Molding Characteristics of Thermoplastics – Shrinkage

Molding Characteristics of Thermoplastics – Shrinkage

Thermoplastics exhibit specific molding shrinkage behaviors due to changes in temperature and internal stresses. The key aspects are:

1. Molding Shrinkage: Shrinkage occurs as molten plastic cools down to room temperature. This is influenced by thermal contraction, molding process parameters, and mold structure.

2. Dimensional Shrinkage: The initial size reduction that occurs after demolding due to thermal and physical-chemical changes in the plastic part.

3. Post-Shrinkage: Secondary shrinkage caused by residual stresses released after demolding. Most of it happens within 10 hours, stabilizing around 24 hours, though full stabilization takes longer.

4. Post-Treatment Shrinkage: Additional shrinkage caused by heat treatment applied to stabilize part dimensions.

5. Directional Shrinkage: Due to polymer chain orientation, shrinkage is anisotropic—greater along the flow direction (with higher strength) and less perpendicular to it. This non-uniform shrinkage may lead to warping, deformation, or cracking.