As a first comment, it is worth noting that kinematic constraints imposed in the process do not really enable a full development of viscoelastic effects.

Several modeling approaches can be considered. The constant-viscosity Newtonian and generalized Newtonian models are the simplest approaches, and these are recommended when only the viscosity data is available. The generalized Newtonian fluid model is an obvious choice, because shear is an important component of the flow.

In addition to the Newtonian and generalized Newtonian models, differential viscoelastic models (Maxwell, Oldroyd-B, PTT, Giesekus, FENE-P, DCPP, and Leonov) are all good candidates for pressing simulations. Generally, the White-Metzner model is not recommended, because the rheological properties are dependent on the local kinematics in this model. Although multi-mode simulations are available, single mode transient viscoelastic calculations are recommended (at least in first instance) because of the computational cost in Ansys Polyflow.

The flow involves a typical time scale corresponding to the pressing. The relaxation time should be of the same order of magnitude as this time scale. For the viscosity factor, select a value corresponding to the shear viscosity, as obtained for a shear rate that is typical for the process.

For a strongly strain-hardening material (for example, LDPE), you can use the Maxwell or Oldroyd-B model, as long as the strain rate remains relatively low and therefore prevents the development of high stresses. Alternatively, if you use the PTT or the Giesekus model, you should consider specifying a low value for or , respectively: typically, between 10^-3 to 10^-2. You can also use the DCPP model with a large number of branches (). For filled materials, the Leonov model could be recommended, although it is computationally expensive.

You can use data on the elongational viscosity if it is available. If the resulting values for the nonlinear parameters are not in agreement with the expected behavior of the melt, they can be fixed as noted in Assigning a Value to a Parameter.