The shrinkage of plastics signifies the volume contraction of polymers during the cooling step of the processing of polymers. This contraction is partly due to the difference of?density of polymers?from the melt state and the cooled, rigid state.
Most of the plastic molded part shrinkage occurs in the mold while cooling. A small amount of shrinkage occurs after ejection as the part continues to cool and after that the part may continue to shrink very slightly until the temperature and moisture content stabilize. In higher shrink materials such as?acetal?and?nylon, the post-mold shrinkage can be significant.
If the regions of the part shrink unequally (called as warpage) stresses are created within the part which, depending on part stiffness, may cause the part to deform or change shape and hence leading to cracks in parts during long term use.