Abstract
Pharmaceutical tablets are the most popular dosage form for drug delivery. The tablets are generally produced by compacting dry powders. During pharmaceutical powder compaction, the tablets produced need to sustain their integrity during the process and have to be strong enough to sustain any possible load experienced during the post-compaction processes, such as coating, packing and handling. Hence, any defects, such as chipping, capping and laminating, are not tolerable during pharmaceutical powder compaction. However, such defects are common problems during the tabletting process. Therefore, understanding the failure mechanisms of these defects has attracted considerable attention. In this paper, only the mechanisms of capping were considered. Previous studies on capping during pharmaceutical powder compaction have been reviewed. Capping mechanisms have been further explored by conducting a combined experimental and computational study on pharmaceutical powder compaction. An instrumented hydraulic press (also known as compaction simulator) has been used to investigate the powder behaviour during the compaction. In addition, an instrumented die has also been used, which enable the material properties to be extracted for some real pharmaceutical powders. Close attentions have been paid to the occurrence of capping during tabletting. An X-ray Computed Microtomography system has also used to examine the internal failure patterns of the tablets produced using the compactions simulator. Furthermore, pharmaceutical powder compaction has also been analysed using finite element (FE) methods, in which the powder was modelled as an elastic-plastic continuum medium following Drucker-Prager-Cap yield criteria and the material properties were determined from the uniaxial compaction with an instrumented die. In both experimental and numerical studies, cylindrical tablets with different surface curvatures, including Flat-face round tablets and convex tablets, were considered. From the experimental observation, it is clear that different capping patterns were obtained for different shaped tablets: cone-shaped capping for flat-faced tablets and normal capping with essentially horizontal failure surface for convex tablets. It was also observed in the experiments that capping takes place at the early stage of decompression (unloading), i.e., the top punch begins to withdraw. Close examination of FEA results reveals that the capping is associated with an intensive shear band developed at the early stage of unloading for all cases considered. Therefore, the combined experimental and numerical studies demonstrated that the intensive shear bands developed at the early stage of unloading are responsible for the occurrence of capping.