Meaning
Injection moulding operators govern volumetric shrinkage through the post-filling phase, a critical window that runs immediately after the gate freezes and holds melt under sustained clamping tonnage. The cycle demands precise hydraulic pressure adjustments to compensate for thermal contraction as amorphous polymers cool inside the cavity. Operators establish this secondary pressure plateau during tool setup based on melt density curves and wall thickness profiles.
A material specification defines inherent polymer flow and shrinkage rates under standard test conditions, whereas a part specification dictates exact dimensional tolerances and mechanical performance required for the final moulded component.
Gate Solidification
Thermal closure of the runner inlet dictates the absolute boundary of the post-filling phase. Melt density inside the core stabilises once this narrow restriction solidifies and cuts off external material feed from the injection unit. Hydraulic pressure applied by the screw no longer influences internal cavity pressure after that physical seal forms.
Regrind economics introduce variability into this transition because thermal degradation lowers melt viscosity and shortens the allowable time window for holding pressure. Part dimensions drift outside acceptable limits if operators fail to account for the shifted solidification timing caused by recycled polymer batches.
Dimensional Stability
Volumetric contraction accelerates during the post-filling phase as cooling progresses from the cold tool walls toward the hot centre of the rib sections. Insufficient holding pressure during this interval produces sink marks, internal voids and excessive warpage in semi-crystalline thermoplastics. Moulders verify process stability by comparing nominal datasheet values against actual measurements taken from components produced across a continuous production run.
Machine capability limits and barrel temperature fluctuations prevent operators from holding the exact nominal pressure curve on every single cycle, which leads to minor batch-to-batch variations in finished part weight.
Clamp Control
Clamping tonnage must counteract the internal cavity pressures generated during the post-filling phase to prevent parting line flash and protect tool faces from excessive wear. Process engineers program proportional valves to ramp down hydraulic output gradually rather than dropping pressure instantly, a technique that prevents sudden density shocks in high-flow engineering resins. Correctly tuned pressure profiles eliminate residual stresses that otherwise compromise the impact strength and fatigue life of structural components.
