Meaning
Mathematical representation of polymer melt rheology provides the foundation for computer-aided mould flow simulation. The cross-WMS model combines the Cross viscosity equation with the Williams-Landel-Ferry temperature dependence to calculate the viscosity of a thermoplastic material over a wide range of shear rates and temperatures. This formulation is highly valued in injection moulding design because it captures the transition from Newtonian behavior at low shear rates to power-law thinning at high shear rates.
Accurate predictions from this algorithm allow engineers to optimize gate locations before steel is cut.
Equation Variable
Several material parameters must be determined through capillary rheometry to populate the mathematical equations. The cross-WMS model uses coefficients that represent zero-shear viscosity, transition shear stress, and temperature sensitivity. These variables are unique to each specific polymer grade and cannot be generalized across different resin suppliers.
Simulation Accuracy
Design software relies on high-fidelity mathematical approximations to prevent moulding defects like hesitation or air traps. By employing the cross-WMS model, the software simulates how the polymer flows through thin-walled sections of the cavity. This analysis prevents costly re-tooling by identifying injection pressure issues during the early phase of part design.
Process Optimization
Processing conditions depend directly on the thermal and shear limits of the specified plastic resin. Using the cross-WMS model during simulation helps in determining the optimal injection speed and melt temperature. This step ensures that the moulding window is wide enough to tolerate minor variations in regrind usage.
