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To predict how well the powder will flow, we exam- ined the typical stress levels in the feed hopper above the tablet press., which required measuring the bulk density of the ibuprofen mixture, as well as the wall friction angle of smooth Type 304-2B stainless steel (figures 5 and 6). This information was used to compute the stress level in the hopper before and during flow. As shown in Figure 7, the stress increases and reaches its maximum at a point between one-third and one-half the way down the conical hopper. The stress level then decreases as the material approaches the outlet. The increase occurs as material converges in the flow channel, and the ratio of the stress against the wall to the stress in the vertical direction is about 2.2. This increases the stress in the hopper. However, as the stress against the wall increases, the wall's friction tends to support more of the powder's weight, and stress thus decreases as the powder approaches the outlet. The lowest pressure in the conical section is near the outlet and it can be an order of magnitude lower than the lowest strength measurement obtained when arching potential is tested using tradi- tional methods. We can use the data from the two strength curves to compute the expected unconfined yield strengths in the entire hopper (Figure 8). Using only the higher-stress measurements gives strength values close to 300 Pa. The data for the new strength measurements at lower stress values show more variability in the hopper, ranging from 26 to 39 Pa, depending on the location. We computed the arching tendency at each location in the conical hop- Tablets & Capsules May 2014 31 Figure 3 Critical arching dimensions of 50 percent ibuprofen mixture using low- and high-pressure data 0 600 500 400 300 200 100 0 2000 50% ibuprofen mixture Flow factor Arching condition High pressure tests 4000 6000 8000 10000 12000 Major principle stress (Pa) Unconfined yield strength (Pa) Figure 4 Components of a typical tablet press Feed frame Hoppers Rotary die Figure 5 Density of 50 percent ibuprofen mixture Figure 6 Wall friction angle of 50 percent ibuprofen mixture 650.0 600.0 550.0 500.0 450.0 40 35 30 25 20 15 10 5 0 0 2 4 6 8 10 12 14 16 Major principle stress (kPa) Wall stress (kPa) 0 5 10 15 20 25 Wall friction angle (deg) Bulk density (kg/m 3 ) g 5 g0 * (1 1 s/s0) b g0 (kg/m 3 ) s0 (Kpa) b 474.31 0.632 0.0742972 Figure 7 Local stress in typical tablet press feed hopper 0 100 200 300 400 500 Flow stress Initial stress Major principle stress (Pa) 0 200 400 600 800 Height (mm) h-Johanart_28-32_Masters 5/14/14 10:17 AM Page 31