Peak Offset Times as an Indication of Stress Relaxation During Tableting on a Rotary Tablet Press

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Abstract
During powder compaction on a Manesty Betapress, peak pressures, Pmax, are reached before the punches are vertically aligned with the centres of the upper and lower compression roll support pins. The interval between the time taken to reach this alignment and the time to reach Pmax is defined as the peak offset time, toff. The duration of toff depends on the ability of the compacted powder to relieve stress and is an indication of the predominant mechanisms of particle deformation during consolidation. Thus, at a given Pmax, short toff values are characteristic of materials that consolidate mainly by brittle fracture while longer values indicate an increase in plastic flow. On the Betapress, the decrease in stress during toff occurs under conditions approaching constant strain and toff therefore, is an indirect measure of stress relaxation. Stress relaxation, and hence toff, decreases with increase in Pmax due to the reduction in the porosity of the compact and consequent restriction of plastic flow into the void spaces. In addition to Pmax, the effects of variables such as punch head geometry, press speed and formulation on toff are reported.