A theory of die‐swell

Abstract An elastic‐fluid theory of die‐swell for long dies is presented. The theory predicts a swelling ratio D e / d asymptotically proportional, for large values of swelling, to the cube root of the recoverable shear evaluated at the die wall. Here the recoverable shear is defined to be half the ratio (first normal stress difference/shear stress). Excellent agreement is shown between predicted D e / d and measured D e / d for experiments on melts and solutions for which adequate data are available.