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14 May/June 2021 Tablets & Capsules continuous coater with a target weight gain of 3 to 4 percent w/w and achieved a throughput rate or 130 to 200 kg/h. The most important aspect of coating uniformity is mass uniformity (that is, the amount, or thickness, of the coating applied) rather than color uniformity. It is often possible to achieve good color uniformity (with no visible difference in color from tablet to tablet) without achieving good mass uniformity. An example of this conflict is shown in Table 3. These data show that, while color uniformity is exceptional, mass uniformity is much less so. Regardless of the coating type, achieving exceptional coating uniformity should always be a critical quality attri- bute (even for cosmetic film coatings), primarily because it: • Ultimately impacts the quality (in the broadest sense possible) of the medication being delivered to the consumer; and • Affects product and process costs—the poorer the coating uniformity, the more coating that may have to be applied (a material cost) and the longer the coating process may have to be (a processing cost). Certain equipment designs are better than others at uniform coating distribution, but many batch tablet coaters in operation today struggle to achieve anything close to good coating uniformity, and RSD values of 15 to 20 per- cent or more are not uncommon, especially as the process scale increases. While many factors affect coating uniformity, one that receives little attention is mixing, or tablet movement in the pan. Tablet movement in a coating pan occurs both axially (in the front-to-back direction) and radially (the turnover of the tablet bed as the pan rotates), both of which are facilitated to some extent by baffles on the pan wall. Prob- lematically, both movement types suffer as scale increases, with the greatest detriment being to radial movement, where a "dead zone" is often created in the middle of the tablet bed. This is because the bed depth increases considerably as scale increases, influencing how often tablets are exposed to the spray zone. To begin, however, it is necessary to define coating uniformity. Two aspects to consider are: • Inter-tablet uniformity, which relates to how uniformly the coating is applied to different tablets; and • Intra-tablet uniformity, which relates to how uniformly the coating is distributed around the surface of each individual tablet. Both characteristics can be affected by tablet shape and size, with the greatest effect being on intra-tablet uniformity, as described by Wilson and Crossman [5]. Cunningham, et al. [6] in discussing continuous film-coat- ing processes, provided examples that show how such processes might improve coating uniformity. One exam- ple addressed color uniformity on tablets coated with a water-soluble tablet coating applied at 20 percent w/w solids, while a second example dealt with improved consistency (in terms of finished product functional characteristics) of a nutraceutical enteric coating applied at 10 percent w/w solids to softgel capsules. The first example used an O'Hara continuous coater with a target weight gain of 3 percent w/w and achieved a throughput rate of 1,100 to 1,300 kg/h. The second example was performed in a Thomas Engineering Table 3 Coating uniformity of tablets coated in an O'Hara FC 350 batch coater containing 300 kilograms of tablets and using a high-solids coating (data courtesy of Ashland) Coating solids (% w/w) Color uniformity (ΔE)* Mass uniformity (% RSD) 20 0.3 17.1 35 0.3 20.5 * ΔE is a color difference measurement calculated from color measurements made on individual tablets. Generally, a ΔE value of less than 2.0 represents a color difference that cannot be distinguished by the human eye. Table 4 Comparison of coating mass uniformity obtained in batch and continuous coaters (data courtesy of Ashland) Parameter O'Hara FC350 batch coater O'Hara FC C500 continuous coater Pan diameter (in) 60 19 Solids (%) 20 35 20 35 Pan load (kg) 300 45 (average at any given time) Coating time (min) 95 50 7 (average residence time) 4 (average residence time) Throughput rate (kg/h) 106* 144* 420 700 Coated tablet weight gain variability (% RSD) 17.09 20.53 16.66 15.96 * Allows for 15 minutes loading, 15 minutes preheating, actual spraying time, 15 minutes cooling, and 30 minutes unloading