Meaning
Dimensional distortion of a plastic part that occurs after it has been ejected from the mould and cooled to room temperature.
Causative Factor
Non-uniform shrinkage across different sections of the geometry creates internal stresses that the part cannot resist. Post-moulding warpage typically results from temperature gradients in the tool or variations in wall thickness. Semi-crystalline resins like polypropylene are more prone to this because the crystal structure occupies less volume than the amorphous melt.
High packing pressures can mitigate some of the shrinkage but may also introduce molded-in stress.
Assembly Failure
Parts that deviate from their intended shape often fail to fit into mating components or housings. Addressing post-moulding warpage usually requires modifications to the cooling layout or the gate location. In some cases, a fixture is used to hold the part in the correct orientation while it reaches thermal equilibrium.
This adds cost and cycle time to the production process.
Material Selection
Glass fibers are frequently added to resins to reduce the total shrinkage and improve stiffness. While these fibers limit the overall movement, they can also cause post-moulding warpage if the fibers align in a way that creates anisotropic properties. Engineers must balance the need for reinforcement with the requirement for flat surfaces.
