Investigation of static zones and wall slip through sequential ram extrusion of contrasting micro-crystalline cellulose-based pastes

The shape and size of static regions in the extrusion flow of a viscoplastic material through axisymmetric square entry dies was studied experimentally using a simple flow field analysis technique. A white microcrystalline cellulose/water paste was extruded sequentially with a graphite-stained black analogue paste, revealing static regions in the corners of the system extending less than 30% of the barrel radius into the centre of the channel. Slow (relative to the bulk) slip flow along the die face was evident in the experiments, implying the existence of a stick-slip transition on this surface. Finite element simulations using the Papanastasiou regularised Bingham plastic viscosity model and the linear Navier wall slip law were found to reproduce the experimental results poorly, capturing only separately the slip dominated bulk flow and the static corner region for different values of the slip coefficient beta. The effect of severe wall slip was to reduce the distinction between unyielded and yielded material making the definition of the yield surface challenging. Investigation and implementation of a stick-slip wall condition is believed to be required for further work in this area. (C) 2014 Elsevier B.V. All rights reserved.