Meaning
Maximum cavity pressure achieved during the solidification stage of the injection moulding cycle governs the final weight and shrinkage of the plastic part. The peak packing pressure occurs after the cavity has been filled with polymer melt, as the screw continues to push material in to compensate for thermal contraction. This value must be controlled within narrow limits to prevent part defects and ensure dimensional consistency across the production run.
The pressure is monitored using cavity pressure sensors.
Pressure Control
Processors adjust the pack pressure and hold time to achieve the required density in the part. During this phase, the peak packing pressure must be high enough to overcome the freeze-off of the gate while avoiding overpacking. If the pressure is too high, the part will stick in the mould or have high internal stress.
If it is too low, the part will have sink marks and shrink beyond limits. The optimal pressure depends on the polymer’s viscosity and the gate size. This setting is a key parameter in process optimization.
Dimensional Stability
Parts moulded with consistent pressure exhibit uniform shrinkage and warp less after demoulding. This uniformity is critical for parts that must fit together in assemblies. When the peak packing pressure remains stable, the weight of the parts varies very little from shot to shot.
This stability is used as a primary indicator of process capability. If the pressure drifts, it indicates a change in material viscosity or check ring leakage. This tracking is essential for quality assurance.
Defect Prevention
Overpacking causes flash and stress cracks, while underpacking causes void formation and sink marks. By holding the peak packing pressure within the correct window, these defects are avoided. This control ensures that the finished parts meet the mechanical and cosmetic specifications.
