07723 fm

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,
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Library of Congress Catalog Card Number: 2002014824
ISBN: 1-884207-72-3
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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
1. 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 1 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 2 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 1 and 2 are, by and large, overviews of
the rest of the book.
Designers of plastic parts should pay particular
attention to Chs. 3 and 4 which provide insight into
part design and material selection, respectively.
Chapter 3 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. 4. 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 5 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. 3 and 4. 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 6 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. 6.
Post-mold shrinkage is discussed in some detail in
Ch. 7. 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 8 is a troubleshooting chapter. In some
ways, it is a more detailed overview of Chs. 1 through
7 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. 8 may be helpful in narrowing the
search for a solution to a problem.
Chapter 9 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. 9.
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