An Evaluation of Expressions Predicting Die Swell

Die swell measurements for polystyrene samples of narrow molecular weight distribution and their blends have been made using a capillary viscometer. For the capillaries studied, the swelling ratios were nearly independent of L/D (length/diameter) but strongly dependent on the distribution of molecular weights. The broader the distribution, the larger the swelling ratio for the same extrusion pressure. The recoverable shear at the wall, which is defined to be half the ratio of the first normal stress difference over the shear stress, was estimated using Graessley's correlation between experimental and Rouse shear compliance. The results were compared to predictions from the theories of Nakajima and Shida, Bagley and Duffey, Graessley et al., and Tanner.