Meaning
Equations of state that relate the specific volume of a polymer melt to its temperature and pressure provide the thermodynamic foundation for injection moulding process simulations. The modified tait equation approximates this behavior across both molten and solid states. Molders use these calculated volumes to predict how the polymer contracts as it cools inside the mold cavity.
Accurately modeling this transition prevents incorrect shrinkage predictions.
Mathematical Formulation
Multiple empirical parameters are fitted to experimental pressure-volume-temperature data for each specific resin grade. The mathematical formulation of the modified tait equation includes a transition temperature that shifts linearly with pressure. This transition separates the liquid region from the solid or glassy region.
Different parameter sets are applied on each side of this boundary to capture the sudden density change during solidification.
Process Simulation
Flow analysis software uses these density calculations to determine pack and hold times. The process simulation of the modified tait equation determines the rate of polymer flow required to compensate for thermal contraction. If the hold pressure is removed too early, the part develops sink marks.
Proper calculations ensure consistent part weight.
Volumetric Calculation
Shrinkage predictions depend on these volumetric outputs. Accurate equations prevent molding defects. Cavity pressure remains controlled.
