Meaning
Molded plastic parts experience varying contraction rates across their geometry during the cooling phase. A severe thermal expansion gradient across the part thickness leads to uneven shrinkage and internal stresses. This differential shrinkage causes the part to warp or twist after it is ejected from the mold cavity.
Cooling Control
Mold temperature regulation must be uniform to ensure consistent cooling across both halves of the tooling. When a thermal expansion gradient is present between the core and cavity surfaces, the part bends toward the hotter side. Conformal cooling channels reduce this temperature difference.
Defect Prevention
Thick wall sections cool much slower than thin ribs, which creates localized thermal variations during solidification. This thermal expansion gradient leads to sink marks and voids in the thick regions of the part. Optimizing the packing pressure and packing time helps compensate for this localized contraction.
Material Selection
Semicrystalline polymers exhibit higher shrinkage rates than amorphous resins due to the molecular alignment during crystallization. Managing the thermal expansion gradient is more difficult in materials like polypropylene or polyamide than in polycarbonate. Fiber-reinforced grades reduce the overall thermal contraction of the resin.
This modification helps maintain tighter tolerances across the production run and prevents post-molding distortion under variable operational temperatures.
