Measurement of the wall slip behaviour of a solid granular soap in ram extrusion

16 Jan 2018

The wall slip behaviour of a solid granular soap was investigated in the context of ram extrusion, with particular focus on determining the sensitivity of the wall shear stress to the pressure within the soap, as well as the slip velocity. Two novel slip measurement devices were used to infer the shear stress: a two stage extrusion die with inbuilt pressure gradient measurement; and a combined compression-translation assembly to measure the frictional force between the soap and the wall. Both devices provided differing measurements of the pressure sensitivity, and gave shear stress estimates in agreement with a Benbow and Bridgwater (1993) analysis of the extrusion behaviour, which cannot gauge pressure dependence. The influence of the wall material on the slip and extrusion behaviour was also investigated, using three geometrically identical extruders constructed from polycarbonate, stainless steel and tungsten carbide. There was found to be a non-negligible relationship between wall material, wall surface roughness, and the Benbow-Bridgwater extrusion parameters, in which the wall shear stress was greatest against the smoothest, cemented tungsten carbide wall in contrast to a rougher stainless steel and rougher-still polycarbonate wall. Keywords: extrusion, wall slip, soft solids