An activated rate theory approach to the hydrostatic extrusion of polymers

Abstract

An analysis of the mechanics of the hydrostatic extrusion process for polymers is presented, in which the predicted extrusion pressure is considered to be influenced by the effects of strain, strain rate and pressure on the material flow stress, as well as by the billet—die friction. The extrusion behaviour of both crystalline and amorphous polymers is discussed with reference to experimental results for linear polyethylene, polyoxymethylene and polymethylmethacrylate. Particular attention is paid to the method of incorporating the flow behaviour of the polymer into the analysis. A modified form of the Eyring equation for an activated rate process is proposed, in which the effects of strain rate and pressure on the flow stress are assumed to be separable, but related to strain by the large strain dependence of the stress activation volume. Moreover, a direct equivalence between the pressure effect and the friction between the polymer and the die is proposed for hydrostatic extrusion, following previous work on the adhesive mechanism for friction in polymers. This results in a formally identical analysis for both crystalline and amorphous polymers, in which the strain rate sensitivity, pressure sensitivity and friction coefficients all increase markedly with material strain during the process.