Shrinkage Injection Molding

 

The shrinkage of plastics designates 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. 

Material selection:

Semi-crystalline polymers (e.g: PBT or PP) always show a higher shrinkage than amorphous polymers (e.g: PS or PC) because when cooled down semi-crystalline polymers will see part of their macromolecular chains re-arranged to form crystallite, that is a well-organized structure, leading to less space needed for the same amount of atoms.

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).

The shrinkage rate is strongly depending on the polymer itself but also on the processing conditions applied and part design.

A standard method to measure “mold shrinkage”, ie contraction compared to the injection molding tool, is the ASTM D955. Other international standard methods related to plastic shrinkage are: ISO 294-4 (for thermoplastics) and ISO 2577 (for thermosets).