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38 October 2018 Tablets & Capsules T twin-screw feeder, due to its ability to handle relatively cohesive and sticky materials. As a direct result of its operating principle, a LIW feeder requires: 1) a periodic refill to recharge its supply hopper and 2) isolation from the process environment to permit accurate and continuous weighing. This means that, as part of the larger process environment, the LIW feeder must perform a balancing act of sorts. On the one hand, it must connect to and interact with the process to receive and discharge material; on the other hand, it must be isolated from the process to ensure maximum weighing accuracy. In operation, an LIW feeder continually circles the simplified control loop pictured on the left in Figure 2, constantly attempting to drive mass-flow error to zero. The time it takes to complete one loop represents the interval during which the feeder measures weight loss and determines any required adjustment to its speed. Using the simplified control loop as a template to orga- nize the typical locations and causes of feeding problems, the right side of Figure 2 separates feeding problems orig- inating in the feeder from problems originating in the external process. Since a feeder's main mission is to control flowrate, the trend for measured mass flow can indicate a perfor- This article describes how to use a loss-in-weight feeder's trending capabilities to help determine the cause of feeder performance problems. he pharmaceutical industry is undergoing a paradigm shift from batch to continuous processes, including direct compression, blending, wet and dry granulation, coating, and hot-melt extrusion. At the heart of these processes is the loss-in-weight (LIW) feeder, which delivers excipi- ents, active pharmaceutical ingredients (APIs), and lubri- cants directly to the continuous process. The pharmaceu- tical industry is slowly coming to understand the basic rules of operation for LIW feeders and how to optimize their performance, but operators are often unfamiliar with how to use a feeder's trending capabilities to understand and even predict feeder performance. Current LIW feeders display many parameters associ- ated with feeding performance and machine status, rang- ing from feedrates and motor-drive commands to span settings and alarm limits. While these parameters allow you to monitor and manage the feeder's operation, ana- lyzing the trends for some of these parameters can also help to identify conditions external to the feeder that may be limiting its performance. Sharpening your ability to find external process causes affecting feeder perfor- mance can reduce unplanned downtime, improve process efficiency, and, most important, improve the output of the process. Working from the feeder To use a feeder's trending capabilities to identify an external cause of a feeding problem, you must first under- stand the feeder's operating principle. As the feeder dis- charges material, the combined weight of the feeder and material decreases. Load cells monitor this weight decrease, and the feeder's control system constantly adjusts the operating speed to produce the desired gravi- metric feedrate—equivalent to the rate of the system's weight loss, as shown in Figure 1. A typical LIW feeding technology for continuous pharmaceutical processes is a Using trending capabilities to troubleshoot continuous feeder performance problems Sharon Nowak Coperion K-Tron feeders

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