Determining the compression behaviour of pharmaceutical powders from the force-distance compression profile

The force-displacement curves obtained from an eccentric tablet machine were examined in a new way. The tendency of the material for plastic deformation, fragmentation and elasticity is expressed as numerical values, which are comparable between different materials. The dependence of these numerical values on the compression pressure was modelled. The accuracy of the displacement measurement was improved by filtering out noise from the measurement data by a novel method. The plastic deformation of the material near the force maximum of the compression cycle could be seen accurately from this precise displacement data. The elastic deformation of the tablet machine was also defined very precisely from the running machine. Tablets were made with an eccentric tablet machine using fixed lower and upper punch adjustments. This ensured that the speed of the upper punch and the theoretical height of the tablets were the same for all compactions. Therefore, only the properties of the materials determined the differences in the shape of the compression curves. The test materials used were alpha-lactose monohydrate, two grades of microcrystalline cellulose, maize starch and dicalcium phosphate dihydrate. The results showed that the use of accurate displacement measurement is essential in order, to see the small variations in the shape of the force-displacement curve near the force maximum of the compression cycle, and it made it possible to dynamically calibrate the elastic deformation of the eccentric tablet machine during compression. It turned out that the numerical values obtained with the new method described the plastic, brittle and elastic properties of the tested materials satisfactorily in a wide compression pressure range.