For the purposes of this chapter, a film is a sheet of polymer with a thickness that is several orders of magnitude smaller than its other dimensions. It is also assumed that the film is flat, so that effects due to film curvature are ignored.

Consider the problem of modeling the flow between a die exit and a take-up roll, where entrance and exit effects are ignored. The problem is truly three-dimensional, since the film will show necking along lateral sides as well as a thickness variation along the principal direction of stretching. However, because of the very small thickness, a 3D model would not be feasible: it would require a very large mesh, which would result in a costly computation. Instead, a special 2D model has been developed in Ansys Polyflow. This model can be compared to the planar stress approximation in elasticity, which reduces 3D problems to 2D.