All thermoplastics contract as they cool, but the amount and direction depend on polymer structure, grade, sheet orientation, forming strain, and cooling history. Semicrystalline materials generally show stronger and more temperature-sensitive shrinkage than amorphous materials.
Positive tools can become tighter as the part shrinks around them. Negative tools may allow the part to pull away from the surface. Directional orientation in the extruded sheet can make shrinkage unequal even in a symmetric part.
Control strategy
Shrinkage should be measured on production sheet after the tool reaches a stable temperature. Supplier values can guide initial design, but they do not replace trials on the actual grade, gauge, and process.
Tool compensation, balanced cooling, controlled release, and part fixtures may all be required. Dimensions should be specified at a defined conditioning time and temperature. Measuring a hot part and comparing it with a room-temperature drawing creates misleading process adjustments.