Extrudate fracture and spheronisation of microcrystalline cellulose pastes

An experimental study of the ram extrusion and subsequent spheronisation of water-based microcrystalline cellulose (MCC) pastes has been performed. The effects of extrusion velocity, die geometry, and paste water content on the integrity of the extrudate produced were investigated. The apparent severity of extrudate fracture increased with decreasing die length/diameter ratio (L/D), and increasing extrusion velocity. The spacing between the fractures was approximately constant for a given set of process conditions, and increased with increasing emph{L/D} and decreasing extrusion velocity, whilst the flare of the extrudate fracture segment increased with decreasing emph{L/D} and increasing extrusion velocity. Two types of fracture shape were identified, emph{viz}. knuckle-bones and cups, the occurrence of which is explained in terms of the relative extent of the radial and axial strain release at the surface of the extrudate upon exiting the die. The shape and size distributions of the corresponding spheronised pellets were analysed and related to the geometry of the extrudate fracture segments. Severe knuckle-bone fracture gave spherical pellets with a wide size distribution. Visibly smooth extrudates gave non-spherical pellets with an apparently narrow size distribution (as measured by sieving). Extrudates displaying the cup type of fracture generated the best quality pellets in terms of both sphericity and size distribution.