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