Meaning
Moulding cycle segments apply sustained hydraulic pressure to the polymer melt to compress the material and compensate for thermal shrinkage. The packing phase begins immediately after the injection stroke is completed and continues until the gates freeze. Applying packing phase pressure prevents the formation of internal voids and sink marks on the part surface.
Shrinkage Compensation
Polymers contract as they transition from a hot liquid to a cool solid inside the mould cavity. Additional melt must be forced into the cavity during the packing phase to fill the volume left by this contraction. Without this extra material, the part dimensions would fall below the required tolerances.
Process Optimization
Moulders must balance the magnitude and duration of the applied pressure to optimize part weight and dimensions. Excessive packing pressure can overpack the gate area, leading to high residual stresses and sticking in the mould. Too little pressure results in poor surface replication and low part weight.
Engineers use pressure-time curves to determine the exact moment the gate freezes.
Gate Solidification
The thickness of the gate determines the duration of the effective pressure application. Once the gate freezes, no more material can enter the cavity. This limitation sets the maximum duration of the packing cycle.
