Meaning
Secondary stage pressure regulation controls the force applied to the melt after the cavity has been filled to prevent the plastic from flowing back out. This program is the holding pressure curve, which is set to compress the polymer as it shrinks during the initial cooling phase. It operates until the gate freezes, preventing voids and sink marks.
Optimized hold times directly influence the weight and dimensional precision of the final product.
Shrinkage Compensation
As plastic cools, it shrinks and requires additional material to be packed into the cavity to maintain dimensions. The holding pressure curve adjusts the pressure over time to match this volumetric contraction. Multi-stage approach prevents overpacking near the gate while ensuring sufficient pressure reaches the far end of the part.
Gate Solidification
The duration of the holding stage must exceed the time required for the polymer in the gate to freeze completely. Once the gate is frozen, changes in the holding pressure curve no longer affect the part, and the cycle can transition to the cooling phase. Measuring part weight at different holding times reveals the exact moment of gate freeze.
Residual Stress
High pressure maintained for too long after gate solidification does not add material but increases internal stress. An optimized holding pressure curve minimizes this residual stress by reducing pressure as solidification progresses. Pressure adjustments prevent warpage and reduce the tendency of the part to crack during assembly.
