Velocity profiling inside a ram extruder using magnetic resonance (MR) techniques

The extrusion of soft-solid materials is an important industrial process. Magnetic resonance imaging (MRI) has been used to acquire in situ velocity profiles during the ram extrusion of two opaque materials representative of those often extruded in industry; a mixture of 5 cSt poly-dimethylsiloxane (PDMS) added to a commercial soap (DoveTMTM) (a soft solid), and a stiff biscuit dough (a paste). Radial velocity profiles have been acquired in both the axial and radial directions at different positions within the barrel (25 mm diameter) and die land, during extrusion through two 8 mm diameter cylindrical dies (length 8 and 16 mm), and through a multi-holed die. At a height of 24 mm above the die entrance, both materials exhibit plug flow with pure slip at the barrel wall. Nearer to the die, the flow has converged towards the centre of the barrel and static zones are observed at the barrel walls. The shape of the velocity profiles was found to be independent of ram speed (0.6–1.2 mm/s), and die length. The shape of the velocity profiles is material dependent, with greater shear effects evident in the biscuit dough and compression effects evident in the soap material.