Intra-tablet coating variability is the variation of coating mass over the surface of an individual tablet. Exposure times of different surfaces of a tablet (e.g., the cap and bands of a biconvex tablet) are directly related to the orientation of the tablet within the spray zone. We define the orientation index for a biconvex tablet as the ratio of the measured band-to-cap exposure areas to the ideal band-to-cap area ratio, with the ideal being the ratio of the band-to-cap surface areas for the given tablet. Therefore, a value of unity for the orientation index implies zero intra-tablet coating variability. Orientation index values larger than one indicate that more band is exposed to the spray than what is considered ideal, and values less than one indicate that more cap is exposed.

The different rates of coating mass accumulation on the caps and bands of biconvex tablets are examined using video-based techniques. Caps of directly compressed MCC tablets of different cap concavity are painted white. Three three pan speeds for two different biconvex shaped tablets and one flat faced cylindrical tablet are examined. The value of the orientation index measured from the video experiments are correlated to spray coating experiments where tablet weight gain is monitored by periodically sampling tablets and cutting the tablet along different planes to measure the coating film thickness. This goals of the work are to determine how tablet shape and pan speed influence intra-tablet coating variability.