Coating at any scale involves the understanding of the interactions between the product being coated, the film coating formulation, the equipment installation and the processing parameters. When a film coating process is transferred and scaled from the laboratory to production equipment many factors must be well thought-out.
The variable inputs to a film coating process derived from differences in equipment installations include but are not limited to :
1. air volume
2. inlet air temperature
3. humidity control
4. nozzle orifice size
5. numbers of spray guns
6. the distance from spray gun to spray gun
7. the distance of the spray guns to the tablets
8. Spray rate
9. Atomizing air pressure
10. the coating pan dimensions
11. Rotating speed of pan
The spray rate is an significant parameter since it impacts the moisture content of the formed coating and the quality and uniformity of the film. The spray can not be too low or too high. It’s also depend on the viscosity of the coating solution. A low coating liquid spray rate causes incomplete coalescence of polymer due to insufficient wetting, which could effect in brittle films. A high coating liquid spray rate may result in over wetting of the tablet surface and subsequent problems such as picking and sticking. If the spray rate is high and the tablet surface temperature is low, films are not formed during the spraying but the post drying phase, and rapid drying often produces cracks in the films.
Atomizing air pressure
In general, increasing the spraying air pressure decreases the surface roughness of coated tablets and produces denser and thinner films. If spraying air pressure is excessive, the spray loss is great, the formed droplets are very fine and could spray-dry before reaching the tablet bed, resulting in inadequate droplet spreading and coalescence. If spraying air pressure is inadequate, the film thickness and thickness variation are greater possibly due to change in the film density and smaller spray loss. In addition, with low spraying air pressure big droplets could locally over wet the tablet surface and cause tablets to stick to each other.
Inlet air temperature
The inlet air temperature affects the drying efficiency (water evaporation) of the coating pan and the uniformity of coatings. High inlet air temperature increases the drying efficiency of the aqueous film coating process and a decrease in the water penetration into the tablet core decreases the core tablet porosity, tensile strength and residual moisture content of coated tablets. Too much air temperature increases the premature drying of the spray during application and, subsequently, decreases the coating efficiency. Measuring the pan air temperature helps to manage the optimum conditions during the coating process and, consequently, enables predicting possible drying or over wetting problems which may result in poor appearance of the film or may have unfavorable effects on the moisture and heat sensitive tablet cores.
Rotating speed of pan
Increasing the rotating speed of the pan improves the mixing of tablets. The pan speed affects the time the tablets spend on the spraying zone and the homogeneous distribution of the coating solution on the surface of each tablet throughout the batch. Increasing the pan speed decreases the thickness variation and increase the uniformity of coatings. But please be attention, too much rotating speed of the pan will cause the tablet to undergo unnecessary attrition and breakage. Hardness of tablet should be appropriate with the speed.