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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