A Spectral Element Simulation of Gravitational Flow During Plastic Pipe Extrusion

Abstract

When plastic pipe is extruded, it emerges from an annular die and passes through a sizing sleeve to set its outer diameter. The pipe is then solidified in a cooling tank by spraying the outer surface with cold water. For thick walled pipe, material on the inside stays molten for a long time, and flows under its own weight. This gravitational flow (or sag) therefore governs the final pipe wall thickness distribution. Thus the solidification of extruded plastic pipe involves heat transfer with phase-change coupled with gravitational flow. The inside thermal boundary condition is arguably adiabatic. For the outside boundary, one can use either Newton’s law of cooling or an isothermal condition. In this paper, a commercially available spectral element code for simulating unsteady incompressible fluid flow with heat transfer, has been used to simulate sag flow. The model predictions of the solid pipe thickness distribution compared well with process data. Also, the effects of different heat transfer parameters on the thickness profile are evaluated. The extrusion temperature is found to have the greatest effect on the pipe wall profile.