Velocity profiles of the exit region of molten polyethylene extrudates

The objective of the work described herein is the experimental investigation of the velocity field of polymer melts flowing through a capillary in the regons of flow prior to and after the capillary exit. The fluids studied are branched polyethylene melts in steady laminar isothermal flow. The technique employed for the determination of the Eulerian velocity profiles is one that utilizes phototomicrogroaphy of the reflected light from tracer particles dispersed in the flowing medium. Axial acceleration of the fluid elements just before the capillary exit was observed. It was found that this accelearation is more pronounced in melts of low bulk viscosity. This observation region, non‐viscometric. The translation of the velocity profiles of the fluids studied, from one resembling a parabola to that of “plug” flow, involves inflection points with minima in the velocity vector v (r, z). These minima appear near the surface of the extrudates and can not be accounted for by an existing theory. It was also found that the density of the viscoelastic fluids studied is a function of the axial position, in the region of flow investigated. The density decreases before the exit and, before it reaches an equilibuiu value at an axial position downstream equal to one or two diameters, increases beyond that value upon exit. This phenomenon is attributed to an “overshoot” in the process fo elastic recoil of the high polymer melts fron a strained structure to a random one.