Meaning
Injection moulding sequences require a period of sustained force after the cavity fills to prevent shrinkage during cooling as the polymer transitions from a melt to a solid. High pressure packing phase refers to this specific interval where the screw maintains forward displacement to push supplemental material into the part. Compensation for volumetric contraction depends entirely on the stability of this force and the duration for which it holds.
Pressure Dynamics
Maintaining constant hydraulic or electric input during the high pressure packing phase ensures that the gate remains open while the core of the component cools. Variations in this force affect final part density and the level of internal stress locked within the molecular structure. A lack of sufficient force results in sink marks or voids, while excessive force induces flash at the parting line or causes parts to stick in the mould.
Correct calibration balances the need for volumetric replenishment against the risk of over-packing the tool.
Thermal Constraints
Cooling rates dictate the effective duration of the high pressure packing phase because the gate must freeze before the screw retracts to ensure no material backflows. Thick wall sections require extended hold times to allow heat dissipation through the cavity walls without creating internal shrinkage. Thin components solidify rapidly, which demands a short duration to prevent unnecessary stress concentration near the injection point.
Settings remain dependent on the specific thermal conductivity of the resin and the geometry of the runner system.
Material Economics
Regrind ratios alter the viscosity of the melt, which forces a modification of the high pressure packing phase settings compared to virgin resin performance. Processors adjust the hold time to account for the changes in crystallization behavior and flow resistance presented by a specific percentage of recycled content. Efficient production relies on finding a stable processing window that maintains dimensional tolerances across different material batches.
Proper control of this phase minimizes scrap rates and stabilizes part mass consistency over long production cycles.
