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22 May 2019 Tablets & Capsules the same example material, the stress, strain, and per- centage increase in particles smaller than 160 microns for a 3-foot-diameter bin are shown in Figures 10 through 12. The stress in the smaller bin is about one fourth the stress in the larger bin. The strain is only slightly smaller, but this combination causes the increase in particles smaller than 160 microns to be about 2.1 percent, which is only about 43 percent of the increase expected in the larger bin. This is one rea- son particle breakage usually isn't a serious problem in typical pharmaceutical bins. Now let's consider a typical feed frame in a tablet press, as shown in Figure 13. Material enters the feed frame through an inlet at the top, and two rotating paddle wheels distribute the material evenly across the die fill zone. As the turret plate rotates beneath the feed frame, the material drops into the empty die cavi- ties below, filling the dies. In this case, the stress is low, but the total strain is high. The rotation of the paddles causes significant strain on the material in the gaps between the paddles and the turret plate and sides of the feed frame. The strain is a function of the rotational speed of the pad- dles compared with the material throughput rate. It's not uncommon for the total strain experienced by the material in the feed frame to be between 200 and 1,000 cm/cm. The stress level acting on the material in the feed frame zone may only be 0.45 kilopascals, but the particle breakage from the large strain can be sig- nificant, as shown in Figure 14. Modifying the process to mitigate particle breakage Once you've quantified the amount and type of par- ticle breakage expected in your process, the final step is to look for potential ways of modifying the process to mitigate the unwanted breakage. For example, sup- pose the test data indicated that your material was very sensitive to breakage due to impact but not very sensitive to breakage from stress or strain effects. You can then analyze the process and look for impact points where transport or handling velocities are large and modify the process to limit those impact velocities and reduce the unwanted breakage. Suppose that two of these impact zones were caused by free fall and one zone was caused by transport veloci- ties between process vessels. Using the breakage test data, a description of the process, and the details of the impact zones, engineers can estimate the breakage occurring at each zone and determine a course of action to limit the impact velocities in the most critical zones. Conversely, suppose the material testing showed only a very moderate breakage due to impact events, significant breakage due to stress events, and little breakage due to strain events. In this case, you would look at the process and estimate the stress acting in each piece of process equipment, determine the set of variables describing each piece of process equipment, and change those variables in the pieces of equipment Figure 11 Expected strain in 3-foot-diameter bin 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Distance (meters) Strain (cm/cm) 0 1 2 3 4 After filling After flow Figure 12 Expected increase in minus-160-micron material in 3-foot-diameter bin 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Distance (meters) Increase in minus-160-micron particles (%) 0 1 2 3 After filling After flow Figure 10 Expected stress in 3-foot-diameter bin 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Distance (meters) Stress (kilopascals) 0 5 10 After filling After flow