Stress relaxation in the extrusion of pastes

Stress relaxation following the sudden halt of ram extrusion was studied over a range of die geometries and extrusion speeds using an alumina paste (solids volume fraction, φ=0.62) and a talc paste (φ=0.51). Distinctly different relaxation characteristics were observed. The alumina paste exhibited a rapid decay to give residual stresses in reasonable agreement with a model based on the pseudo-plastic rheological characterisation described by Benbow and Bridgwater. Although similar forces were required to extrude the talc paste, it showed a heavily damped decay with two time constants. The talc paste also continued to extrude slowly after the ram had stopped. This behavior was attributed to differences in φ and the contribution, and dissipation, of liquid pore pressures. However, increasing the solids volume fraction in the talc paste led to behaviour similar to that exhibited by the alumina paste.