Numerical and experimental investigation of capping mechanisms during pharmaceutical tablet compaction

Capping is a common problem in the pharmaceutical tabletting process in which catastrophic failure of the powder compact can occur. It is of great interest to the pharmaceutical industry to understand the fundamental reasons why capping occurs and how it can be avoided. Recently, a combined numerical and experimental study on pharmaceutical powder compaction revealed that cone capping, which is a typical failure mechanism during the production of flat-faced cylindrical tablets using powders of low tensile strength, is due to the formation of a narrow band with localised, intensive shear stresses running from the top edge towards the bottom centre of the tablet [C.Y. Wu, O. Ruddy, A.C. Bentham, B.C. Hancock, S.M. Best, J.A. Elliott, Modelling the mechanical behaviour of pharmaceutical powders during compaction, Powder Technology, 152 (2005)107–117.]. In this paper, the results of further studies are reported in an attempt to explore possible methods to alleviate the propensity for tablets to cap. These methods have been systematically investigated using finite element methods (FEM), and include using lubrication to reduce the die-wall friction, employing different tooling kinematics (speeds and compression profiles), making tablets with different thicknesses, and making convex tablets using punches with curved surfaces. It has been found that none of these methods could avoid the development of intensive shear bands during unloading, which implies that capping cannot necessarily be avoided using these methods. In addition, physical experiments using a compaction simulator have also been carried out, in particular, for making convex tablets with different curvatures. The tablets produced were examined using X-ray microtomography (XMT), from which the failure patterns were identified and found to be in very good agreement with the numerical analysis. The combination of experimental and numerical studies has demonstrated that: (i) capping takes place during the decompression (unloading) phase, and (ii) the intensive shear bands formed during decompression are responsible for the occurrence of capping.