Flow Behavior in Contractions

There are three phenomena associated with the flow behavior of polymeric fluids which must be considered in the design of extrusion dies pressure drops in contractions (or expansions), die swell, and melt fracture. The latter two bear a direct relation to extrudate uniformity, while the flow behavior in contractions may be related only indirectly to extrudate uniformity. In this section, for illustrative purposes, we present results based primarily on studies in the capillary geometry. One must recognize that the extension of results 

As far as expansions are concerned, there is even less information. We assume that the pressure rise in an abrupt expansion is the same as the drop in an abrupt contraction. Certainly this will not be the same because of the viscoelastic nature of polymeric fluids. 

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