Shrinkage is the reduction in the dimensions of a plastic part, compared with the mould dimensions. Shrinkage occurs as the polymer cools. Shrinkage can vary in different directions.

Crystalline and semi-crystalline materials are particularly prone to thermal shrinkage; amorphous materials tend to shrink less. Excessive shrinkage can be caused by the following factors:

• Low injection pressure.
• Short pack-hold time or cooling time.
• High melt temperature.
• High mould temperature.
• Low holding pressure.