Tablets remain the dominant oral dosage form because they combine accurate dosing, very high stability and acceptance, and cost-efficient, high-throughput manufacturing. Tablet coating or film coating further expands this platform by adding functional performance (e.g., enteric protection, modified release), stability (light/moisture barrier), and patient-centric attributes (taste masking, smoother swallowing, visual identification).
Why Coating Uniformity Matters (and What It Is Not)
As coating functions become more complex, e.g., multi-layer systems, combination products, or individualised dosing concepts, uniformity becomes a critical quality requirement. Variations in coating thickness can translate into variability in release, taste masking, or barrier performance; QbD-based approaches (Quality-by-design) have been shown to improve coating uniformity using structured experimentation and analytics.
Uniformity is commonly quantified as RSD (relative standard deviation) of coating thickness or coating mass. Importantly, colour difference (ΔE) is not equivalent to coating uniformity; ΔE indicates perceptible colour differences and should not be used as a substitute for thickness/mass uniformity. High ΔE values describe a noticeable colour difference. The Optimal Process: Mixing, Spraying, and Drying Simultaneously Modern drum coating is essentially the controlled coupling of three sub-processes:
Mixing / Tablet Motion:
It is essential that the tablet cores move smoothly and gently under the spray cones. The cores must not be subjected to excessive mechanical stress to prevent damage.
For over 20 years, L.B. Bohle (Ennigerloh, Germany) has used an enlarged coating drum with welded-in mixing spirals (length/ diameter ratio >1) to great success. These spirals ensure continuous and gentle mixing of the tablet bed. Homogeneous mixing is achieved within minutes and maintained throughout the process. The flat tablet bed reduces mechanical stress. Due to the continuous guidance of the mixing spirals, the tablets are not subject to strong acceleration. Consequently, tablet breakage and twinning are prevented.
Spraying:
The geometry of the optimal drum in a tablet coater creates a large spray area within the moving tablet bed. This enables the use of more spray nozzles than would be possible with shorter drums, resulting in a larger total spray area and a higher spray rate. As well as the coating suspension, the type of nozzle, the number of nozzles, and the distance between them are particularly important factors.
Several solutions for adjusting the distance between the nozzles are available, e.g. the spray angle and the atomisation pressure.
Typically, the amount of suspension mass in film-coated tablets is 5–15% of the core mass. Film thickness is particularly important, not only for active ingredient coatings, but also for thin colour (protective) coatings, as it must be uniform. Uneven film application within a batch can result in colour variations that degrade product quality or lead to poor compliance. Compared to conventional tablet coaters with L/D ratios of less than 1, systems with enlarged drums allow process times to be up to 40% shorter due to higher spray rates.
