Calculating Specific Mold Cavity Dimensions Based on Material Shrinkage Variance Parameters
Calculate mold cavity dimensions using ISO 294-4 divisor formulas biased toward steel-safe limits to absorb material shrinkage variance before machining.
Packing Pressure Decay
Meaning
The progressive decrease of hydrostatic pressure inside a mould cavity takes place as the polymer freezes from the cold steel walls inward toward the component core. Injection moulders evaluate packing pressure decay through cavity pressure sensors to optimize the duration of the holding phase, compensating for volumetric resin shrinkage during solidification. The term stops applying once the gate freezes solid, ending all pressure transmission from the machine screw to the cavity interior.
Inaccurate packing pressure transitions cause high residual stresses, sink defects, and wide dimensional tolerances across production batches.
Solidification Mechanics
Molten polymer shrinks significantly as its temperature drops from processing temperatures down to mould demoulding limits. Insufficient holding pressure accelerates packing pressure decay, which prevents secondary melt from entering the cavity to make up for volume loss. Amorphous resins shrink between zero point four and zero point eight percent, while semicrystalline materials can shrink over two percent.
Fast pressure decay inside thick part sections allows cooling polymer to pull away from cavity walls, forming cosmetic sinks or internal vacuum voids. Sustaining packing pressure until the gate solidifies retains part density, ensuring reliable assembly dimensions.
Gate Freeze Termination
Gate dimensions and local cooling channel positions determine the exact time when packing pressure decay ceases to matter. Cavity pressure traces show a continuous decline during packing pressure decay until the narrow gate section freezes, which traps residual pressure inside the part cavity. Premature screw retraction before gate freeze allows pressurized melt to flow backwards into the hot runner, dropping part weight and causing cosmetic sink.
Holding pressure past the gate freeze point adds no plastic to the part, unnecessarily extending cycle times and increasing electrical consumption. Pinpointing the gate seal point through systematic weight studies establishes the ideal holding time.
Stress Accumulation Limits
Overly extended hold times combined with slow pressure decay pack excessive polymer molecules into cold gate regions. High pressure maintained against freezing flow fronts causes high packing pressure decay gradients, leaving high residual stresses near the gate. Highly stressed sections crack easily when exposed to external impacts or aggressive chemical cleaners during use.
Over-packed components also adhere to cavity walls, causing scuff marks or pin push marks during mechanical ejection. Balancing pressure decay rates maintains part density while avoiding internal stresses that lead to long-term environmental stress cracking.