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extrudate swell

Extrudate swell off a fluid is another phenomenon which is typical for viscoelastic fluids. The above photos compare the behavior of a Newtonian material with a polymeric fluid exciting from a die. The ratio of the diameter of the jet to the capillary diameter is 113% for the Newtonian case (low Reynolds number) whereas it can be as high as a few 100% for the case of polymeric fluids. This is a very important phenomenon when extruding products. The die-swell will need to be accounted for (think about how you would make a square duct by extrusion). Again the die swell arises mainly because of differences in the normal stresses along and perpendicular to the stream lines in the tube. They relax as the fluid exists from the tube causing a contraction in the longitudinal direction.