PEAK OFFSET TIMES AS AN INDICATION OF STRESS-RELAXATION DURING TABLETTING ON A ROTARY TABLET PRESS

During powder compaction on a Manesty Betapress, peak pressures, P(max), 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 P(max) is defined as the peak offset time, t(off). The duration of t(off) 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 P(max), short t(off) values are characteristics 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 t(off) occurs under conditions approaching constant strain and t(off) therefore, is an indirect measure of stress relaxation. Stress relaxation, and hence t(off), decreases with increase in P(max) due to the reduction in the porosity of the compact and consequent restriction of plastic flow into the void spaces. In addition to P(max), the effects of variables such as punch head geometry, press speed and formulation on t(off) are reported.