Novel ultrasonic in-die measurements during powder compression at production relevant speed.

Abstract

A novel ultrasonic instrumentation was successfully implemented in a compaction simulator. A through-transmission set-up was realised with longitudinal and transverse transducers being alternately positioned inside Euro-D-modified punches. Key features of the data acquisition are described. Considerable attention was paid to an accurate displacement measurement and a synchronic acquisition of the ultrasonic signal. Vivapur 102 and Di-Cafos A150 were chosen for evaluation. In contrast to other published instrumentations, production-relevant powder densification speeds were feasible whilst featuring outstanding measurement precision. Maximum ultrasonic speed was achieved at maximum density. Materials differed considerably regarding the slope of the decompression phase, which might be suitable for assessing elasticity and speed sensitivity of powders or formulations without compressing twice. The developed set-up furthermore enables in-die measurements of apparent Young's modulus and apparent Poisson's ratio (i.e. their change throughout the course of the tableting process). Young's modulus increased upon densification and values match with literature data. Poisson's ratio increased linearly as a function of solid fraction for plastically deforming Vivapur 102, whereas it was practically constant for brittle Di-Cafos A150. Increased mechanistic understanding of deformation factors (e.g. rearrangement, fragmentation, elasticity) and estimation of mechanical compatibility of mixtures, is feasible. Moreover, in-die Young's modulus and Poisson's ratio are valuable for compression simulations based on finite or discrete element method.