Issue link: https://www.e-digitaleditions.com/i/962670
Tablets & Capsules April 2018 13 uniformity. If the guns are aimed too low, the coated tablets may not have enough time to tumble and dry before other tablets tumble on top of them. This can cause over-wetting of the tablet bed and potentially lead to coating defects such as sticking and picking. After the spray-gun angle and direction is set, ensure that the guns are the correct distance from the tablet bed. For most aqueous coating solutions, Freund-Vector recommends a distance of 8 to 10 inches (203.2 to 254.0 millimeters) between the spray-gun nozzle and the tablet bed. For processes using solvent-based solutions, that distance should be decreased to 6 to 8 inches (152.4 to 203.2 millimeters) to compensate for the solvent's increased evaporation rate. In either case, locating the spray guns too close to the tablet bed can cause spray blowback, which will decrease your yields. The next step is to adjust the guns' atomization and pattern air. The spray guns use high-pressure air to atomize the coating solution into fine droplets during spraying. The amount of air pressure determines the droplet size—higher atomizing air pressure results in smaller droplets, and lower atomizing air pressure results in larger droplets. The amount of atomizing air you should use depends on the type of coating solution you're applying. High- viscosity solutions require higher atomizing air volumes than lower viscosity solutions to achieve a similar droplet size. If your atomization air is too high, the droplets will be smaller, and a higher percentage of the liquid in the droplet will evaporate before the droplet reaches the tablet surface. This can cause the tablet surface to appear rough and is referred to as "spray drying" or "orange peel." If your atomization pressure is too low, the droplets may be too large and might not have enough time to spread out across the tablet surface and dry before coming into contact with another tablet, which can lead to sticking and picking problems. The goal is to create a uniform droplet that will spread over the tablet surface and dry without sticking to other tablets. Once you've achieved the proper droplet size, increase the guns' pattern air to create a uniform spray curtain across the tablet bed. For multiple-gun coaters, the spray from each spray gun should be a uniform fan without any overlapping between guns, as shown in Figure 2a. If the pattern air pressure is too low, you'll create dead spots in the spray zone where the coating spray won't reach the tablets (Figure 2b). If the pattern air is too high, the spray pattern will overlap between adjacent spray guns, which can cause streaking (Figure 2c). The simplest way to test a spray gun's atomization and spray pattern is to remove the spray bar from the machine and spray some solution onto a large piece of paper while matching the distance between the spray nozzles and the tablet bed in your coater. This will allow you to check the droplet size and see if there are any dead zones or overlapping spray patterns.