A comparison of ram extrusion by single-holed and multi-holed dies for extrusion–spheronisation of microcrystalline-based pastes

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The use of multi-holed dies as an alternative to single-holed dies for generating extrudates for spheronisation was investigated both in terms of extrusion and spheronisation performance. A model 45 wt% microcrystalline cellulose (MCC)/water paste was employed in ram extrusion tests with square-ended dies with 1, 6, 33 and 137 holes, all of diameter 1 mm and length 2 mm. The extrudates generated using the multi-holed dies yielded pellets with comparable sphericity to those using the single-holed die. Multi-holed dies could also be operated with lower paste flow rates before encountering liquid phase migration (LPM). The characteristic processing velocity for the onset of LPM was determined for each die configuration and supported the hypothesis that LPM was caused by suction effects. A simple model of the flow pattern in a lab-scale Fuji-Paudal frontal screen extruder is presented which yields estimates of velocities and shear rates involved in these devices. The pressure required to extrude the paste through multi-holed dies was compared with the model proposed by Benbow and co-workers. The paste rheology was characterised using the Benbow–Bridgwater approach, employing 1, 2 and 3 mm diameter dies of various lengths. The Benbow et al. model under-predicted the observed extrusion pressure, which was attributed to its failure to account for the redundant work contribution in these complex flows.

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