Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature. The shrinkage of ed plastic parts can be as much as 20 percent by volume, when measured at the processing temperature and the ambient temperature.
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 semi-crystalline materials having a greater difference in specific volume between the processing state (point A) and the state at room temperature and atmospheric pressure (point B) than amorphous materials (see figure below).
pvT curves for amorphous and crystalline polymers.