Meaning
Unbalanced heat extraction across different faces or sections of a plastic part during the solidification phase of injection moulding represents a major source of residual stress. Differences in heat transfer occur when one side of the mould cavity remains hotter than the opposing side. This situation of asymmetric cooling arises from poorly designed cooling channels, uneven steel thickness, or incorrect flow rates of the water circuit.
When the polymer shrinks at different rates on each surface, the resulting internal forces bend the part out of its intended shape.
Shrinkage Gradient
Differential solidifying rates on opposite surfaces of a moulded part produce unequal volumetric contraction through the wall thickness. When asymmetric cooling is present, the side that cools more slowly continues to shrink after the faster-cooling side has already set. This uncoordinated contraction pulls the rigid structure toward the hotter side, generating internal tensile stresses.
Warpage Risk
Mechanical distortion of the part geometry leads to assembly failures and dimensional non-compliance during quality inspections. While a moulder can adjust setpoints to achieve a datasheet tensile modulus, the presence of asymmetric cooling forces the actual part to twist under the influence of its own internal stress. Moulders often face high scrap costs because they must reject warped parts that fail to meet strict dimensional tolerances.
This instability can be particularly problematic in semi-crystalline resins where crystallization kinetics are highly sensitive to local cooling rates.
Mold Optimization
Resolving thermal imbalances requires the careful design of conformal cooling channels and the independent control of water temperatures on each half of the tooling. Adjusting the flow rate to maintain turbulent flow within the cooling lines increases the rate of heat transfer. Using different temperature setpoints for the core and cavity halves can balance the cooling times of each side.
This proactive thermal management ensures that both outer surfaces of the part solidify at the same rate, which prevents warpage and preserves the dimensional stability of the component across long production runs.
