Part Shrink Explained

Contraction of Plastic Molded Parts

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.

The shrinkage of molded plastic parts can be as much as 20% by volume, when measured at the processing temperature and the ambient temperature.

This volume contraction of polymers often leads to wrapped parts and dimension differences between manufactured parts and the mold (the die if we consider extrusion as the processing technique). 

Shrinkage is a rate, so it is expressed in percent, %.