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Tablets & Capsules September 2020 13 • Tablet reject gates on the discharge chute, which can be especially helpful for multitip minitablet production. Compression-force monitoring Tablet presses often use strain gauges to monitor the amount of compression force applied to the tooling. This monitoring allows for automatic tablet weight control, reject functions, and overload protection to safeguard the tooling from excessive loads. When compressing minitablets, the tablet press may apply only 0.5 to 2 kilonewtons of force. A strain gauge designed for a 100-kilonewton range may be accurate only to 1 kilonewton. The gauge's accuracy depends partly on the signal-to-noise ratio, and signal amplifica- tion to detect low forces can increase noise levels. As a result, such a gauge may not be sensitive enough to con- trol tablet weight effectively. This issue crops up most often with poorly compactible formulations. A high-sensitivity force-control system may help when using a tablet press dedicated solely to minitab- lets. The natural result of higher sensitivity is reduced range, so your tablet press force monitoring may be reduced to only 20 kilonewtons or so, depending on the kit. If your tablet press must run both minitablets and standard-sized tablets, consider using high-sensi- tivity strain gauges for precompression only and using software that can toggle weight control between main and precompression. When you compress tablets to a fixed thickness as set by the pressure-roll positions on the tablet press, a direct relationship exists between tablet weight and com- pression force, as shown in Figure 1. This relationship is described by the equation: F = C 0 W C1 where F = compression force C 0 = compression coefficient W = tablet weight C 1 = compression exponent This equation is helpful in determining the ideal set- tings for the weight control limit. To determine the C 0 and C 1 values for your formulation, first collect data by varying the depth of fill in 2 percent increments from 90 to 110 percent of the set point values and recording the average tablet weight and compression force for each fill depth for a group of 20 tablets, as shown in Table 4. After you collect the data, use a least squares power curve to determine the equation. Then simply plug in the desired high and low values for the tablet weight to determine the reject-force values. Lower punch pulldown Compared to standard tooling, microtip tooling has much thinner punch tips, which can be susceptible to bending or buckling under high compression loads. You can prevent this by using lower punches with a punch-tip support shoulder and matching dies with a counterbore to • Plastic rather than stainless-steel stripper plates, which allow a lower height without risking cata- strophic damage; • Teflon coating of the discharge chute, which reduces the coefficient of friction to decrease tablet damage; • Chute designs optimized for minitablets, which may include air assist and special reject and sam- pling gates; and Figure 1 Tablet weight versus compression force for fixed-thickness tablets Force (kilonewtons) 3 2.5 2 1.5 1 0.5 0 Weight (grams) 0.11 0.115 0.12 0.125 0.13 0.135 y = 1713.5x 3.2322 Table 4 Average tablet weights and compression forces for 20 tablets based on depth of fill Depth of fill (millimeters) Average tablet weight (grams) Compression force (kilonewtons) 6.20 0.1134 1.5 6.34 0.1153 1.6 6.48 0.1177 1.7 6.61 0.1194 1.8 6.75 0.1219 1.9 6.89 0.1236 2.0 7.03 0.1259 2.1 7.17 0.1283 2.2 7.30 0.1296 2.3 7.44 0.1312 2.4 7.58 0.1331 2.6 Set points: Tablet thickness 3.5 millimeters Press speed 30 rpm Target weight variation ±10% Target weight 0.125 gram Target depth of fill 6.9 millimeters Target weight lower limit 0.1125 gram Target weight upper limit 0.1375 gram Predicted low force limit 1.46 kilonewtons Predicted high force limit 2.80 kilonewtons