  Extrusion is a widely used process for forming pastes into designed shapes,
and is central to the manufacture of many industrial products. The extrusion
through a square-entry die of a model paste of non-Brownian spheres suspended
in a Newtonian fluid is investigated using discrete element simulations,
capturing individual particle contacts and hydrodynamic interactions. The
simulations reveal inhomogeneous velocity and stress distributions, originating
in the inherent microstructure formed by the constituent particles. Such
features are shown to be relevant to generic paste extrusion behaviour, such as
die swell. The pressure drop across the extruder is correlated with the
extrudate velocity using the Benbow-Bridgwater equation, with the empirical
parameters being linked directly to particle properties such as surface
friction, and processing conditions such as extruder wall roughness. Our model
and results bring recent advances in suspension rheology into an industrial
setting, laying foundations for future model development, paste formulation and
extrusion design.
