Meaning
Molecular tension dissipation occurs when oriented polymer chains return to a disordered, lower-energy state after flow has ceased. This reduction in tension is shear stress relaxation, and it must occur before the polymer solidifies to avoid locked-in stresses. If the cooling rate is too fast, this relaxation is interrupted, leaving high residual stresses.
Warpage Mitigation
Locked-in shear stresses from the filling stage are a primary cause of part deformation after ejection. Encouraging shear stress relaxation by using higher mould temperatures or slower cooling rates helps to maintain part flatness. Thin-walled parts are particularly prone to twisting and warpage when relaxation is incomplete.
Mechanical Integrity
Parts containing high levels of unrelaxed stress are brittle and more prone to cracking under mechanical load. Allowing sufficient shear stress relaxation before solidification ensures that the polymer chains are in a stable, isotropic state. Uniform properties improve the impact resistance and fatigue life of the finished product.
Process Adjustments
Extending the melt temperature and holding time can help reduce the residual stress level in the cavity. Faster solidification prevents shear stress relaxation from being completed, which increases the likelihood of part failure in service. Processors must balance the reduction in cycle time against the risk of locking in internal stresses.
Tuning the transition from filling to packing represents a reliable method to achieve this balance.
