Understanding the correlations between tablet flow dynamics and coating uniformity in a pan coater: Experiments and simulations

Coating uniformity is crucial for pharmaceutical solid dosages to ensure sufficient stability and to program drug release behavior. The present study aims to thoroughly investigate the correlation between the coating uniformity and the tablet flow dynamics with a novel computational model. The model was developed on discrete element method (DEM) coupled with the ray-tracing algorithm and was validated with experiments. Firstly, tablet flow dynamics and patterns were fully recorded and clearly categorized. By increasing the peripheral speed of the coating pan, the tablet flow pattern distinctly transited from slumping, rolling, cascading to cataracting, and the coefficient of variation to inter-tablet coating weight (CVI) was significantly reduced from 1.9 to 1.0, indicating the coating uniformity dramatically increased. But further increasing the peripheral speed, the tablet pattern approached the centrifuging status and the CVI value was reversely increased to 2.0, suggesting a reduction in uniformity. Interestingly, huge speed variations of the individual tablet (0–100 cm/s) occurred even in the same tablet pattern, this allowed an active tablet region for guiding the location of the coating nozzles. These results were consistent within the scale-up pan coaters with different transition values. Additionally, the case study showed that properly adjusting the shape and location of baffles could effectively accomplish the coating pattern (rolling and cascading) at a slower peripheral speed. These findings suggest that tablet flow dynamics has a strong impact on the coating uniformity and needs to be carefully manipulated with useful strategies such as tuning the operating parameters or applying internal baffles. The mathematic model developed by the current study has paved a new way to comprehensively understand the tablet coating mechanics.