Meaning
Secondary pressure application ensures that the mould cavity is entirely filled and the polymer is compressed to compensate for thermal shrinkage. During volumetric packing the machine maintains a specific pressure level to force additional material into the cooling part. This step is necessary because plastic contracts as it transitions from a liquid to a solid state.
The process begins immediately after the initial fill is complete and continues until the gate freezes.
Density Compensation
Maintaining high pressure during the cooling phase increases the final mass of the moulded component. Effective volumetric packing prevents the formation of internal voids and surface sink marks. If the pressure is removed too early, the center of a thick wall section pulls away from the mould surface.
Part Mass
Monitoring the weight of finished parts provides a reliable way to check the consistency of the process. Inconsistent volumetric packing results in variations in part dimensions and mechanical strength. A lighter part often indicates that the gate froze before the cavity reached the required density.
Void Prevention
Proper execution of this phase is required for parts with high aesthetic or structural requirements. The machine must hold the pressure until the narrowest point of the flow path reaches its glass transition temperature.
