Meaning
Viscosity calculation equations combine shear rate dependence and thermal expansion behavior to model polymer melt flow during injection moulding. Implementation of the cross wvf model fits non-Newtonian melt viscosity across wide temperature and pressure ranges, incorporating the Williams-Landel-Ferry equation to describe glass transition behavior near solidifying temperatures. Accuracy breaks down above extreme shear rates where viscous dissipation generates local thermal gradients uncaptured by standard fitting parameters.
Mold filling simulation software relies on these rheological constants to predict cavity pressure drop, clamping force requirements and gate freeze timing. Resin suppliers supply these parameters derived from capillary rheometry data across multiple temperature scans.
Rheological Fitting
Zero shear viscosity and transition shear stress parameters define the upper plateau and shear thinning slope of the polymer melt. Material testing fits experimental flow curves across three decade ranges of shear rate. Correct parameter selection prevents underestimating pressure drops through narrow cold runner gates.
Temperature Dependence
Shift factors move viscosity curves along the rate axis as melt temperature varies during cavity filling. The WLF formulation tracks rapid viscosity increases near the glass transition temperature, capturing the sharp resistance rise during skin layer freezing.
Simulation Accuracy
Accurate viscosity models prevent miscalculating fill time and clamp tonnage requirements during mold design. Simulated injection pressures match sensor readings when rheological parameters reflect actual batch resin properties.
