Rheological behavior of polymer melts in monodimensional flow through low-angle convergence and spiralling dies

An experimental study was carried out to examine the rheological behavior o f polymer melts in laminar flows through channels with small-angle converge nces (2-3 degrees) and spiralling walls (6 degrees /mm along the flow direc tion), using an extrusion grade of low-density polyethylene. The results sh ow that convergence is the main factor responsible for the observed deviati ons from steady state laminar shear flow, particularly with respect to addi tional pressure requirements to maintain a constant mass output. The swell ratio, calculated from the dimensions of the channel at the exit, was found to be considerably higher for flow-through converging channels. Only a sma ll increase in swell ratio could be attributed, on the other hand, to the r otational elements of the die configuration. It is inferred that even for s mall angles of convergence the increased level of swelling is associated wi th elongational stress components, which were grouped together and denoted as the "additional stress." This was obtained from the difference between t he calculated average shear stress at the wall, using the recorded pressure at the die entry, and the value of the average shear stress at the wall ca lculated from the shear viscosity data for the melt. (C) 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 99-107, 2001.