Flow visualisation of polymer melts in abrupt contraction extrusion dies: quantification of melt recirculation and flow patterns

This paper is the first part of a series of three in which we present exper imental and analytical data covering the development of entry profiles, nor mal and shear stress fields, velocity fields and the in-process extensional viscosity measurement of branched and linear polyolefins. Here, we report accurate in-process measurements of the natural flow profiles of selected b ranched and linear polyolefins obtained from screw-driven extrusion flows t hrough an abrupt contraction geometry. Recirculation areas, vortex centres and detachment lengths were quantified and their dependence on flow rate an d process temperature studied. Significant recirculation areas were observe d for the branched melts. Both the size and the development of these featur es were dependent on flow rate and temperature. Recirculation areas of the linear polymers were less prominent and independent of flow conditions. Cor relation has been observed between rheological characteristics of the low d ensity polyethylenes and their entry behaviour. (C) 2000 Elsevier Science B .V. All rights reserved.