Die Swell and Normal Stresses: An Explanation

Newtonian fluids show a small increase (about 10%) in cross-section diameter on emerging from a capillary at low Reynolds numbers (Re≪1). Highly viscous elastic fluids, such as polymer melts, exhibit a far greater increase, sometimes between 200 and 300%. This phenomenon is often referred to as “Barus effect” or “die swell” and is usually described quantitatively in terms of the swelling ratio (d/D), which is the ratio of extrudate diameter to …