Dynamic Pack Pressure

Pressure Profile

The holding phase of the injection cycle relies on step-down pressure curves rather than a constant hold. As the outer layers of the polymer freeze against the cold steel of the mould, the required force to push additional material decreases. Dynamic pack pressure lowers the hydraulic or electric force in steps to match this growing resistance.

This prevents the region near the gate from becoming over-stressed and brittle. It also saves energy by reducing pump demand when the cavity is mostly solid.

Shrinkage Control

Thermoplastics experience high volumetric contraction as they transform from liquid melt to solid parts. If the dynamic pack pressure is too low, the core of the thick sections will pull away from the walls, creating sink marks or internal voids. Moulders must hold the pressure high enough to force extra melt into the shrinking core until the gate is completely frozen.

Once the gate is solid, further packing pressure is ineffective and only increases cycle time.

Internal Stress

High residual stresses are frozen into the part when holding pressures are kept too high for too long. This molecular stress leads to premature part failure, warpage, or cracking when the component is subjected to mechanical load. By utilizing a dynamic profile, the moulder can taper off the pressure as the polymer cools, allowing the molecules to relax before the part is ejected from the mould.

This relaxation is particularly important for polycarbonate and other amorphous resins that are prone to stress-induced cracking. Proper profiling also prevents the part from sticking to the cavity walls, which would otherwise lead to ejector pins punching through the warm material during ejection.