Issue link: https://www.e-digitaleditions.com/i/492796
Tablets & Capsules April 2015 23 so it covers a larger portion of the tablet bed. The higher the air pres- sure, the flatter the cone. How to adjust. Figure 1 illustrates some common spray patterns. Which pattern you use depends on the num- ber of guns, their spacing across the boom or manifold, the gun-to-bed distance, and other factors. Estab - lishing correct pattern-air settings requires some trial and error, but there are a couple general rules. First, avoid overlapping the sprays from each gun. While some droplets will always travel outside the pattern, there should not be a hard overlap. Second, as the number of guns in creases, the need for pattern air decreases. What can go wrong. Excessive pattern air makes the cone too flat, which can lead to over-wetting and thus to tablet picking and/or twin- ning (photos). Too little pattern air will prevent the spray from fully cov- ering the tablet bed, which can lead to erosion and picking. The best practice is to make no assumptions when it comes to pat- A successful aqueous film coating operation—one that produces high- quality tablets—requires the correct interaction of 10 or more variables, also known as critical operating para- meters (Table 1). But one parameter, pattern air, is often misunderstood, given token recognition, or over- looked. The lapse isn't academic. On the production floor, I see manufacturing batch records that fail to cite it or that provide only a range for the set- ting. In other cases, the pattern-air setting isn't correctly matched to the atomizing-air set point. Yet other times it makes perfect sense to omit pattern air from the record because the equipment doesn't allow you to adjust it (photos). What it does. When it can be adjusted, pattern air works in tandem with atomizing air. Both are supplied through the spray gun's air cap, and you adjust both by varying the air pressure, typically expressed as pounds per square inch, bar, or kilo- grams per square centimeter. As the name implies, pattern air shapes the spray, mostly by flattening its cone tern air. Verify the setting you're using or plan to use. If either picking or twinning becomes a problem and you're hard pressed to pinpoint the cause, check the spray gun configura- tion and the pattern-air settings and verify that the sprays don't overlap. Getting pattern air set correctly doesn't take long and can improve tablet coating quality immediately. T&C Fred A. Rowley is the owner and chief lecturer at Solid Dosage Training and Troubleshooting, 497 Phoenix Circle, Vallejo, CA. 94589. Tel. 925 352 5724. Website: www.soliddosageindia. com. He is also an editorial advisor to Tablets & Capsules. For further reading You'll find more information about the operation and troubleshooting of tablet coating systems from Rowley and other experts listed under "Coat - ing" in Tablets & Capsules' article index in the November 2014 issue or at T&C's website, www.tabletscapsules. com. Tech tip: Understanding pattern air Table 1 Critical operating parameters for film coating operations Air volume Pan pressure Inlet-air temperature Humidity Pattern air Gun-to-gun distance Gun-to-bed distance Spray rate Atomizing air pressure Pan rotation speed Figure 1 Common spray patterns Neither the traditional spray manifold (left) nor the modern one allows you to adjust the pattern air Overlapped sprays can cause over wetting and lead to picking (left) and twinning. A B C D E Fred A. Rowley Solid Dosage Training and Troubleshooting