Meaning
Physical dimensions of a thermoplastic part decrease as the material cools from its processing temperature to ambient conditions. The thermal shrinkage is the contraction of a polymer part caused by the reduction in molecular volume and the increase in density during the transition from a melt to a solid.
Material Influence
Semicrystalline resins like acetal or nylon exhibit higher contraction rates than amorphous resins like polycarbonate because the formation of crystal structures packs the molecules more tightly. This thermal shrinkage is accounted for during the design of the injection mould by making the cavities slightly larger than the desired final part. Fillers such as glass fibres or minerals reduce the overall contraction by replacing a portion of the polymer with dimensionally stable material.
Processing Variable
Holding pressure and cooling time directly affect how much the part will contract once it is ejected from the tool. Higher packing pressures force more material into the cavity to compensate for the volumetric loss associated with thermal shrinkage. Uneven cooling across the part geometry leads to differential contraction, which is a primary cause of internal stress and structural warping in complex mouldings.
Tolerance Control
Precise calculation of contraction rates allows for the production of components that meet strict assembly requirements.
