Polyamide Cavity Pressure Influence on Moulded Shrinkage Rates
Increasing peak cavity pressure compresses polyamide melt during solidification, directly counteracting phase-change volumetric shrinkage and tightening component tolerances.
Tait Equation of State
Meaning
Density modelling provides the mathematical link between pressure and volume for amorphous polymers above the glass transition temperature. The tait equation of state characterizes this relationship through a specific semi-empirical function that adjusts for high hydrostatic compression levels. It defines the volume reduction of a material when subject to force, preventing unrealistic density predictions at the extreme pressures typical of injection mould cavities.
Processing Dynamics
Shrinkage control depends on accurate knowledge of how the specific volume of a resin behaves during the cooling cycle. The tait equation of state enables injection moulding software to calculate the density changes as melt flows through the gate and occupies the cavity volume. Designers use these calculations to predict part dimensions and weight before metal is cut for the mould.
Excessive gate freeze off or premature packing pressures occur when the software relies on inaccurate parameters for the polymer under compression. Deviations in these values force technicians to adjust holding time or melt temperature to compensate for unexpected volumetric contraction, increasing the production cost of every shot.
Material Specification
Resin suppliers publish coefficients for this model as part of the thermal properties provided on a technical datasheet. Moulders check these constants to ensure the simulation software accurately represents the actual performance of the plastic under load. A discrepancy between the values in the simulation and the actual material response leads to discrepancies between part prints and the physical moulded dimensions.
Virgin material maintains a predictable response based on these constants, whereas regrind alters the base polymer density and changes the way the material responds to pack pressure.
Operational Performance
Tooling engineers require precision when setting the pressure profile for complex geometries with thin sections and deep ribs. The tait equation of state dictates the required clamp force by predicting the total volume occupied by the compressed melt. An overestimation of the final part density results in excessive pressure application that risks flashing the parting line or damaging the mold core.
Accurate simulation of these volume changes prevents premature tool wear and ensures the final component maintains the required structural integrity.