Determining Optimal Injection Molding Cavitation Counts and Processing Parameters
Optimal cavitation balances machine hourly rates, clamp tonnage, and shot capacity against thermal cooling limits and cross-cavity dimensional distribution.
Cushion Retention
Meaning
The remaining volume of molten polymer left in the barrel after the injection and hold phases must be maintained to ensure pressure transfer. This residual melt is monitored through cushion retention, which is the ability of the machine to hold a constant screw position at the end of the holding cycle. It ensures that the screw does not bottom out against the barrel head during injection.
Process Consistency
Maintaining a stable cushion guarantees that the pressure from the hydraulic ram is continuously applied to the polymer in the mould cavity. If the screw moves forward during the holding phase, the melt pressure drops and the part shrinks unevenly. This leads to sink marks, dimensional deviations and internal voids.
Operators track this variable to verify the repeat stability of the injection cycle.
Mechanical Wear
Failure to hold the cushion often indicates physical wear on the non-return valve or the barrel wall. As the screw is forced forward, worn components allow polymer to slip backward into the screw flights. This leakage makes cushion retention impossible and results in shot-to-shot weight variations.
Replacing worn rings and check valves restores the sealing capability of the assembly.
Material Influence
Different resin formulations and the inclusion of regrind material affect the seal behavior during the injection stroke. High-viscosity resins require more force to backflow, which can mask minor valve wear, while low-viscosity polymers flow easily and demand tighter mechanical clearances. Regrind can introduce abrasive particles that accelerate the wear of the screw tip and the check ring.
Moulders must monitor the cushion position closely when changing material lots to prevent scrap. This constant surveillance prevents part defects on high-volume runs.