Modelling the mechanical behaviour of pharmaceutical powders during compaction

The mechanical behaviour of pharmaceutical powders during compaction is analysed using Finite Element Methods (FEM), in which the powder is modelled as an elastic–plastic continuum material. The Drucker–Prager Cap (DPC) model was chosen as the yield surface of the medium, which represents the failure and yield behaviours. Uniaxial compaction experiments were also carried out using a compaction simulator with an instrumented die. The objectives of these experiments were two-fold: (1) to investigate the pharmaceutical powder behaviour during compaction, for which the variation of relative density of the powder bed with applied pressure is analysed; and (2) to calibrate the DPC model with the experimental measurements, from which realistic powder properties are generated and fed into finite element analysis (FEA). The relationship between relative density of powder bed and applied pressure is also obtained from FEA and compared with the experimental data. Good agreement between the experimental and FEA results is observed, which demonstrates that FEA can capture the major features of the powder behaviour during compaction. Furthermore, close examination of the evolution of the stress distribution during unloading reveals that there is a narrow band existing from the top edge towards the bottom centre of the tablet, in which there are localised, intensive shear stresses. It is in this band that potential failure regions, such as cracks, can initiate. This has been demonstrated with experimental evidence from X-ray microtomographical images and photography of fractured tablets. It is therefore demonstrated that FEA can predict the possible mechanism of failure, such as capping, during compaction.