Effect of Peak Cavity Pressure on Unreinforced Polyamide Linear Shrinkage Rates
Elevating peak cavity pressure compresses unreinforced polyamide melt, suppressing specific volume and reducing post-moulding linear shrinkage rates.
PVT Behavior
Meaning
The relationship between the specific volume, pressure, and temperature of a polymer governs how the material behaves as it transitions from a hot melt to a cold solid. Understanding this pvt behavior is necessary for predicting volumetric shrinkage and determining the correct holding pressure during the injection molding process. Since polymers compress under pressure and expand when heated, this thermal relationship dictates the amount of plastic needed to fill a cavity.
Mold design software relies on these material curves to simulate mold cooling and part ejection.
Phase Transition
As the temperature of the melt drops, the specific volume decreases, causing the polymer to contract inside the tool. In amorphous resins, this contraction occurs gradually as the material cools toward its glass transition temperature. Semi-crystalline resins, however, undergo a rapid change in specific volume during the crystallization phase.
This sharp change makes semi-crystalline materials more sensitive to cooling rates and pressure control.
Shrinkage Prediction
Holding pressure is used to compensate for this natural volume reduction. If the pressure is released before the gate freezes, the polymer will flow back out of the cavity, altering the pvt behavior and increasing the final shrinkage. This backflow results in undersized parts and internal voids.
Tooling Design
Mould designers utilize these volume-pressure-temperature curves to calculate the correct steel dimensions of the mold cavity to achieve the target part sizes. When the pvt behavior is neglected, the mold cavity might be cut too small, requiring expensive tool modifications. This behavior is also affected by the cooling circuit layout, which must maintain uniform temperatures across the tool face.
Proper cavity cooling ensures that the shrinkage is uniform, minimizing part warp.