Die swell

Die swell occurs in instances of polymer extrusion, in which a stream of polymeric material is forced through a die, a specialized tool in manufacturing to shape or cut polymeric materials.

[1] Die swell is a phenomenon directly related to entropy and the relaxation of the polymer within the flow stream.

As the polymer spends time inside the die and is subject to the much increased flow rate, the polymers lose the spherical shape, becoming longer due to the increased flow rate.

Physical entanglements may relax, if the time scale of the polymer within the die is long enough.

When the polymer stream leaves the die, the remaining physical entanglements cause the polymers in the die stream to regain a portion of its former shape and spherical volume, in order to return to the roughly spherical conformation that maximizes entropy.

Pre-entrance die: slow flow rate polymers have a roughly spherical conformation..Narrow die: flow rate increases, polymers begin to length in response. Exiting die: Return to slow flow rate. Polymers regain spherical conformation.