Issue link: https://www.e-digitaleditions.com/i/1101841
Tablets & Capsules April 2019 11 developing on the metal surface of the elements, which can then dry and flake off, leading to surface defects on the tablets and contamination of the batch. Uniform movement of the tablet bed is always an advantage, even for very different filling levels. In Tablet feeding The tablet feeding process transfers uncoated tab- lets from a storage container to the tablet coater drum. An optimized feeding process should be quick but gentle enough to prevent damage to the tablets. Pneumatic transfer processes should be avoided because they subject uncoated tablets to excessive mechanical stress and damage, making them a high risk to good manufacturing practice (GMP). Manufacturers use many types of containers to supply uncoated tablets to coaters. Whichever type of feeding container you use, ensure that the container lifting unit and transition chute are customized to suit the coater's feed opening. When planning a new coating line, con- sider the tablets' angle of flow during feeding and the available height of the installation space. Uncoated tab- lets with poor flow characteristics may require a flow-aid mechanism to ensure complete tablet discharge from the feeding container. For tablets containing highly potent active ingredients, the coater feeding process is subject to a high risk of contamination. Such applications require containment valves and enclosed lines between the feeding container and coater to contain any dust generated during feeding. The coating process Once the tablets are loaded into the coating drum, the coating process can begin. Tablet coating consists of three sub-processes, or steps: mixing, spraying, and dry- ing. During film coating, the coater executes each of these steps simultaneously; during sugar coating, there are usually separate cycles for adding the sugar solution, coating and mixing the tablet cores, and drying. Mixing. Thorough mixing is a basic requirement for tablet coating. To ensure an even surface quality through- out the batch, the coater must expose all the tablets to the spray guns with a similar frequency so that they receive approximately the same amount of coating solution. To achieve this, coater manufacturers incorporate baffles on the inside of the drum that act as mixing elements. When developing a coater, the manufacturer carefully shapes these elements and adapts them to the drum geometry, so they create a gentle mixing action in the tablet bed as the drum rotates. While older coaters were often equipped with large, sometimes height-adjustable, replaceable mixing ele- ments, the mixing elements on modern coaters are sig- nificantly lower and fixed, as shown in Figure 1. These lower mixing elements provide a more uniform tablet-bed surface and prevent a wave-like, tumbling action in the tablet bed. The uniform tablet-bed surface prevents tab- lets from being sprayed at different distances from the spray nozzles, which can excessively moisten tablets at close distances or cause the spray droplets to dry in the air at greater distances. Because the mixing elements are continuously covered by the tablet bed, they are not directly exposed to the coating spray. This prevents a partial film coating from Figure 1 Coating drum mixing elements past and present b. 2018 a. 1990 Figure 2 Mixing elements specifically designed to create a uniform tablet-bed surface and prevent low- or no-flow recesses in the drum