Analysis and improvement of die design for the processing of extruded plastic pipes

Basket dies, also known as screen pack dies, have been widely used in centre-fed extrusion of plastic pipes as an alternative to conventional spider-supported pipe dies. The aim of introducing basket dies was to eliminate the production problem of ‘weld-lines’ caused when using spider pipe dies. However, a phenomenon has been observed experimentally when processing LDPE and HDPE using basket dies; there are obvious striations running the whole length of the finished thin-walled pipe, resulting in an undesired surface appearance. This work will demonstrate the analysis and comparison of the performance of these two pipe dies. Computer simulations of polymer melt through the dies were performed using the commercially available finite element package, FIDAP®. A non-isothermal flow of polymer melt with the temperature-dependent Carreau model for the fluid flow properties has been studied. Through analysis of the simulation results, the phenomenon is explained and the stagnation zones are predicted within the basket die. Experimental study on the samples produced using basket dies provided some validation of the simulation results. A solution to improve the surface appearance of the products is also proposed.