Die Swell and Delayed Die Swell

Abstract

A jet of viscoelastic fluid which is extruded from a pipe can swell to three or four times the diameter of the pipe from which it is extruded. The old name for this is “die swell” or more recently extrudate swell, since the extrudate rather than the die, swells.* The phenomenon of die swell is one important signature of the nonlinearity of flow in non-Newtonian fluids. It is a very important problem in industry because so many processes involve the extrusion of viscoelastic fluids, say plastic in their molten state. Die swell is not well understood. There are a number of approximate approaches and numerical simulations based on assuming different constitutive models. These approaches are well represented in the books of reference and will not be discussed here.