The elastic relaxation of starch tablets during ejection

Tablets are invariably formed by uniaxial compression in rigid dies and the tablet mechanical integrity is highly dependent on its ability to withstand the physical dimensional changes resulting from the elastic relaxation due to the release of the elastic strains incurred during the tablet formation. Therefore, a complete understanding of the continuous tablet elastic relaxation behaviour will provide designers of the solid drug delivery system, important information on the probable mechanical performances of the designed tablets and the likelihood of failures at each stage in the tablet preparation process. This current work has successfully studied online Starch 1500 tablet elastic relaxation behaviour during ejection, which is one of the main stages in a tablet preparation through the use of novel non-contact laser measuring devices. The starch tablets were observed to undergo ‘recompression’ or height reduction in the early stages of ejection, followed by height expansion as they moved further towards the die exit and during emergence. Online diametrical measurements of the tablets during emergence indicated the existence of periodic diametrical expansions–contractions, which has been attributed to the diametrical elastic relaxation behaviour as well as the topographical nature of the circumferential tablet surface. The amplitudes of these expansion–contraction cycles, are believed to be dependent upon the corresponding stored elastic energy of that particular strip of circumferential surface in contact with the die walls. It is also found that the starch tablets formed at a lower compaction velocity predominantly dissipate the internal stored elastic energies by plastic flow and maintain its stored elastic energy during the ejection stage in relative to those formed at a higher compaction velocity.