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Tablets & Capsules January 2017 11 To achieve consistent weights, you should run powders that are formulated to cohere into slugs. You must also set the tamping stations correctly. Here's a good starting point, which can be fine-tuned as needed for your powder: Set the last—station five—so the pins are level with the top of the dosing disc. Then set each preceding station 2 to 3 millime- ters deeper. See Table 1. This setup is effective for most product types, but not when the powder doesn't compress into a slug. In those cases, try using a thicker dosing disc and applying less com- pressive force, which may reduce weight variation caused by "splash-out," where powder squeezes out of the bore past the pin during compression. The changes, however, won't produce a clean-cut slug, so weight variation is still likely at the transfer station and during capsule closing. Material The formulation of the fill material is always important, and although preparing a formulation to fill capsules is much simpler than preparing one to make tablets, many people still over-formulate for capsules. The most common mistake is adding too much flow agent, such as silicon diox- ide. A small amount is effective when it's used sparingly to distribute powder within the bowl, but adding more than necessary creates a problem that's difficult to reverse. Operators often complain that the powder begins flowing like water inside the bowl, at which point it will no longer compress. Instead, it moves past the periphery of the tamp- ing pin during penetration instead of remaining in the bore (spalsh-out). A better option to over-lubricating is retrofitting the bowl with one or more attachments—whisks, diverters, or a cone—to force the powder into the area it belongs. The attachments allow you to overcome powder flow issues and avoid the risk of creating a product that won't compress, which leads to weight variation. Another way to fight weight variation on a tamping- style machine involves the wiper block at the powder trans- fer station. The wiper block's function is to clean the disc before the slugs are transferred into the capsule bodies. Directly after the wiper block is station one, where com- pression starts. Because of this station's proximity to the wiper block, the product bed is often uneven on the trailing side of station one (photo). In some cases, raising the tamp- ing pins of station one all the way—in effect not using that station—will prevent weight variation at the onset. Only the four remaining tamping stations are used to achieve the proper weight. Machine Machine readiness is the variable you control the most. Make sure the capsule filler is in prime condition to run capsules. Pay special attention to three major components: the tamping pins, tamping springs, and dosing disc. Tamping pins. When new, the pins should have a mirror finish and their working end, the tip, should be squared off 90 degrees to vertical. The mirror finish prevents product from adhering, and the precisely machined tip prevents product from escaping from the disc bore during compres- sion. When the pins become worn and/or damaged, replace them. Tamping springs. The springs are located above the tamping pins to provide both support and relief to the pins during compaction. If you're over-compressing, the springs may break, which will lead to poor compaction and the weight of the powder slugs will vary. Dosing disc. The bores of the dosing disc often get damaged by tamping pins that are misaligned at setup or by a worn guide ring that allows the pins to move laterally (side-to-side). If weight variation stems from a damaged or worn dos- ing disc, there's an easy way to pinpoint the problem dur- ing a production run. As shown in Figure 3, most tamping- style machines have 12 lower segments mounted to the index table and six stations on the dosing disc. Because there are twice as many segment stations, each location on the dosing disc corresponds to two segments 180 degrees apart. As shown in the diagram, location F on the dosing disc transfers product into segments four and 10, while location E transfers into stations three and nine, and so on. If the dosing disc has a damaged bore at location B, it will affect both segment six and segment 12. To identify Table 1 A simple method of setting tamping pins Station Penetration 5 Level with disc 4 3 mm 3 6 mm 2 9 mm 1 12 mm Disc wiped clean Compression begins Powder recovery zone The wiper block precedes station one, where slug compression begins. That arrangement can lead to an uneven product bed on the trailing side of the station. You can resolve the problem by bypassing the first station.