Coatings are applied to pharmaceutical tablets for a number of reasons ranging from aesthetic purposes to providing protection of the tablet cores from the environment or controlling the release profiles and ultimately bioavailability of the drug (i.e. functional coatings). Usually these coatings are sprayed onto tablet batches contained within horizontal rotating pans. A successfully coated tablet batch will have a prescribed coating thickness on each tablet's surface with little inter- and intra-tablet variability.

In this paper the inter-tablet coating variability in a horizontal axis pan coater is investigated using the discrete element method. Measurements of the fractional spray zone residence time (fSZRT), defined as the ratio of time spent by a tablet in the spray zone to the total coating time are made for three tablet batch sizes and four pan rotational speeds. The average fSZRT is found to be independent of pan speed and total coating time for spherical tablets and is equal to the ratio of the number of tablets exposed to the spray to the total number of tablets, a result that is consistent with analyses. Increasing fSZRT by increasing the number of tablets exposed to the spray or reducing the tablet load decreases the time required to achieve a specified average coating mass and decreases inter-tablet coating variability. The simulations also show that the tablets are randomly mixed in the investigated coating pan and thus the relative standard deviation of the tablet coating mass is inversely proportional to the square root of the number of coating “trials”. Analyses are also presented for predicting the time required to achieve a minimum coating mass for a specified fraction of the tablets as a function of the process parameters.