Meaning
Process engineering methodologies establish secondary injection pressure levels and hold times to compensate for volumetric polymer shrinkage during cavity cooling. Mould technicians perform pack pressure optimization to achieve target part weights and dimensional tolerances without causing tool flash or part sticking. Injection moulding machines transition from velocity control to pressure control at gate freeze to pack additional melt into the cooling cavity.
Setting pack pressure too low leaves internal voids while excessive packing induces residual stress.
Process Parameter
Second-stage pressure settings control the rate of melt delivery while the part core remains molten. Systematic pack pressure optimization establishes pressure profiles that maintain density uniformity across varying wall thicknesses, preventing localized sink marks near thick structural ribs. Incremental pressure study curves map part weight against packing duration to pinpoint exact gate freeze-off timing.
Shrinkage Control
Cavity shrinkage in semi-crystalline polymers requires sustained packing pressure to prevent internal void formation. Standard pack pressure optimization protocols for polyoxymethylene and polyamides address volumetric contraction up to two percent during phase transition. Proper pressure application balances wall density and minimizes differential post-mould shrinkage.
Defect Prevention
Optimized packing schedules prevent mold over-packing, reducing ejector pin push defects and mold damage. Machine operators rely on pack pressure optimization to stabilize component dimensions across varying ambient shop temperatures.
