Meaning
Specific volume reduction occurs as the polymer cools from its molten processing temperature to its solid state. This density increase is volumetric contraction, and it must be compensated for by packing additional melt into the cavity during the holding stage. If packing is insufficient, the part will exhibit sink marks or internal voids.
Packing Compensation
Maintaining high pressure during the cooling phase forces extra material into the mould to fill the space left by cooling. This step compensates for volumetric contraction, ensuring that the part replicates the cavity dimensions exactly. The packing pressure must be maintained until the gate solidifies to prevent material from flowing back out.
Further packing has no effect on the final part density once the gate is frozen.
Residual Defects
Inadequate packing allows the center of thick sections to shrink away from the walls, creating internal bubbles. Sink marks are another common outcome of uncontrolled volumetric contraction, reducing the mechanical strength of the part and creating cosmetic blemishes. Thick-walled designs are particularly prone to these defects and require long cooling times.
Simulation Modeling
Mould flow analysis uses PVT curves to predict where and when shrinkage occurs during the cycle. By simulating the volumetric contraction, engineers can optimize gate locations and cooling channels to ensure uniform shrinkage across the part. Uniform shrinkage reduces internal stress and minimizes the risk of post-moulding warpage.
