Polymer Melt Flow Behavior in High Shear Injection Molds
High shear molding relies on pseudoplastic shear thinning, requiring capillary rheometry over melt flow index to control viscous heating and pressure losses.
Cross-WMM Model
Meaning
Rheological viscosity equations define melt flow resistance across shear rates in injection moulding simulations. The cross-wmm model combines the Cross viscosity equation with Williams-Landel-Ferry temperature dependence to calculate non-Newtonian melt behavior during fill operations. Standard datasheets quote single-point melt flow index values that fail to capture shear sensitivity during cavity filling.
Numerical flow software uses this mathematical relationship to calculate local viscosity variations from thin gate shear fields to wide cavity core regions.
Rheological Formulation
Mathematical parameters inside constitutive viscosity equations establish how polymer chains disentangle under applied shear stress. Zero-shear viscosity defines the plateau at minimal shear rates, while the power-law index captures structural thinning during high-velocity cavity injection. The cross-wmm model incorporates a temperature shift factor that scales the relaxation time of molten resin across processing windows.
Shift parameters derived from capillary rheometry data allow numerical solvers to maintain convergence across non-isothermal boundary layers during dynamic filling simulations.
Viscosity Prediction
Accurate shear-thinning calculations prevent misjudging injection pressure requirements in thin-wall plastic parts. When numerical software relies on simplified power-law representations, pressure predictions in low-shear regions deviate from actual machine transducer readings. Applying the cross-wmm model reveals shear heating effects inside narrow runner channels before molten plastic reaches the cavity.
Moulding engineers rely on these viscosity predictions to set velocity profiles that avoid polymer degradation.
Tooling Calibration
Flow simulation software requires empirical rheometer data to construct valid mathematical coefficients for specific resin grades. Fitting the cross-wmm model to lot-specific capillary data prevents underestimating pressure drops across long production runs.