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


Type

Article

Change log

Authors

Bryan, MP 
Rough, SL 
Wilson, DI 

Abstract

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 micro-crystalline 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 _. 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.

Description

Keywords

Viscoplastic fluid, Micro-crystalline cellulose, Ram extrusion, Wall slip, Static zone, Yield surface

Journal Title

Journal of Non-Newtonian Fluid Mechanics

Conference Name

Journal ISSN

0377-0257
1873-2631

Volume Title

220

Publisher

Elsevier BV