Issue link: https://www.e-digitaleditions.com/i/292644
gauge, which was kept at the machine's side so operators could quickly determine whether they had the right punch head and profile. The moral of the story: Understand the importance of configuring the tablet press correctly and use the right tools to verify your setup. What to check Today, we can do better than a go/no-go gauge. But first, everyone working on a press should know the critical dimensions of a tablet compression tool—be it a punch or die—and understand how they can affect tablet quality. On a rotary tablet press, the "working length" of the punch is the most critical dimension. It is defined as the distance from the head flat to the lowest measureable area of the punch cup (Figure 1). The right length helps ensure consistent tablet hardness, weight, and thickness, which are the fundamental physical attributes we track to ensure tablet quality. Naturally, that means establishing procedures to inspect in-process tooling. It's also impor- tant to inspect new tools, typically by spot-checking them against the matching report that should accompany every tooling shipment. Make sure that each punch is within the allowable tolerance: 0.002 inch across a set of upper or lower tools and no more than a 15 percent varia- tion in the cup depth. When inspecting in-process tools, check for die-bore wear. If the bore is too worn, it can lead to excessive tablet flashing, capping, and laminating. If you didn't know about checking the die bore for wear, you're not alone. Until recently, it was difficult if not impossible to inspect this area because the tools for doing so were either ineffective or unaffordable to many people. Tra - ditionally, an optical comparator was the tool of choice for conducting quick and simple checks for punch-tip wear, head profiles, surface details of tips, etc. But a com- parator only shows a silhouette of the die bore and does not measure wear. Another traditional tool is the micrometer, but it doesn't allow you to check tip wear properly because it only measures the largest area of the punch tip. A micrometer can also measure the tip diameter of new tooling, but it should not be used on in-process tooling because it doesn't effectively measure small variations in diameter caused by wear. A better option is to use an instrument dedicated to the task, such as a tooling condition monitor [1]. In addi- tion to determining the extent of die-bore wear, the instrument can measure the clearance between the punch tip and bore, which corresponds to die-bore wear. Exces - sive clearance between the punch tip and the die bore typically leads to tablet flashing. You should also examine the punch tips themselves for wear. Is the punch cup in good condition? Look closely for evidence of a j-hook, which can cause picking (Figure 2). Other critical di men - sions that affect tablet quality are found on the tablet press itself. The pressure rollers, ejec- tion cams, fill cam, and punch retainers all play an integral role in en - suring tablet quality. Furthermore, these are all wear parts, and you and your co-workers should understand that their service life de - pends on how you run your press and what you run on it. The more you run the press and the more aggressive the products, the more frequently you must inspect. Some of these components require removal from the press for inspection, but with others you can check for signs of wear or damage while they are on the machine. A pressure roll, for example, can be inspected for run-out using a dial indicator fixed to a magnetic base. Run-out refers to the degree of inconsistency on the surface as a part rotates and it can be checked by applying the indica- tor to the roller's face and spinning it by hand, as illus- trated in the photos (next page). When run-out exceeds the specified tolerance, the likelihood of premature tool- ing wear grows, and that leads to inconsistent tablet qual- ity, typically in thickness and hardness. 34 April 2014 Tablets & Capsules Figure 1 Working length is the most critical dimension. Cut depth Working length Head flat Overall length Figure 2 A j-hook can cause picking. J-hook g-Natoliart_33-35_Masters 4/3/14 2:40 PM Page 34