Design of Polymer Melt Extrusion Dies to Avoid Non-Uniformity of Flow

Abstract

A well designed annular extrusion die used in the manufacture of tubular film, pipe or cable covering, should provide a tube of melt that is uniform thermally, geometrically and kinematically. The mandrel of such a die must be supported in some way, either by a spider in an ‘in-line’ die or by a flow deflector in a ‘cross-head’ die. There may be two problems associated with the subsequent disturbance to the flow; firstly weld lines are formed in the extrudate where the flow separates then rejoins around the spider legs or flow deflector (1) as shown in figures 1 and 2. The weld introduces an area of weakness, which may cause premature failure in pressurised pipe. By using a compression zone in the die exit region it is possible to minimize the effect, depending on the residence time of the melt in the compression region. Another method of reducing the weld line effect is to use mixing devices in the die such as breaker plates, spreader plates (2) or spiral spreaders.