Injection Molding Packing

Density Control

High compression of the melt increases the part weight and ensures the uniformity of the molded component’s dimensions. During injection molding packing, the flow of polymer is minimal, focusing instead on increasing the melt density within the cavity. Shrinkage occurs as the polymer cools, and this phase forces fresh melt into the mold to offset that volume loss.

This action holds the polymer firmly against the cavity walls, ensuring accurate replication of the tool surface.

Quality Impact

Incorrect pressure levels during this stage lead directly to dimensional instability or surface blemishes on the finished part. Excessively high pressure during injection molding packing causes over-packing, which creates molded-in stress and leads to part warpage or ejector pin damage. Low packing pressure results in sink marks and internal voids, especially in thick sections of the part.

Moulders adjust this variable to find the narrow window where the part meets dimensional tolerances without experiencing stress-induced cracking.

Cycle Optimization

Determining the gate freeze-off time is required to optimize the duration of this process step. Process technicians run a gate freeze study by weighing parts produced at different packing times to identify the point where part weight stops increasing. This measurement ensures that the packing phase is not extended beyond the point of gate solidification, which would waste cycle time and increase production costs.

Efficient cycle design depends on this precise timing, as running the machine longer than necessary increases energy usage and reduces hourly part output. By terminating the packing phase immediately after the gate freezes, the cooling phase can proceed under optimal conditions, maximizing production efficiency.