INTRODUCTION As they cool, crystals form which shrink much
Most mold makers and molders face the problem more than the amorphous regions and shrink at
of mold shrinkage of polypropylene (PP) and different rates. This imbalance results in a net
polyethylene (PE). For many, it is difficult to increase in shrinkage and introduces a sensitivity
understand why, with all the resources and to other molding parameters which have
technology available, one can't get more additional effects on the shrinkage.
accuracy in mold shrinkage data than a “typical”.
This Technical Tip briefly explains what mold Another factor influencing shrinkage is the visco-
shrinkage is and why it's impossible to establish elastic characteristic of high-molecular-weight-
a constant value for PP and PE. polymer melts. The long, molecular weight
chains are literally stretched and placed under
WHAT IS MOLD SHRINKAGE? stress as they fill the mold. As the stresses are
In some respects, mold shrinkage can be relieved during cooling, the chains want to relax,
compared to linear thermal contraction or analogous to stretching a rubber band and slowly
expansion. A mass of molten polymer cooling in letting it return to its original size. This
a mold contracts as the temperature drops. relaxation also influences the shrinkage,
Holding pressure is used to minimize shrinkage especially in different flow directions. Both the
but is only effective as long as the gate(s) average molecular weight and the molecular
remains open. If the polymer is homogeneous, weight distribution are key material factors that
all parts should shrink essentially the same control this facet of mold shrinkage.
amount even after the pressure is removed or the
gates freeze off. This is generally the case with WHAT PROCESS AND DESIGN PARAMETERS
amorphous polymers such as polystyrene, INFLUENCE MOLD SHRINKAGE?
polycarbonate, ABS, etc. Published values for The relative proportion of crystalline to
mold shrinkage of these materials are very low amorphous component changes the shrinkage.
and do not exhibit a broad range, generally in the This is a very critical variable with polyethylene
order of less than 0.010 in./in. but is not as significant with polypropylene, as
evidenced by the much narrower range of specific
gravity, another property affected by the degree
COMPARATIVE MOLD SHRINKAGE VALUES of crystallinity.
Material Type Shrinkage/in/in Since it is related to the physical contraction due
Polypropylene Semi-Crystalline 0.010 – 0.025
to cooling, mold shrinkage is greatly affected by
Polyethylene Semi-Crystalline 0.015 – 0.040
the thermal history of the molding. The melt
Nylon (6-6) Semi-Crystalline 0.007 – 0.018
temperature, cooling rate, part thickness and
Acetal Semi-Crystalline 0.018 – 0.025
ABS Amorphous 0.004 – 0.009 gate dimensions can be critical variables which
Polycarbonate Amorphous 0.005 – 0.007 control the amount of shrinkage as shown in the
Polystyrene Amorphous 0.004 – 0.007 following graph obtained from a laboratory study
PPO Amorphous 0.005 – 0.008 using a 6 in. x 6 in., variable-thickness-plaque
mold and a polypropylene impact copolymer.
Shrinkage data obtained under controlled
Why are PP and PE (and nylon and acetal)
laboratory conditions and using relatively simple
different? Unlike amorphous polymers, PP and
mold geometries should be used only for
PE are not homogeneous but semi-crystalline,
comparison purposes and should not be applied
with a structure containing both amorphous and
crystalline components. to actual tool design.
(Continued on Page 2)
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Mold Shrinkage (Continued)
One facet often overlooked in mold design is the SUMMARY
need for uniform filling and cooling. In a part Mold shrinkage of semi-crystalline polymers
having a complex geometry, even with relatively such as polypropylene and polyethylene, is a very
uniform wall thickness, it is not unusual to complex problem influenced by the inherent
observe different shrinkage rates in different structure of the material and also by the heat
sections of the part. This may be due to non- transfer dynamics that occur during the injection
uniform cooling and/or non-uniform filling molding process. Consideration of these factors,
patterns. The use of modern computer analysis in conjunction with flow and cooling analysis, are
to study the filling and cooling patterns is an important means of minimizing shrinkage and
extremely useful tool to identify these problems ensuring it is as uniform as possible throughout
and provide guidance for their minimization or the part.
Contour Plots of Constant Mold Shrinkage as a Function of Gate Area and Part Thickness
Part Thickness (mils)
0 0.005 0.01 0.015 0.02 0.025 0.03
Material: 12 MFR Polypropylene Impact Copolymer Gate Area (sq. in.)
Mold: 6 in x 6 in adjustable depth plaque
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conditions of use of our products are beyond our control, establishing satisfactory performance of our products for the intended
application is the customer's sole responsibility. All uses of Equistar products and any written or oral information, suggestions or
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Lyondell Chemical Company
1221 McKinney, Suite 700
P.O. Box 2583
Houston, Texas 77252-2583
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