Abstract
Extrusion of solid-liquid particulate pastes is a well-established process in industry for continuously forming products of defined cross-sectional shape. At low extrusion velocities, the solids and liquid phases can separate due to drainage of liquid through the interparticle pores, termed liquid phase migration (LPM). The effect of wall friction, die shape and extrusion speed on LPM in a cylindrically axisymmetric ram extruder is investigated using a two-dimensional finite element model of paste extrusion based on soil mechanics principles (modified Cam-Clay). This extends the smooth walled model reported by Patel et al. (2007) to incorporate a simplified Tresca wall friction condition. Three die entry angles (90°, 60° and 45°) and two extrusion speeds are considered. The extrusion pressure is predicted to increase with the Tresca friction factor and the extent of LPM is predicted to increase with decreasing ram speed (both as expected). The effects of wall friction on LPM are shown to be dictated by the die shape and ram displacement: there are few general rules relating extruder design and operating conditions to extent of LPM, so that finite element-based simulation is likely to be needed to predict the onset of LPM accurately.
Highlights
Particulate pastes are used widely to manufacture products such as agrochemicals, pharmaceuticals, ceramic parts, mortar and solder pastes using techniques including ram extrusion, screw extrusion and injection moulding (Wilson and Rough, 2012)
Liquid phase migration (LPM) was only observed at the smaller ram speed, as was observed in the absence of wall friction
The decrease in PeL with ram speed eventually exceeds the increase in PeS at z⁄ ! 0.32. These results demonstrate that LPM causes significant variation in Pe via competing effects on PeL and PeS, and that these changes are coupled to the die shape, friction factor and current ram displacement
Summary
Particulate pastes are used widely to manufacture products such as agrochemicals, pharmaceuticals, ceramic parts, mortar and solder pastes using techniques including ram extrusion, screw extrusion and injection moulding (Wilson and Rough, 2012). Some pastes are viscoplastic, reflecting the use of a highly viscous binder or (less frequently) a rate-dependent particulate matrix (Mascia and Wilson, 2007). Others, such as mortar pastes, reflect ‘frictional’ rheology more reminiscent of dry particulate assemblies (Perrot et al, 2006b). In the latter case, the rheology is dominated by friction at the interparticle contacts as opposed to viscous shear in the liquid binder.
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