Volumetric Shrinkage Compensation

Pressure Management

Holding pressure forces additional melt into the cavity as the polymer solidifies and contracts. Adjusting the volumetric shrinkage compensation process involves setting the correct pressure profile and pack time to ensure the gate freezes before pressure is released. If the pressure is removed too early, the molten polymer can backflow into the runner system, causing severe sink marks.

Conversely, over-packing can create high residual stresses and make part ejection difficult.

Quality Control

Part weight is a direct indicator of whether the correct amount of material has been packed into the mould. Monitoring weight variations across a production run helps verify the consistency of the volumetric shrinkage compensation process. Variations in melt temperature or screw recovery can affect the volume of the injected shot, leading to inconsistencies.

Closed-loop process control systems can adjust the holding pressure dynamically to maintain a stable part weight.

Dimensional Accuracy

Amorphous and semi-crystalline polymers display different shrinkage behaviors that must be accounted for during tool design. The volumetric shrinkage compensation must be tailored to the specific resin’s thermodynamic behavior, which governs how density changes with pressure and temperature. Semi-crystalline polymers require higher holding pressures and longer cycle times due to their higher density change during crystallization.

A well-compensated process ensures that the finished parts meet the required dimensional tolerances and do not warp.