Meaning
Thermal gradients occurring across a moulded part during the solidification phase generate varying volumetric shrinkage and residual stresses. A differential cooling rate typically arises when the mold wall temperatures differ between the core and cavity halves or across thin and thick regions. This thermal imbalance leads to warpage, sink marks, or dimensional instability in the finished plastic component.
Thermal Variation
Heat extraction from molten polymer depends heavily on local wall thicknesses and coolant channel placement. When some regions of the plastic part cool much faster than adjacent zones, the differential cooling rate alters the local rate of crystal growth in semi-crystalline resins. This non-uniform solidification creates varying density zones across the part geometry.
Shrinkage Deviation
Volumetric shrinkage changes based on how quickly the polymer transitions from a melt to a solid. Faster cooling restricts the polymer chains from packing closely together, whereas slower cooling allows for greater compaction and density. This localized shrinkage discrepancy pulls on the surrounding material, causing the part to bend or twist after ejection from the tool.
Stress Distribution
Internal forces develop when the outer skin of the part solidifies while the inner core remains molten. As the core eventually cools and contracts, it pulls against the already rigid outer layers, creating tensile stresses in the center and compressive stresses on the surface. These molded-in stresses can cause premature part failure under low mechanical loads or generate delayed warping during subsequent storage.
Applying conformal cooling channels in the injection mold helps to equalize the thermal history across the entire part surface, which minimizes the differential cooling rate and stabilizes part dimensions.
