We propose a theory to predict the neck-in behaviour of polymer materials in the film casting process. To calculate the neck-in length, we employ the Dobroth-Erwin model 2) which assumes that the film deformation is planar in the central part, and uniaxial at the edge. We derive an equation for the edge line of the film and an expression for the neck-in length. For Newtonian fluid, the neck-in length is shown to be given by L/√2 , where L is the length of the air gap. For viscoelastic fluid, the neck-in length depends on the draw ratio. As an example, the neck-in length is calculated for Maxwell model, and the result is compared with experimental data.
2003 The Society of Rheology, Japan