Computer simulation of steady-state pipe extrusion

Extruded pipe is made by forcing a molten polymer through an annular die and cooling it; the final properties of the pipe depend on a number of variables, including in particular the cooling step, during which crystallization takes places. An accurate prediction of temperature profile and crystallinity distribution within a semi-crystalline polymer (polypropylene) is of importance in determining the processing conditions. In this study the solidification of the melt during cooling in pipe extrusion has been analyzed by means of a computer simulation. The mathematical model, which involves a steady-state nonlinear energy equation incorporating the crystallization kinetics proposed by Nakamura, simulates the behavior of polymer melt in the cooling chamber during formation of a thick walled pipe. An investigation was carried out into effects of melt inlet temperature, take-off speed, and pipe wall thickness on the temperature profile as well as crystallinity distribution. NACHOS II, a computer program based on finite element method designed for solution of two-dimensional fluid dynamics and heat transfer, is employed for the numerical solution. The numerical model gives therefore a basis for the prediction of the crystallinity distribution and adjustment of the operation conditions.