LyondellBasell
Technical Tip
tech.topic
Mold Shrinkage
! INTRODUCTION
Most mold makers and molders face the problem
of mold shrinkage of polypropylene (PP) and
polyethylene (PE). For many, it is difficult to
understand why, with all the resources and
technology available, one can't get more
accuracy in mold shrinkage data than a “typical”.
This Technical Tip briefly explains what mold
shrinkage is and why it's impossible to establish a
constant value for PP and PE.
! WHAT IS MOLD SHRINKAGE?
In some respects, mold shrinkage can be
compared to linear thermal contraction or
expansion. A mass of molten polymer cooling in
a mold contracts as the temperature drops.
Holding pressure is used to minimize shrinkage
but is only effective as long as the gate(s)
remains open. If the polymer is homogeneous,
all parts should shrink essentially the same
amount even after the pressure is removed or the
gates freeze off. This is generally the case with
amorphous polymers such as polystyrene,
polycarbonate, ABS, etc. Published values for
mold shrinkage of these materials are very low
and do not exhibit a broad range, generally in the
order of less than 0.010 in./in.
COMPARATIVE MOLD SHRINKAGE VALUES
Material
Type
Shrinkage/in/in
Polypropylene
Polyethylene
Nylon (6-6)
Acetal
ABS
Polycarbonate
Polystyrene
PPO
Semi-Crystalline
Semi-Crystalline
Semi-Crystalline
Semi-Crystalline
Amorphous
Amorphous
Amorphous
Amorphous
0.010 – 0.025
0.015 – 0.040
0.007 – 0.018
0.018 – 0.025
0.004 – 0.009
0.005 – 0.007
0.004 – 0.007
0.005 – 0.008
Why are PP and PE (and nylon and acetal)
different? Unlike amorphous polymers, PP and
PE are not homogeneous but semi-crystalline,
with a structure containing both amorphous and
crystalline components.
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As they cool, crystals form which shrink much
more than the amorphous regions and shrink at
different rates. This imbalance results in a net
increase in shrinkage and introduces a sensitivity
to other molding parameters which have
additional effects on the shrinkage.
Another factor influencing shrinkage is the viscoelastic characteristic of high-molecular-weightpolymer melts. The long, molecular weight
chains are literally stretched and placed under
stress as they fill the mold. As the stresses are
relieved during cooling, the chains want to relax,
analogous to stretching a rubber band and slowly
letting it return to its original size. This relaxation
also influences the shrinkage, especially in
different flow directions. Both the average
molecular weight and the molecular weight
distribution are key material factors that control
this facet of mold shrinkage.
WHAT PROCESS AND DESIGN
PARAMETERS INFLUENCE MOLD
SHRINKAGE?
The relative proportion of crystalline to
amorphous component changes the shrinkage.
This is a very critical variable with polyethylene
but is not as significant with polypropylene, as
evidenced by the much narrower range of
specific gravity, another property affected by the
degree of crystallinity.
!
Since it is related to the physical contraction due
to cooling, mold shrinkage is greatly affected by
the thermal history of the molding. The melt
temperature, cooling rate, part thickness and gate
dimensions can be critical variables which control
the amount of shrinkage as shown in the
following graph obtained from a laboratory study
using a 6 in. x 6 in., variable-thickness-plaque
mold and a polypropylene impact copolymer.
Shrinkage data obtained under controlled
laboratory conditions and using relatively simple
mold geometries should be used only for
comparison purposes and should not be applied
to actual tool design.
(Continued on Page 2)
LyondellBasell
Technical Tip
tech.topic
Mold Shrinkage
(Continued)
One facet often overlooked in mold design is the
need for uniform filling and cooling. In a part
having a complex geometry, even with relatively
uniform wall thickness, it is not unusual to
observe different shrinkage rates in different
sections of the part. This may be due to nonuniform cooling and/or non-uniform filling
patterns. The use of modern computer analysis
to study the filling and cooling patterns is an
extremely useful tool to identify these problems
and provide guidance for their minimization or
elimination
! SUMMARY
Mold shrinkage of semi-crystalline polymers such
as polypropylene and polyethylene, is a very
complex problem influenced by the inherent
structure of the material and also by the heat
transfer dynamics that occur during the injection
molding process. Consideration of these factors,
in conjunction with flow and cooling analysis, are
important means of minimizing shrinkage and
ensuring it is as uniform as possible throughout
the part.
Contour Plots of Constant Mold Shrinkage as a Function of Gate Area and Part Thickness
Part Thickness (mils)
180
0.016 in/in
160
0.014 in/in
140
120
0.012 in/in
100
80
0.010 in/in
60
0
0.005
0.01
Material: 12 MFR Polypropylene Impact Copolymer
Mold: 6 in x 6 in adjustable depth plaque
0.015
0.02
0.025
0.03
Gate Area (sq. in.)
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