7 Factors Affecting Post-Mold Shrinkage and Warpage

7 Factors Affecting Post-Mold Shrinkage and Warpage
Most part shrinkage takes place within a very short
time after the part is molded, typically within sixteen
to forty-eight hours after demolding. The reduction in
volume during this initial time period is a result of solidification and thermal contraction as the molded part
cools to room temperature. This rapid size change is
influenced by the variables discussed in Chs. 2–6: material properties, part geometry, the runner and gate
systems, melt temperature, mold temperature, injection pressure, holding pressure, and so on.
The same variables affect post-mold shrinkage, occurring more than forty-eight hours after demolding.
Especially important phenomena in post-mold shrinkage are temperature and moisture conditions during
molding, along with in-service exposure after manufacturing. This chapter reviews the factors of greatest
influence on post-mold shrinkage.[39]

7.1

Effects of Temperature on
Dimensions

Time and temperature conspire to allow molded-in
stress relaxation and some slight additional crystallization in semicrystalline materials after the molded part
is ejected. Some semicrystalline materials such as acetal, PBT, and PB can shrink as much as 0.5% after
molding. The longer the time and the higher the ambient temperature, the greater the tendency for the molded
part to shrink after molding.
Plastics, by their very nature, have more thermal
expansion and contraction than metals. When plastics
are constrained by being attached to a metal part, they
may crack or totally fail if exposed to widely varying
temperatures. This type of failure is due to the frequent change in stress from tension to compression and
back again under the influence of the temperature
variations.
In molding operations, the plastic material is cooled

from the outer surface. Solidification occurs against
the mold surface and the solidification front proceeds
from that surface toward the center of the thickness of
the plastic part. Several factors affect the rate of heat
transfer from the plastic to the mold. The mold temperature is the most significant factor and most subject to the control of the molder. The higher the mold
temperature, the slower the plastic will cool because

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the temperature gradient between the molten plastic and
the mold wall is lower.
Higher mold temperatures slow the cycle and increase the in-mold shrinkage, but reduce long-term or
post-mold shrinkage. The net result is that the parts
molded in a hot mold need little or no annealing and
exhibit little or no post-mold shrinkage. For example,
in molding Delrin® at moderate temperatures, good stability can be obtained with a mold temperature of 90°C
(194°F). For more severe conditions, the mold temperature for Delrin may need to be as high as 120°C
(250°F).[33]
The cooling efficiency of the mold contributes to
the cooling rate of the plastic part. For example, if cooling channels in the mold are placed very near the molding surface, the heat transfer into the cooling water is
quite rapid near the water channels but somewhat
slower between water channels. This results in a variation of the temperature of the surface of the mold from
a minimum immediately over the water channel to a
maximum half-way between the channels. The variation in mold temperature across a large, flat surface
that results from cooling channels placed too near the
surface may cause a visible “ripple” on the surface of
the part.
Placing the cooling channels at a greater distance
from the molding surface results in a more uniform
surface temperature. At one time it was advocated that

cooling channels not be placed in the inserts but instead be placed in the holder blocks or the plates immediately behind the mold inserts. This resulted in very
uniform temperatures on the mold surfaces initially,
but the continuous, very slow heat-transfer ultimately
caused a rise in the mold surface temperature. This
“uniformity” theory actually can result in a reduction
of mold-temperature consistency.
If there are mold details that are difficult to cool,
remote cooling lines increase that difficulty and increase
the mold surface-temperature variations. In addition,
if there are mold cycle-time variations, as there frequently are with manually operated molding machines,
the mold surface temperature drops more during any
delays (such as when the operator sprays the mold surface, smokes a cigarette, drops a part, extracts a stuck
part, etc.). After a delay such as this, the next few parts
are molded in a cooler mold than those molded during
a consistent cycle.

Ch. 7: Factors Affecting Post-Mold Shrinkage and Warpage


98
In some cases, it is impossible to maintain absolutely uniform mold surface temperature. Very small
and long core-pins cannot be effectively cooled throughout their length. Usually, most of the cooling around
such core pins is from the outside surface of the part
around the cored hole, with little of the heat transferred
through the core pin. A similar problem exists in the
vicinity of sharp, inside corners of a molded part. This
type of uneven cooling shifts the neutral axis toward
the hot side of the part and increases the tendency toward warpage.
As the plastic part cools, it pulls away from the
mold surface due to volumetric shrinkage. The lower

the packing pressure, the sooner the separation occurs.
As the plastic pulls away from the mold wall, there is a
sharp reduction in heat transfer from the plastic to the
mold wall. This happens because dead air space is an
excellent insulator. A vacuum is an even more effective insulator and a vacuum is often present as the plastic shrinks away from the cavity wall because there is
no source for air until the mold opens. Inadequate packing pressure can cause significant variations in the cooling rates thus cooling inconsistency across the surface
of a molded part as a result of this type of separation.
In summary, higher mold or melt temperature results in less post-mold shrinkage. However, higher mold
temperatures are often localized because of inefficient
cooling. Localized hot spots cause shrinkage variation
and warpage. Post-mold annealing can accelerate the
post-mold shrinkage and minimize later size change.
Parts molded in cooler molds can be annealed (stress
relieved) to achieve better mechanical properties and
stability in the final part. Fixturing may be required to
stabilize parts during the annealing process.
Fixturing is a complex process and should only be
used when molded parts require very tight tolerances
and exposure to high temperatures for prolonged periods while in use. Attempts to reach good dimensional
stability by annealing parts molded in a cold mold are
likely to lead to high post-molding shrinkage and may
introduce stresses causing uncontrolled deformation.
This is especially true for semicrystalline materials such
as acetal or nylon.
Post-mold shrinkage of acetal parts molded at a
variety of mold temperatures when exposed to different temperatures for 1000 hours are shown in Fig. 7.1.
The annealing procedures for the parts showing the
least shrinkage in the charts in Fig. 7.1 were subject to
the following guidelines:
• Parts should be exposed to air or an inert

mineral oil at 160 ±3°C for 30 minutes
plus 5 minutes per mm of wall thickness.

Ch. 7: Factors Affecting Post-Mold Shrinkage and Warpage

• Overheating and hot spots should be
avoided.
• Parts should neither contact each other
nor the walls of the container.
• Parts should be left in the container to
cool slowly until 80°C is reached.
• Stacking or piling, which may deform the
parts while they are hot, should be delayed until the parts are cool to the touch.
• Annealing can also be used to test molded
parts to determine their long-term stability and size change. Annealed parts
closely resemble the ultimate size of the
parts after long-term use.
For maximum in-service stability of the molded
part, mold temperatures should be near the high end of
the plastic supplier’s recommendations. For example,
post-mold shrinkage can be estimated for Delrin® acetal from Fig. 7.1.[33]

7.2

Effects of Moisture on
Dimensions

Post-mold size change also can come about as a
result of absorption or loss of fluids such as water or
plasticizers. The loss of plasticizers causes a plastic

part to become more brittle and to shrink. How many
automobile dashboards have you seen that have lost
color or cracked? This type of failure is caused by the
loss of plasticizers.
Some materials are hygroscopic; that is, they attempt to absorb moisture from the environment. As
they absorb moisture, the material properties change.
Sometimes the materials become tougher, usually there
is dimensional change. Figure 7.2 shows the change in
size due to moisture absorption of Zytel® 101.[9] Size
changes for Delrin® 100 and 500 are shown in Fig. 7.3.[33]
Other moisture absorption curves can be found in the
material-specific data section (Ch.11 of this book).
Nylons are strong materials with good chemical
resistance, but they absorb large amounts of water if
immersed. It is not generally considered a good application for nylon if the part is to be immersed in or
continually exposed to water unless full consideration
is given to the amount of post-mold growth that nylon
can experience in water. Applications using nylon have
failed because the nylon parts that were immersed in
water swelled so much that they did not allow the moving parts to move freely. Some nylons can absorb mois-

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99
ture to such an extent that the totally saturated nylon
part is larger than the cavity in which it was molded.
Figure 7.2 shows the dimensional change of nylon
as it absorbs moisture. The change shown here is not
necessarily equal in flow and cross flow. The measurement direction is not specified but is probably in the

flow direction.[13]
Figure 7.3 implies that the molded part was probably a tensile test (dog-bone) specimen and that the
measurements were along the long or flow-direction
axis. There is no indication that the cross-flow changes
are the same.
The presence of moisture during molding inhibits
a glossy surface. Moisture usually causes surface splaying (which normally manifests itself as silvery streaks
parallel to the flow direction of the plastic, sometimes
as irregularly shaped silver spots) or other imperfections because it inhibits close contact with the cavity
wall and can cause foaming or voids within the molded
part.
Moisture in the plastic pellets as they enter the
heating section of the molding machine often cause plastic-property degradation because of chemical reactions
between the plastic and superheated steam.
Table 7.1 shows the equilibrium water absorption
percentages for several polyamides.[9] Nylons must be
molded dry to avoid material degradation, but in the
dry condition, they tend to be brittle. When they have
absorbed moisture, they become tougher.

Figure 7.1 Post-molding shrinkage of Delrin ® acetal
resins.[33] (Courtesy of DuPont.)

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Figure 7.2 Size change of Zytel ® 101 vs moisture
absorption.[9] (Courtesy of DuPont.)

Ch. 7: Factors Affecting Post-Mold Shrinkage and Warpage

100
The 24-hour absorption levels of water by nylon
compared to the equilibrium levels of water in nylon in
an environment where the relative humidity is less than
about 25% are as follows:
Type of nylon

Figure 7.3 The effect of temperature and moisture content
on the dimensions of Delrin® 100 and Delrin ® 500. [33]
(Courtesy of DuPont.)

Table 7.1. Water Absorption of Nylons in Air and
Water

Nylon 66

24 hours
in water
1.2

Equilibrium
% of water content
9.0

Nylon 610
Nylon 11

0.4
0.3

3.5
2.0

Figure 7.4 shows longer-term water absorption for
Nylon 11 and two other grades.[13] Note that Nylon 6
absorbs significantly more water than the other grades.
In most cases, it is a good idea to condition nylon parts
in hot water before placing them in service to stabilize
the moisture absorption and increase the toughness of
the nylon. Dry nylon as molded is relatively brittle.
Suppose a flat part is exposed to water on one side
and a dry environment on the other. The bow-shaped
warpage as shown in Fig. 7.5 could take place. The
same sort of warpage can take place if one side of a
part is coated with an impermeable layer and the other
side is left uncoated.
Plastics will absorb all kinds of fluids to a measurable level. Inspection of the chemical compatibility
of the plastic in question will give a good indication of
likely absorption of a particular fluid. If a supplier states
that a plastic is compatible with a particular fluid or is
resistant to that fluid, it can be assumed that after two
weeks of immersion, the plastic will absorb an amount
of fluid that is less than 1% of the weight of the part.[13]

Absorption
Polyamides

In Water at 20°C
(%)

In Air at 50%
RH, 23°C (%)

6

8.5

2.8

66

7.5

2.5

6/66

7.5

2.5

6/12

3.0

1.3

6/10

3.0

1.2

Amorph

5.8

2.8
Figure 7.4 The percentage of water absorbed by some
grades of nylon over long periods of time.

Ch. 7: Factors Affecting Post-Mold Shrinkage and Warpage

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101

Figure 7.5 Potential warpage (exaggerated) due to nonuniform exposure to moisture.

Many plastics contain mobile fluids such as plasticizers, antistatic agents, lubricating oils, dyes, etc.
Most users are aware of the problem of plasticizer migration and that plasticizer loss will cause significant
changes in dimensions (shrinkage). The migration of
mobile fluids is accelerated by contact with a wide range
of organic fluids which, having greater affinity for the
plasticizer than the molded plastic, may cause rapid
shrinkage.[13] Some materials contain plasticizers without this being explicitly stated. Flexible grades of cellulosics and nylons (particularly Nylon 11 and Nylon
12) are quite common, and these will be prone to migration-induced shrinkage, just as will any plastic containing mobile fluids.
Figure 7.6 shows the moisture absorption as a percentage of the weight of the part of certain glass-fiber

plastics immersed in water.[40] This figure does not differentiate between hygroscopic and non-hygroscopic
materials, but rather suggests at least some moisture
migration along the glass fibers into the plastic part.
From Fig. 7.7 it is obvious that nylon is hygroscopic and its level of water is strongly affected by the
environment.[35] The more water that is available, the
more nylon absorbs to reach equilibrium.
The time that is required for a plastic part to reach
an equilibrium condition, for any given moisture concentration, is affected by the environmental temperature and thickness of the plastic part. The thicker the
part, the longer it takes for the moisture to migrate
through the plastic and uniformly permeate the part.
Figure 7.8 shows how thicker walls of Zytel® 101 take
longer to reach equilibrium.[35]

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The equilibrium condition for this material is the
same, about 2% to more than 5% moisture, no matter
how thick the walls are. This graph indicates that a
1.5-mm thick wall reaches equilibrium in about 6
months, but the thicker walls may not reach equilibrium in a year.
Figure 7.9 shows another nylon resin that has not
reached equilibrium in thicker sections in a year.[35]
When immersed in water, these same two resins
approach equilibrium more rapidly than at 50% RH in
air. See Fig. 7.10.[35]
Figure 7.11 shows the time required to condition
Zytel® 101 to 3% moisture and to saturation for various wall thicknesses.[35]
Figure 7.12 shows that nylon can increase in size
as a result of moisture absorption as much or more
than it can shrink out of the mold (as much as 0.025

inches per inch).[35]
We have dealt here primarily with size change of
nylon due to absorption of water. The wrong chemical
can affect any plastic. While water is probably the most
common environmental fluid that is likely to be absorbed by a plastic, and some plastics react more
strongly to its presence than others, many plastics react adversely to hydrocarbons that are quite common
in the petroleum and automotive industry. Check the
plastic’s reaction to known or suspected chemicals that
are likely to be present in the expected environment.

Figure 7.6 The percentage of moisture absorption (but not
the size change) of a variety of plastics as a result of
immersion in water.[40] (Courtesy of Hoechst Celanese.)

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102

Figure 7.7 The equilibrium conditions of moisture content
vs relative humidity for a variety of Zytel® nylon resins.[35]
(Courtesy of DuPont.)

Figure 7.8 Moisture content vs time for Zytel® 101F exposed
to 50% RH air at 23ºC.[35] (Courtesy of DuPont.)

Figure 7.9 Moisture content of Zytel® 151 as time passes
when the Zytel is exposed to air at 50% RH at 23°C. Three
different thicknesses are shown.[35] (Courtesy of DuPont.)

Figure 7.10 Moisture content vs time for Zytel® 101 and
Zytel® 151 when immersed in water at 23°C.[35] (Courtesy
of DuPont.)

Ch. 7: Factors Affecting Post-Mold Shrinkage and Warpage

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103

Figure 7.11 Boiling times to condition Zytel® 101.[35]
(Courtesy of DuPont.)

Figure 7.12 The size change of Zytel® 101 in the stressfree (annealed) condition as it absorbs moisture. [35]
(Courtesy of DuPont.)

7.3

of any significant part of the tensile strength of the
material, long-term measurement of deflection (six
months minimum exposure) should be conducted. The
test should be conducted at the highest expected stress
and at the highest expected environmental temperature.
Any significant deflection over time would indicate the
need for additional structural support.
It does happen that product suppliers do introduce
new resins that have had only short-term testing. A
few years ago, a company introduced a new large product line in which the thermoplastic was expected to
carry significant structural loads. The initial short-term

testing of the product yielded outstanding results. However, after six months to a year in the field, the product
sagged to the point that it became unacceptable for the
intended purpose. This ultimately led to bankruptcy of
the company. Had the long-term creep characteristics
of the thermoplastics been recognized, other structural
elements could have been included in the design that
would have produced an excellent product. Unfortunately, the failure to recognize the creep characteristics of the plastic led to the company failure and added
another black mark to consumers’ concepts of plastic.

Creep

While it is not strictly a shrink or warp phenomenon, if a plastic part is loaded to a significant fraction
of its tensile strength, it can be subject to creep failure.
For most practical purposes, plastic can be thought of
as molasses in January in Alaska. Fiber fillers increase
the stiffness of plastics but they do not eliminate the
tendency to creep. As a general rule, it is unwise to use
thermoplastics as load-bearing structures without huge
safety factors or extensive, long-term, elevated-temperature testing. For this type of application, the creep
data for the plastic is much more significant than the
tensile or compressive strength.
Creep is a phenomenon that is foreign to most designers. Most thermoplastics are subject to at least some
creep. Amorphous thermoplastics are similar to glass;
the slow rate of creep has no limit. Semicrystalline
materials are somewhat more rigid and the creep rates
tend to diminish over time. The physical property data
for a given plastic is for short-term loading. Long-term
deflection versus stress is rarely published. Before
marketing a product that is exposed to long-term stress

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Ch. 7: Factors Affecting Post-Mold Shrinkage and Warpage