Effects of punch geometry on powder movement during pharmaceutical tabletting processes

This paper describes an investigation involving an experimental study of the movement of powder during the axially symmetrical uniaxial compaction of microcrystalline cellulose. The influences of simple flat faced cylindrical and concave curved faced cylindrical punch geometries are investigated which form tablets of non-homogeneous structure and possible poor mechanical integrity due to the development of uneven pressure distributions within the compacts. Powder compaction occurs by the action of a compressive stress acting on a powder bed and the constrained powder moving according to the direction of the resulting forces generated by the initial application of the forming force. Changing the geometry of the punch and die set used will inherently change the direction of the resultant forces causing major changes in the direction of powder movement. The, experimental; study adopted a metal shot tracer method in conjunction with X-ray imaging, that has been designed to show how inherently changing the geometry of the punch, causes different amounts of powder movement to take place leading to the formation of a non-homogeneous structure within the tablet. Radial powder movements have also been investigated using the same metal shot tracer technique, The results presented show that the tablets were highly non-homogeneous in nature with high density regions, present in the "top corners" and "middle bottom half", with respect to its compression axis of the flat faced tablets. High density regions were also noted in the "corners" of the convex curved faced tablets where powder was in contact with the die wall. Significant amounts of radial powder movement, with respect to the load axis were observed. This was particularly so for the convex curved faced variety, inducing large amounts of stored stress and strain, therefore, increasing the chances of mechanical failure. (C) 2004 Elsevier Ltd. All rights reserved.