Polypropylene is semicrystalline and can show substantial, direction-dependent shrinkage. Warpage develops when crystallization, orientation, wall thickness, and cooling are not balanced across the part.
The sheet may also sag strongly over a narrow heating range, making small temperature differences important. Fillers, copolymer type, color, and recycled content change both forming and shrinkage behavior.
Correction
The exact grade should be processed within the supplier’s recommended window. Mold temperature and cooling must be stable across the tool, and the part should remain supported until it can retain geometry. Thickness differences should be reduced through pre-stretch, plug design, and heater zoning.
Shrinkage should be measured by direction and after a defined conditioning period. A fixture can improve shape, but persistent springback indicates that orientation or thermal imbalance remains in the part.