Issue link: https://www.e-digitaleditions.com/i/648486
blend is compressed, typically in a roll compactor, and then milled and sieved. At the tablet press, the granulation flows from a feed hopper and is fed by a gravity or paddle feeder into dies. The granules are usually small, and sometimes that leads to arches or jams in the feeding system. That's because, gener- ally speaking—and there are many other variables that come into play—the smaller the particles, the harder it is for a powder to flow. As a rule of thumb, particles smaller than 50 microns are cohesive and hinder flow. Particles between 50 and 100 microns can be cohesive to easy flow- ing, and particles larger than 100 microns usually flow eas- ily. Of course, the properties of the substance itself and environmental factors (temperature, humidity) can also affect how well a powder flows. Figure 1 shows the flow function of a powder, and the regions of flow are desig- nated as very cohesive, cohesive, easy flowing, and free flowing. Getting the correct amount of powder into the die is critical, because over- or under-filling it will lead to off- weight tablets and other problems. A jam during tabletting that starves the feeder and leads to empty dies can severely damage the press and its tooling. So again, it's worth your time to evaluate the powder's flow properties before and during production. In addition to checking flow behavior, you should measure the powder's bulk density and compare it to a target value. Only when it's on or very near that tar- get can the tablet press apply the correct amount of pres- sure to produce a good tablet. In fact, bulk density measurement defines the optimal force to apply, and if the bulk density changes for any rea- son—due to temperature, humidity, incorrect blending, or out-of-spec ingredients— and you don't adjust for it, the press might apply the wrong amount of force. Once you calibrate the press to apply the correct amount of force, moni- tor the bulk density of the powder. If it increases, the press will likely not apply enough force, causing the the tablet to cap or laminate. If the bulk density decreases, the press is likely to apply too much force, crushing the powder and perhaps damaging the tooling or the press itself. While capsule filling machines operate differently from tablet presses, they too require powders with a consistent bulk density to ensure accurate dosing and smooth operation. By quickly and accurately identifying a change in bulk density, you can adjust the tablet press to apply the correct amount of force. That's where a shear cell can help. It pro- vides accurate, cost-effective, and timely flow measurement data—essentially an assessment of overall flow behavior. 28 March 2016 Tablets & Capsules Figure 1 Flow function graph from shear cell data Unconfined failure strength (kPa) 10 9 8 7 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 11 Major principal consolidating stress (kPa) Very cohesive Cohesive Easy flowing Free flowing By quickly and accurately identifying a change in bulk density, you can adjust the tablet press to apply the correct amount of force.