Function of upstream and downstream conveying elements in wet granulation processes within a twin screw extruder

• Downstream element to kneading block influences size, shape and strength of granules.
• Larger flight pitch results in larger sized granules.
• Upstream element has no significant effect on exiting granules.
• Increasing flow rate improves stability of the granulation process.

Understanding the function of individual screw elements in twin screw wet granulation has become an area of intense study but has yet to contemplate the interactions of adjacent elements. The present work examines the influence of conveying elements around the critical compression zone created by a kneading block. Granulated samples were prepared using nine different screw configurations with flight pitches of 20, 30 and 40 mm and tested at two different degrees of channel fill utilizing a placebo formulation of 20 wt% microcrystalline cellulose in α-lactose monohydrate. Samples were taken directly from the upstream and downstream conveying elements as well as the machine exit and analyzed for particle size, shape, apparent porosity and fracture strength. The results showed that the particle size, shape, and fracture strength of granules produced were significantly influenced by the pitch of downstream conveying elements while upstream conveying elements had no effect on exiting particles. Both zones were found to be insensitive to the degree of fill, though the variance in measurements decreased at higher fill. Similar results were observed when the formulation was adjusted to included 15% ibuprofen (though the ratio of lactose to microcrystalline cellulose remained constant).

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