Film coating processes can be divided into three basic operations:
- mixing
- spraying
- drying.
Each step is carefully engineered and optimized to coordinate with the others. Bohle Coaters offer the best combination of the processing operations on the market.
Mixing
One essential but basic operation in coating is tablet bed mixing. Each tablet moves uniformly under the spray. This movement, however, must be done carefully and with minimal mechanical stress to avoid damaging delicate tablets. The perfect choice is an elongated coating pan with continuously welded spirals as mixing elements. Spirals guarantee constant and careful mixing. Because of the elongated pan, the flat tablet bed ensures minimum static pressure and very careful handling of sensitive tablets.
Spraying
Apart from the coating liquid, two other aspects must be taken into account. First, the spraying equipment: nozzle type, number of nozzles and nozzle-to-nozzle distance.
Second, the process parameters: nozzle-to-bed distance, angle spray rate, atomizing and pattern air pressure.
Coating nozzles are the key to efficient coating. The adjustable oval spray pattern and so-called “anti-bearding caps” are currently state of the art. Another key factor is the design of the nozzle-to-nozzle distance.
Drying
The Bohle coaters offer the best energy transfer and mass transfer (evaporation) analog to a packed-bed reactor. Once the coating liquid has reached the tablet, the coating should dry quickly and evenly. To ensure rapid drying, supply air is directed through the pan perforations into the tablet bed. Significant parameters include the supply air temperature and humidity, supply air volume and pan perforations. Excessive moisture or spray drying can be prevented with the appropriate settings. Spray drying, in particular, frequently accompanies coating.
The drying performance can be influenced and optimized through the type of air flow in the coater. Directing supply air and exhaust air through the tablet bed reduces turbulence and creates a cold environment in the spraying area. This internal dynamic prevents spray drying.
