A polymeric extrudate can expand sideways after leaving a die as the normal-stress differences and stored deformation generated upstream relax. Axial elastic recovery contributes to transverse swelling in an incompressible flow. A Newtonian extrudate can also swell through rearrangement of its velocity profile, so observing swelling alone does not isolate the elastic contribution.
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Die swell is a phenomenon that occurs during the processing of polymers, especially during extrusion. When a molten polymer is forced through a die to take a specific shape (such as in the production of pipes, sheets, or films), it often expands or swells as it exits the die. This swelling is primarily due to the relaxation of the polymer chains as they leave the constraints of the die.