congress Catalog Card Number: 2002014824 ISBN: 1-884207-72-3 Published in the United States of America by Plastics Design Library / William Andrew, Inc 13 Eaton Avenue Norwich, NY 13815 1-800-932-7045 www.williamandrew.com www.knovel.com 10 This book may be purchased in quantity at discounts for education, business, or sales promotional use by contacting the Publisher Plastics Design Library and its logo are trademarks of William Andrew, Inc Notice: To the best of our knowledge the information in this publication is accurate; however the Publisher does not assume any responsibility or liability for the accuracy or completeness of, or consequences arising from, such information This book is intended for informational purposes only Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Publisher Final determination of the suitability of any information or product for any use, and the manner of that use, is the sole responsibility of the user Anyone intending to rely upon any recommendation of materials or procedures mentioned in this publication should be independently satisfied as to such suitability, and must meet all applicable safety and health standards Manufactured in the United States of America Library of Congress Cataloging-in-Publication Data Fischer, Jerry M Handbook of molded part shrinkage and warpage / Jerry M Fischer p cm Includes bibliographical references and index ISBN 1-884207-72-3 Plastics Molding Handbooks, manuals, etc I Title TP1150 F59 2002 668.4'12 dc21 2002014824 William Andrew, Inc., 13 Eaton Avenue, Norwich, NY 13815 Tel: 607/337/5080 Fax: 607/337/5090 Preface Mold makers and molders face the problem of predicting or controlling the shrinkage of the plastic parts they mold It may be difficult to understand why data are not available that are more accurate than those published in various publications like Modern Plastics Encyclopedia The data normally published indicate ranges of shrinkage that can vary by over one hundred percent of the minimum For example, published shrink data for polypropylene may indicate a shrink rate that ranges from 0.010 to 0.020 units per unit length The reasons the published data vary so widely are discussed herein All materials change size when subjected to temperature variations Additional size changes occur if the material is cast; that is, changed from a more or less liquid form to a solid form while in a die This happens with steel, aluminum, brass, lead, silver, and gold Cast metals are heated to liquid form for pouring, and size changes occur as the metals cool again in the product preform Warpage in the finished piece may result from machining operations that add or reduce stresses Working with plastic is similar to working with metal: the same kind of size changes occur during the injection molding process Plastic materials, though, vary more in their physical properties than metals, adding complications to the size changes Plastics have fiber-like molecular chains that affect size change as they are processed Sometimes reinforcing fibers are added to plastics and this further complicates size change As plastics flow, the molecular and reinforcing fibers tend to orient themselves along the flow path, leading to uneven or anisotropic size change The very nature of the injection molding process induces stresses in a molded part as a result of huge temperature gradients between the mold surface and the molten plastic as it enters the mold and cools The skin on the surface of the molded part assumes the temperature of the mold wall almost immediately, while a significant time is required for the core of the plastic to cool If the cooling is not symmetrical, either because the mold walls are at unequal temperatures or because of asymmetrical part configuration, then the part will tend to warp The warpage is caused by uneven cooling that creates tensile stress in the core of the part where it cools last and compressive stresses in the outer layers These stresses can significantly affect the ability of the molded part to perform satisfactorily © Plastics Design Library Fill rates affect the level and orientation of flow, inducing stresses Each molecule of plastic acts a little like a rubber band As it flows into the mold, it stretches Because the mold cools the plastic so rapidly, some of the elongation is frozen-in, causing a different type of molded-in stress Many plastics absorb various liquids and are affected by environmental variables such as solvents and ultraviolet rays Additives such as colorants can affect the size change of plastic parts As with all materials, not all variables affecting the size and warpage of molded plastic parts can be determined with exactitude Just as one cannot see preexisting stresses in metals, one cannot predict exactly the flow patterns and orientations within a molded part This book assembles pertinent published research and practical knowledge about injection molding to aid more accurate prediction of finished part size This book will also help its users understand, prevent, and troubleshoot some warpage Where warpage cannot be prevented, the user will be able to take steps to anticipate and allow for it This book is not intended to be a theoretical report or to include a lot of formulas for predicting plastic behavior during the molding operation That type of information is the purview of academic theorists and software designers This book will be of interest to researchers, equipment designers, and software developers designing material and equipment improvements Here we present general behavioral rules for plastic that will help the product-part engineer and the mold designer anticipate problems that are likely to occur, make intelligent guesses about shrink rates, recognize typical causes of warpage, and take action to avoid or minimize defective product runs Chapter surveys some basics related to the injection molding process The distinguishing characteristics of crystalline and amorphous plastics are discussed, as are the filling, holding, and cooling stages of the molding process Chapter tells how shrinkage is measured and how warpage and molded-in stresses are the result of anisotropic or nonuniform shrinkage It discusses the causes and remedies of nonuniform shrinkage, and how size change occurs over long periods This chapter also discusses the difference between long-term and short-term shrinkage and the variables that affect them Preface iv Chapters and are, by and large, overviews of the rest of the book Designers of plastic parts should pay particular attention to Chs and which provide insight into part design and material selection, respectively Chapter discusses the effect of incorrect or problematic part design Part-thickness variations, ribs, bosses, and other considerations can cause shrinkage variations in spite of the best efforts of the mold builder and mold designer The effects of material choice or change are covered in Ch Plastics are long chains of atoms or molecules that act like fibers or rubber bands When particulate or fibrous materials such as mica or glass fibers are added to a plastic material, they drastically change the shrinkage characteristics of the resin as well as its stiffness, strength, and creep characteristics Runner and gate design and location affect the flow patterns within the mold cavity Cooling channels and methods affect the rate and uniformity of cooling Chapter discusses a variety of mold features and how they may affect the shrinkage and warpage of the molded part A properly designed mold can compensate somewhat for undesirable part design features If you are designing a mold, you need to be fully aware of the potential problems mentioned in Chs and If the part design or choice of material is problematic, it should be discussed with the customer as early as possible Certainly it should be discussed before the mold design is finalized The mold designer should be thoroughly familiar with the molding process and the problems the processor is likely to have The mold designer should anticipate and consider how the molder can deal with as many problems as he possibly can The better the mold designer does his job, the better the mold trials will go and the more satisfied the molder will be Chapter deals with the processing variables and how they affect molded part shrinkage and warpage Finding proper injecting and cooling times are mentioned, as are the advantages and disadvantages of warm versus cool molds The molder is, you might say, trapped between the customer and the mold builder He has to deal with the customer-created part design problems and with the mold builder’s attempts to deal with them This chapter can help because among the molder’s best tools is an extensive understanding of processing options and their effects A brief discussion of other processes such as the gas- or water-assist technique is included in Ch Post-mold shrinkage is discussed in some detail in Ch The effects of temperature, stress, and liquid absorption are covered The molder cannot assume that a part is sized correctly if it is measured as soon as it is Preface cooled to room temperature He must anticipate the additional size change that can occur over time as a result of long-term stress relaxation and environmental factors Chapter is a troubleshooting chapter In some ways, it is a more detailed overview of Chs through because trouble can arise from any of the wide variety of problems mentioned therein If you are somewhat familiar with the molding, mold design, and part design process, Ch may be helpful in narrowing the search for a solution to a problem Chapter covers the current state of the art of computer-aided engineering (CAE) for plastic molding The accuracy of the shrink and warp predictions and the mold design data produced by these programs is discussed Assumptions are made for these analyses, and their effects on the end results, are detailed If an intelligent estimate of average shrinkage is not adequate for a particular molded part, then the wisest course of action is to contract with someone who regularly uses one of several flow/cooling/shrink analysis CAE programs on a regular basis Such a consultant should have several years of experience to intelligently use the software Many assumptions and generalizations are necessary even with CAE software, and considerable experience is necessary to apply appropriate values The old adage, “garbage in, garbage out,” applies here Some of the interesting problems and solutions I have encountered over more than thirty-five years of plastic mold design and problem solving are included in Ch 10 Chapter 11 contains a large selection of data, including a number of tables, graphs, and charts, that will be useful in estimating shrinkage and warpage The balance of the book includes a table of common conversion factors and equivalents; a list of common abbreviations and acronyms used in the plastics industry; a glossary of terms; a reference list of useful books, papers, and other materials; a keywords index If you are new to the molding industry, I hope this book will help you better understand the industry and its problems Awareness of the challenges arising in all phases of the process of designing a part, building a mold, and producing a part is helpful for anyone, whether an OEM (original equipment manufacturer), a part designer, a mold builder, a molder, or an end user Even old hands in the industry may find new or unique ideas herein It is hoped that reference to specific chapters will help solve plastic part problems, whatever they may be Jerry Fischer January, 2003 © Plastics Design Library Acknowledgments I’d like to thank Clive Maier for the original idea for this book, for some of the research, and for his editorial comments Thanks to the publisher Bill Woishnis for his unfailing commitment and to the editors, Millicent Treloar, Jeanne Roussel, and Valerie Haynes for their support and encouragement as I struggled through this tome Sam Miller of General Electric made significant contributions to Ch 4, and especially to the section about PVT diagrams He and General Electric were major contributors to the data section (Ch 11) of the book as well Anne Bernhardt of Plastics and Computers (TMconcepts®) contributed greatly to Ch Most important of all, I appreciate the support I received from my wife, Rose, and my son and partner, Phil, especially for their patience and willingness to give up time I might otherwise have spent with them or in support of our business © Plastics Design Library Acknowledgments