Holding Pressure Profile

Melt Density

Compensating for the volumetric shrinkage of polymers requires a steady and managed flow of pressurized melt into the mold cavity. A well-designed holding pressure profile maintains the density of the molded part by forcing additional resin to flow into the cavity to fill the voids created by cooling. Because polymers shrink at different rates depending on their crystallinity, the profile must be customized for each material.

Semi-crystalline resins require higher pressure and longer duration than amorphous polymers to achieve equivalent part weight. This step is particularly critical when molding thick-walled components where the core remains molten long after the skin has frozen. Process engineers use melt density calculations to determine the exact mass of polymer required to prevent internal shrinkage voids.

Defect Prevention

Proper application of pressure stages prevents common molding problems such as sink marks, voids, and excessive internal stress. If the holding pressure profile is set too high or held for too long, the area near the gate becomes over-packed, causing high residual stresses that lead to warping or cracking. Conversely, terminating the pressure prematurely allows the pressurized melt to flow back out of the cavity, resulting in hollow voids.

Machine Setup

Setting up this parameter on modern injection molding machines involves dividing the holding time into multiple steps of varying pressure levels. Process technicians configure the holding pressure profile on the machine controller by defining the pressure and duration for each step. This multi-step adjustment ensures a gradual decay of pressure as the gate freezes, preventing stress concentrations.

The profile represents a part-specific setting that cannot be copied directly from one mold to another.