Transient Interfacial Macro Gap Prediction in Variotherm Injection Tooling
Coupled thermo-viscoelastic modeling of PVT shrink and tool expansion predicts variotherm macro gap formation and prevents optical distortion.
Volumetric Shrinkage Kinetics
Meaning
Physical measurement of the rate and magnitude of the volume reduction that occurs as a polymer melt cools and solidifies within a mould. Understanding volumetric shrinkage kinetics is essential for designing cavities that produce parts within the required dimensional tolerances. The change in volume is driven by the transition from a disordered melt to a more ordered crystalline or glassy state.
Pressure Influence
High packing pressures are used to force more material into the cavity to compensate for the contracting volume. During the hold phase, volumetric shrinkage kinetics determine how much additional resin must be injected to prevent sink marks. If the gate freezes too early, the pressure cannot be maintained and the part will shrink excessively.
Thermal Effect
Cooling rates significantly impact the final density of semi-crystalline materials. Slow cooling allows more time for the polymer chains to organize, which increases the effect of volumetric shrinkage kinetics on the final part size. Fast cooling might trap the material in a less dense state, leading to post production dimensional changes as the part reaches equilibrium.
Moulders use temperature control units to manage these rates and ensure consistency across a production shift. The relationship between pressure and volume across varying temperatures is mapped using PVT diagrams to predict these changes accurately.
Tooling Correction
Mould designers apply a shrinkage factor to the cavity dimensions based on the expected behavior of the specific resin grade. Accurate data on volumetric shrinkage kinetics prevents the need for expensive recutting of the steel tool after the first trial.