The linear properties and nonlinear shear viscosity provide good insight into the viscoelastic character of the material being extruded. Also, if the nonlinear shear viscosity is not available, it can often be estimated using the Cox-Merz rule [4] or the Gleissle mirror relationship [6] (see Empirical Rules and Principles for details). When feasible, this set of experimental data can be usefully completed with first normal-stress difference data. For most materials, measurement of linear properties for angular frequencies ranging from 0.01 to 100 rad/s , or perhaps up to 1000 rad/s is achievable. If you consider a typical wall shear rate in the extrusion process, it is usually included within this range of measurements. Otherwise, extrapolation should be considered.

For 3D flows, it is practical to select a “computationally light" rheological model, to reduce the computational cost of the Ansys Polyflow simulation. Hence, it is useful to identify a typical shear rate for the flow, and obtain viscometric data one decade on each side of this shear rate. Doing this implicitly reinforces the idea of a constitutive equation for a flow (rather than for a fluid). Consequently the fitted model for a given shear-rate decade will probably not be the best one for another shear-rate decade.