Meaning
Compression force applied within a tool during the solidification phase to offset material shrinkage and ensure dimensional accuracy for the final part. Application of cavity packing pressure occurs immediately after the injection velocity phase is complete and the machine switches to pressure control. The machine pushes additional melt into the tool to compensate for the volumetric contraction that happens as the polymer cools from its melt state to a solid state.
Effective management of this variable defines the final weight and the dimensions of the moulded component. This phase remains active until the gate freezes, preventing further material flow into the cavity or back out toward the screw.
Compression Mechanism
Transfer from the high speed injection stage to the holding phase requires a precise set point known as the switchover position. Once the screw reaches this position, the system maintains the cavity packing pressure at a constant level rather than a constant speed. This sustained force drives more polymer through the gate before it reaches its glass transition temperature to ensure that the internal sections of the part are dense.
If the force is too low, the part will shrink excessively and pull away from the tool walls too early. Conversely, excessive force can lead to overpacking, making the part difficult to eject or causing flash at the parting line. The duration of this phase is limited by the gate freeze time.
After this point, no more material can enter the cavity regardless of the pressure applied.
Resin Stability
Sourcing consistent raw materials is necessary for maintaining a stable process window during the packing stage. When using virgin resin, the viscosity remains predictable, allowing for a repeatable cavity packing pressure response across thousands of cycles. Switching to regrind or recycled content introduces variations in molecular weight distribution and melt flow index.
These changes alter how the material transmits pressure from the nozzle through the runner system and into the cavity. A more viscous regrind batch might require higher set pressures to achieve the same part weight as a virgin batch. Moulders must monitor the cushion size to ensure that the screw does not bottom out.
Such a failure would result in a loss of pressure control and inconsistent part dimensions.
Defect Correlation
Improper pressure settings during the holding phase lead to quality issues such as sink marks and internal porosity. Sink marks appear on thick sections where the internal cooling rate is slower than the surface, pulling the skin inward if the cavity packing pressure is insufficient to hold it. Internal voids occur when the outer skin is rigid enough to resist sinking, but the shrinking core creates a vacuum hole.
Dimensional stability is also compromised, as parts may fall outside of the specified tolerance ranges for length or diameter. High pressure can induce residual stresses that cause part warpage or stress cracking after the part is ejected. Balancing these pressures requires a thorough understanding of the specific resin pressure volume temperature data and the cooling efficiency of the tool.
