Shrinkage & Warpage
Shrinkage varies by material, the rate of temperature change and the thickness of the material. For PP and PE materials, the expected material thickness is the best predictor. Thin parts may shrink as little as 1% and thick parts in excess of 6%. A typical .060" thick wall part will shrink approximately 1.7% as it cools and a .125" thick wall part will shrink about 2%. The shrinkage expectation must be taken into consideration in the mold design by increasing the mold size.

Designs that allow thinning to occur in portions of the part will generally result in warped parts. The thin areas will shrink less before cooling than the thick areas. The variation in shrinkage rates and amounts will then cause the part to warp.

Shrinkage variations occur in every molded product because even if perfectly uniform material thickness is attained, the cooling rate of the plastic will vary. The skin of the material against the mold metal will cool and take a set before the material not actually touching the mold metal. The result is a tendency for outer walls to warp inward. This is often countered by the tendency of the inner wall to warp outward. The use of a rib, tack-off, arc or step can create a structure that helps reduce warpage.