Effects of punches with embossed features on compaction behaviour

Abstract

This work investigated the modifications of compaction behaviour that resulted from using punches with one or two embossed (i.e. raised) ridges across their contact faces, to produce debossed furrows on otherwise flat-faced cylindrical specimens. Single-sided uniaxial compaction of spheronised microcrystalline cellulose was used to prepare specimens of different thickness and density. Based on methods developed previously, small-angle X-ray scattering (SAXS) was used to map local density variations, compressive stress, strain and strain direction over diametral sections from the specimens. The results revealed zones of relatively high compaction below the debossed furrows, with lower compaction along the flanks. High compressive stresses (up to eight times the average peak punch pressure) were indicated just below the furrows, which decreased exponentially with increasing distance below the furrows. Extrapolation towards the surface suggested that even higher stress occurred where the powder was in direct contact with the embossed ridges on the punches. It is suggested that the significant variations in relative density, stress, strain and principal strain direction observed around the debossed features could contribute towards the pressing faults that are sometimes encountered during commercial tabletting. The high stresses may also cause crystallographic changes in susceptible materials. Moreover, although the present work was performed using a pharmaceutical material, it seems likely that similar effects would also arise where powder compaction is used to produce metallic or ceramic items with complex shape.