Polyethylene sheets, especially HDPE, can show high shrinkage, strong memory, and broad warpage when cooling is uneven. Grade density, molecular weight, extrusion orientation, color, and recycled content affect the result.
A hot part may appear correct on the mold and move substantially after release. Large flat areas and asymmetric wall thickness make distortion more visible. Shrinkage onto positive features can also complicate demolding.
Process control
Tool temperature and cooling flow should be balanced and allowed to reach steady state. Material distribution must be measured because thin and thick zones cool and shrink at different rates. The part should be supported through the period when it loses most of its heat.
Design should avoid broad unsupported flat surfaces where possible. Tool compensation must be based on production trials with the actual sheet direction and grade rather than a generic polyethylene shrinkage value.