Investigation of differences in mechanisms of die filling between a compaction simulator and a rotary press

Die Filling is the critical process step in tableting as it determines the tablet weight and its variability as well as impacting tablet strength and defect propensity. Several studies have focused on modeling die filling on rotary presses, however none have investigated the matter on a compaction simulator. Therefore, the aim of this study is to characterize the die filling behavior on a compaction simulator and compare it to a laboratory scale rotary press. Special attention is paid to the complex interplay of process parameters, machine geometry and material properties. Experimental results are supported by a newly introduced physics-based calculation of the course of the exerted differential pressure as a main driver of die filling. On the compaction simulator, suction filling is shown to be more intense due to its geometry and elevated lower punch velocities, rendering paddle speed of the feed frame less crucial. On the rotary press, paddle rotation is necessary to ensure sufficient powder flow into the dies, especially at high production speed, due to a shorter filling time. An alternative fill cam geometry, where the punch is already pulled down to a certain extent before entering the feed frame, reduces the exerted suction pressure in the filling zone, giving generally lower filling yield for materials of limited flowability. The study offers a solid understanding of die filling on a compaction simulator and the underlying mechanisms. Together with the comparative experiments, the foundation for a model for rational scale transfer towards rotary presses is established.