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depth of the void it can create, and that depth should slightly exceed what the final weight cam allows. The deeper void causes overfilling, which promotes tablet weight uniformity. But if the overfill is too great or if the settings of the fill cam and weight cam differ excessively, too much material will recycle to the feeder, where it can become over-blended. And should the feeder get full, the excess material can be lost to the vacuum collection system. In fact, some powders can jam the paddles and drive motor. Note the difference here between single- and bi-layer tablet presses. A single-layer press directs excess powder to the center of the turret and, as the turret rotates, the material can re-enter the leading side of the feeder. But a bi-layer tablet press cannot allow material to rotate with the turret because that would contaminate the other layer. It is thus very important to select a fill cam of the appropriate size or you will diminish product yields. Fill cam selection is based on the tooling geometry, material bulk density, and tablet weight. First, calculate the tablet volume by dividing the tablet weight by the bulk density of the material: Tablet volume 5 Tablet weight Bulk density Next, calculate the weight cam setting by dividing the tablet volume by the cross-sectional area of the punch tip: Weight cam depth 5 Tablet volume Punch tip's cross-sectional area Once you know the weight cam depth, select the fill cam that will allow a slight overfill. Table 1 provides a guideline for selecting fill cams. When making bi-layer tablets, the penetration of the upper punch for layer one dictates the maximum weight of layer two. In other words, the upper punch penetration of the first layer acts as the fill cam, creating the void in the die that accepts the second layer. Be sure to optimize this setting in order to minimize the problems that ensue from excessive overfills as described above. Once the weight cam is dialed in and the proper fill cam selected, you might have to make minor adjustments to the weight cam to compensate for the rotation of the turret, which affects feeder dwell time. Feeder dwell time is the period during which the lower punch remains under the feed system to allow the die to accept a charge of material. The equation on the next page is a simple formula for fine-tuning the weight cam depth based on the initial weight cam depth and tablet weight. 24 May 2014 Tablets & Capsules Upper compression rollers of a bi-layer tablet press Fill cams Table 1 Fill cam selection chart Required weight of fill (mm) Fill cam (mm) 0 to 5 6 5 to 8 9 8 to 11 12 11 to 14 15 15 to 18 19 g-Sedlockart_22-27_Masters 5/14/14 10:16 AM Page 24