The role of the displacement-time waveform in the determination of Heckel behaviour under dynamic conditions in a compaction simulator and a fully-instrumented rotary tablet machine.

The Heckel equation has been used widely to characterize the compression behaviour of pharmaceutical powders, yet very little attention has been paid to the role of the displacement-time profile used to generate this relationship. The objective of this study was to evaluate and compare selected standard waveforms with actual and theoretical tablet press waveforms in the Heckel analysis of representative formulations under dynamic conditions in a compaction simulator and to compare such data with that determined on the same formulation using an actual fully-instrumented rotary tablet press. Increased tableting rate and different programmed displacement-time waveforms with the same gross punch-speed changed the Heckel behaviour of all formulations. The results of this study suggest the pressure-volume relationship determined during powder-bed compression is affected by the instantaneous punch-speed profile of the displacement-time waveform for all materials studied, even though they deform by different mechanisms. It appears that the instantaneous punch-speed profile of the particular displacement-time waveform is a confounding factor of Heckel analysis. Compaction simulators programmed to deliver saw-toothed displacement-time traces have the advantage of constant punch-speed and may be a better choice for characterizing a formulation by Heckel indices and the strain-rate sensitivity index. On the other hand, they also carry the liability of not being a realistic representation of tableting on a rotary tablet press.