There are several viscoelastic models available in Ansys Polymat, as described in detail in Material Data Parameters. These models involve linear and nonlinear parameters, which in turn carry viscometric and extensional properties. It can often be a difficult task to select the best constitutive equation with the most appropriate material parameters. The task can be more easily addressed if it is broken into three smaller questions: how many modes, which constitutive equation, and which parameter settings. These questions implicitly assume that everything is known about the material being modeled, and that all properties are equally important, which is usually not the case. In actual industrial practice, however, only some of the information is known, and assumptions will therefore be necessary.

The purpose of this section is to suggest useful guidelines for the selection of a constitutive model and associated parameters. Two strategies are possible: you can evaluate the numerical values of parameters in order to match some experimental data in a given range, or you can try to fit all viscometric (and possibly elongational) measured data over a broad range. The two approaches lead more or less to the selection of a rheological model for a flow and the selection of a rheological model for a fluid, respectively.

Although you may prefer one of these two approaches, useful guidelines can be found in both of them. Therefore, the guidelines that follow will be presented on the basis of the flow being simulated. For example, the kinematics involved in profile extrusion is significantly different from that in blow molding or thermoforming. Indeed, swelling during extrusion results from a velocity rearrangement and normal-stress difference developed in a shear flow, while blow molding involves an elongational component with a strain-hardening or strain-thinning response from the melt.

Recommendations will be given for the four most-commonly encountered types of flow: extrusion (Guidelines for Extrusion), fiber spinning (Guidelines for Fiber Spinning), film casting (Guidelines for Film Casting), and blow molding/thermoforming processes (Guidelines for Blow Molding and Thermoforming).