Cross-WLF viscosity model

The Cross-WLF viscosity model describes the temperature, shear rate, and pressure dependency of the viscosity for thermoplastic materials.

The viscosity model is given by the following equation:

where:

• is the melt viscosity (Pa s)
• is the zero shear viscosity or the 'Newtonian limit' in which the viscosity approaches a constant at very low shear rates,
• is the shear rate (1/s)
• is the critical stress level at the transition to shear thinning, determined by curve fitting, and
• is the power law index in the high shear rate regime, determined by curve fitting.

The zero shear viscosity is given by the equation:

where

• is the temperature (K)
• is the glass transition temperature, determined by curve fitting,
• 
• is the pressure (Pa), and where
• , , , and are data-fitted coefficients.

The glass transition temperature is given by the equation: where is a data-fitted coefficient.