The influence of punch curvature on the mechanical properties of compacted powders

The influence of changing the overall thickness and face curvature of compacted microcrystalline cellulose powder (a direct compression excipient used extensively in the pharmaceutical industry) prepared at different compaction pressures has been assessed by the diametral compression test. For specimens that failed in tension, the tensile strength of the compacts was calculated from formulae derived from stress analysis. The values for tensile strength indicated that for equivalent compaction pressures, compacts with different thicknesses and face curvatures, strengths were not equivalent. This could be due to differences in the internal structure of the compacts due to different stress distributions during the formation process. The results, however, clearly show that the maximisation of the tensile strength can be achieved for different thicknesses and face curvature. In general, a compact diameter to face curvature ratio of 0.67 corresponding to what are termed “normal” concave tablets, gave the maximum value for the tensile strength.