Cooling and Wall Thickness Uniformity in Plastic Pipe Manufacture

A mathematical model of cooling and solidification, coupled with gravity induced flow during the cooling of extruded plastic pipe is set up and solved by the finite element method. Detailed and comprehensive experimental data are obtained from a commercial extrusion line producing 315 mm O.D. SDR 11 MDPE pipe using vacuum sizing and external spray cooling, including temperatures measured within the pipe wall using embedded thermocouples, which are carried down the entire cooling line. The computer simulations are successfully fitted to experimental temperatures by adjusting transfer coefficients for the outer surface. The deduced values are: tank (1) (vacuum) 750 W/m2 K; (2) 200 W/m2 K; (3) 150 W/m2 K. Heat loss to the bore is shown to be negligible. Predicted final pipe wall thicknesses – starting with a concentric extrudate – agree within 4 % with measured values. Wall thickness variation is predicted to be 16.6 %, compared with 13.9 % measured.