Unique design principles assure a good coating uniformity

L.B. Bohle has developed and improved its coater design in the past few years to achieve good coating uniformity

Pharmaceutical film coating represents nowadays an important process step in the pharmaceutical industry. Derived originally from dragee pan processing, coating technology has developed continuously in the past few decades. The majority of coating processes are performed for drug release modification, drug stability improvement against light or moisture and taste masking. Furthermore patient compliance issues play an important role, as swallowability improvement or a simpler identification due to a different colour.

Finally API coating is gaining more and more importance since it enables fixed dose combinations or the combination of incompatible drugs. Also different drug release characteristics can be realised by applying for example sustained release coatings in addition to immediate release coating layers. Such formulations sometimes consist of up to four coating layers, which leads to long processing times.

To successfully develop and produce such formulations, coating uniformity is a prerequisite and a quality attribute, since coated tablets have to pass the test on uniformity of dosage units according to the pharmacopoeias1,2. As a coating process consists of simultaneous spraying, mixing and drying processes, a good coating uniformity can only be achieved with the choice of the proper parameters. To meet these requirements L.B. Bohle developed and improved its coater design in recent years.

The Bohle Coating Technology incorporates three unique design principles that assure a good coating uniformity.3

Bohle pan geometry with an L/D ratio >1 offers a large tablet bed surface, which allows the incorporation of a high number of spray guns (see Figure 1). Compared with conventional pan geometries that can be found in the market, process time can be reduced by up to 40% due to larger throughput of coating suspension. Moreover, the thinner tablet bed induces a minimum shear onto the tablets, which allows the coating of even very weak tablets.

Figure 1: Bohle pan geometry vs conventional coating geometry

The unique helical baffles (patented) consist of two layers of baffles (see Figure 2). They are responsible for the continuous and homogeneous axial mixing within the tablet bed. In addition the drum rotation maintains the radial mixing. Both movements are a guarantee for a dead zone free tablet bed. Thus a homogeneous mixture in the tablet bed is usually achieved within a few minutes. Due to the constant tablet movement the tablets do not experience any acceleration peaks which could cause tablet damage or even twinning.

Figure 2: Bohle helical baffle design

The air principle in Bohle Film Coaters utilises the drying capacity where it is most needed: in the tablet bed (see Figure 3). Thus the drying capacity of the inlet air is used most effectively without heating the rest of the coater inner parts. The inlet air comes from below the tablet bed and is directly sucked through the rotating tablet bed into the exhaust air funnel. This set-up offers another advantage: The spray guns are not being heated during coating and remain cool. Therefore spray losses are reduced to a minimum, which leads to coating efficiencies of >95% which is especially beneficial for API coating processes.

Figure 3: Bohle air principle

Besides these unique features that make the Bohle Coating Technology so successful, the coating machines can be equipped with CIP systems. At high pressure values the cleaning takes place with a cleaning lance which effectively cleans the coater after production in an automated mode.

The Bohle coating equipment is available form lab scale to large production scale. Furthermore different configuration lines are available to meet different customer needs on all continents. Besides the classical batch mode machine types, semi continuous coaters are also available.


1. European Pharmacopoeia, Monograph 2.9.40. Uniformity of dosage units, 7th ed., Directorate for the Quality of Medicines of the Council of Europe, Strasbourg, 2011.

2. United States Pharmacopoeia, Monograph <905> Uniformity of Dosage Units, 35th ed., U.S. Pharmacopoeial Convention, Rockville, 2011

3. Optimization of inter-tablet coating uniformity in an active coating process, Just S. et al., Poster presentation AAPS Annual Meeting, Chicago 2012

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