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Tablets & Capsules May/June 2021 9 weight gain often necessitates slower than optimal tablet feed rates. Fortunately, this dilemma has essentially been resolved by the introduction of high-solids coatings (up to 35 percent w/w solids). For example, recent coating trials using an O'Hara FCC 500 continuous coater (nominal throughput rate of 500 to 600 kg/h) achieved a throughput rate of only about 250 kg/h using a traditional HPMC-based coating applied at 12 percent w/w solids but achieved a throughput rate of approximately 700 kg/h using a newer, high-solids coating (Aquarius Genesis, Ashland) applied at 35 percent w/w solids without sacrificing coated tablet quality or coating uniformity. Such a result has positive implications for both manufacturing capacity and processing costs. To address the growing interest in fully continuous manu- facturing processes (powder enters at one end and packaged final product emerges from the other end), coating equip- ment suppliers have recently introduced fully continuous coaters that can handle the output from a typical high-speed tableting process (approximately 300,000 to 400,000 tablets per hour). An example of such a pan-coating process is the O'Hara FCC 75, with a typical throughput rate of about 50 to 75 kg/h. Semi-continuous coaters While fully continuous coating equipment was adopted first in the pharmaceutical industry, the introduction of semi-continuous coaters has provoked interest. The Koco coater was an original concept introduced by LL Bohle consisting of several coating pans lined up in series. More recently, the company has adopted a single-pan approach that uses a similar design but employs a short processing time (discussed later). size, tablet shape, and coating function), and the nature and solids content of the coating formulation. Fully continuous coaters Initially, continuous film-coating processes were based on the concept of an elongated side-vented coating pan, as shown in Figure 2, where uncoated tablets are continuously fed into the rotating pan at one end, pass beneath a bank of spray guns, and emerge completely coated at the other end, with a typical process dwell time of about 15 minutes. Potential advantages to this concept include: • High throughput rates (up to 1,000 to 2,000 kg/h) compared to a large-volume batch coater (typically 300 to 500 kilograms in 2 to 3 hours); • Less in-process material handling (batch loading and unloading, reduced product warm-up and cool- down times); • Reduced product exposure to stressful process con- ditions (heat, moisture, and mechanical duress)— typically 15 to 20 minutes compared to 1 to 3 hours for a batch process; • Improved coating uniformity (discussed further later); • Smaller equipment footprints in manufacturing areas; and • Lower manufacturing costs [2]. However, the advantage of shorter dwell times cre- ates challenges with respect to the coating formulation design. While the possibility of achieving improved coat- ing uniformity potentially allows for lower coating levels, a residence time inside the coating pan of only about 15 minutes means that, with traditional coating formulations (typically hydroxypropyl methylcellulose [HPMC] based, applied at 12 percent w/w solids), achieving the required Figure 1 Current concepts for continuous coating processes a. Fully continuous process c. Rapid-turnaround batch process b. Semi-continuous process Figure 2 Schematic of a typical continuous lm-coating process (courtesy of Freund Vector) Supply air handler Air in Main unit Control panel Rotating drum Product handling options Powder applicator Exhaust air duct Charge conveyor/hopper Inlet air ducts Dust collector Exhaust fan Solution pump Solution tank