Why do they occur?
Shrinkage is inherent in the injection molding process. Shrinkage occurs
because the
density of polymer varies from the processing temperature to the
ambient temperature
(see Specific volume (pvT diagram)). During injection
molding, the variation in shrinkage both globally and through the cross section
of a part creates internal stresses. These so-called residual stresses (see
Residual stress) act on a part with effects similar to externally applied
stresses. If the residual stresses induced during molding are high
enough to
overcome the structural integrity of the part, the part will warp upon
ejection
from the mold or crack with external service load.
Shrinkage
The shrinkage of molded plastic parts can be as much as 20 percent by volume,
when measured at the processing temperature and the ambient temperature.
Crystalline and semi-crystalline materials are particularly prone to thermal
shrinkage; amorphous materials tend to shrink less. When crystalline materials
are cooled below their transition temperature, the molecules arrange themselves
in a more orderly way, forming crystallites. On the other hand, the
microstructure of amorphous materials does not change with the phase change.
This difference leads to crystalline and semi-crystalline materials having a
greater difference in specific volume ( ) between their melt phase and solid
(crystalline) phase. This is illustrated in Figure 1 below. We’d like to point
out that the cooling rate also affects the fast-cooling pvT behavior of
crystalline and semi-crystalline materials.
FIGURE 1. The pvT curves
for amorphous and crystalline polymers and the specific volume variation
between the processing state (point A) and the state at room temperature and
atmospheric pressure (point B). Note that the specific volume decreases as
the pressure increases.
Causes of excessive part shrinkage
Excessive shrinkage, beyond the acceptable level, can be caused by the
following factors. The relationship of shrinkage to several processing
parameters and part thickness is schematically plotted in Figure 2.
- low injection pressure
- short pack-hold time or cooling time
- high
melt temperature
- high mold temperature
- low holding pressure.
Problems caused by part shrinkage
Uncompensated volumetric contraction leads to either sink marks or voids in
the molding interior. Controlling part shrinkage is important in part, mold, and
process designs, particularly in applications requiring tight tolerances.
Shrinkage that leads to sink marks or voids can be reduced or eliminated by
packing the cavity after filling. Also, the mold design should take shrinkage
into account in order to conform to the part dimension. Part shrinkage predicted
by C-MOLD offers a useful guideline for proper mold design.
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