Meaning
Sustained hydraulic force applied after initial cavity fill forces additional polymer melt into the mold to offset thermal shrinkage during cooling. Setting appropriate packing pressure determines final part density, dimensional accuracy, surface sink depth and internal stress concentration. The stage ends when the cavity gate freezes solid, preventing further melt transfer between the runner system and the moulded component.
Resin datasheets list generic processing guidelines, but optimal packing levels must be established experimentally on the shop floor for each specific part and tool geometry. Insufficient packing causes sink marks and dimensional under-sizing, while excessive pressure induces flash and mold sticking.
Volumetric Compensation
Cooling polymer contracts inside the cavity, creating space that incoming liquid melt fills under controlled pressure profiles. Maintaining force during phase change holds part surfaces against cool steel walls, ensuring high fidelity reproduction of tool surface textures.
Gate Freeze
Melt solidifies rapidly inside narrow gate openings, isolating the cavity from the injection unit. Pressure applied after gate seal accomplishes no further volumetric packing and only adds stress to runner systems.
Overpacking Stress
Excessive pressure packs extra material into regions near the gate, creating severe residual stresses and causing ejection sticking. De-moulding forces rise, leading to ejector pin push-through or post-moulding warpage in flat sections.
