Dynamic Cavity Pressure Integral Correlation with Part Dimensions in Injection Moulding
Dynamic cavity pressure curve integration over hold time governs volumetric polymer density and predicts final part dimensions before tool ejection occurs.
Amorphous Polymer Contraction
Meaning
Volumetric shrinkage occurs when a molten polymer cools and transitions from a disordered liquid state to a solid state with reduced specific volume. In non-crystalline materials, amorphous polymer contraction represents this uniform reduction in volume during solidification, which lacks the sudden density transition seen in semi-crystalline resins. The contraction is governed by the gradual decrease in free volume between polymer chains as the melt temperature drops below the glass transition point.
Thermal Behavior
Cooling rates determine the final density of the moulded part because rapid cooling traps the polymer in a state of higher free volume. When amorphous polymer contraction is restricted by cold mold walls, internal stresses develop within the part. These residual stresses often lead to post-moulding warpage or dimensional instability over time.
Dimensional Impact
Tooling designers compensate for amorphous polymer contraction by making the cavity slightly larger than the target part dimensions. Because amorphous polymers shrink less and more isotropically than semi-crystalline resins, they allow for tighter dimensional tolerances in the finished product. Uncompensated contraction leads to sink marks in thick sections of the part.
Process Control
Injection pressure resists the natural volumetric reduction of the material by forcing additional melt into the cavity during the holding phase. This pack stage offsets amorphous polymer contraction by maintaining high density until the gate freezes. Ineffective pressure transmission results in hollow voids inside the thickest sections of the part.
If the holding pressure is removed before gate solidification occurs, the pressurized polymer can flow backward out of the cavity, increasing the final contraction rate.