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

Abstract

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.

Flow visualisation and modelling of solid soap extrusion

“…A possibility is that the wall slip behaviour of soap is pressure-dependent, in that the increased extrusion pressure in the 215 smaller extruders would act to increase τ W due to enhanced material-wall contact. Pressure-dependent slip is somewhat supported by experiments reported separately(Bryan et al, 2017), though the magnitude of the effect here is larger.This type of slip is not often suggested for fluid-like materials but it is possible that soap behaves more like a solid in this context, which typically show normal 220 stress-dependent friction. Also possible is that some difference in the surface-material interaction affects the τ W values in the three extruders.…”

“…Also possible is that some difference in the surface-material interaction affects the τ W values in the three extruders. It was not possible to study this further in the context of the present work, thoughBryan et al (2017) does present data from extruders constructed of different materials and with different surface finishes, finding that this can affect τ W measurements by a similar magnitude.While table 2 shows that there is substantial variation in Benbow-Bridgwater parameters reported in the literature for materials which are all classed as soap, the current findings indicate that even greater care must be taken when comparing values originating from different geometries. It is also worth repeating…”

“…The parameters for the wall interaction behaviour were determined through separate experiments conducted by the author (Bryan et al, 2017), with values of β = 4.8 × 10 5 Pa/(m/s) 0.34 and n = 0.34 chosen as representative of the results seen experimentally. This model does not include a slip yield stress, viz.…”

Linking fresh paste microstructure, rheology and extrusion characteristics of cementitious binders for 3D printing

“…It can be easily noticed from Figure that τ w is different for the two die sizes used—the larger the L d / D d ratio, the higher the wall shear. This is because the wall slip of cement paste is pressure‐dependent, as has been observed for other similar systems . It is conceivable that the increased extrusion pressures in smaller diameter dies will increase τ W due to higher degrees of particle‐wall contact.…”

Effect of Particle Size and Shape on Wall Slip of Highly Filled Powder Feedstocks for Material Extrusion and Powder Injection Molding

“…Wall slip effect on the extrudability and windability of filaments was recently confirmed by Hasib et al 23 for feedstocks based on polylactic acid and Ni–Cu powders. Furthermore, the relation between yield stress and shear stress at the wall affected by wall slip was investigated in some studies 24–26 ; the wall shear stress higher than the yield stress potentially leads to the phase separation and inhomogeneous print quality. In contrast, wall slip phenomenon might prevent clogging of the nozzle, 27 but binder rich areas at filament walls might not ensure quality of sintered parts.…”