Radial voidage variation in randomly-packed beds of spheres of different sizes

Abstract

Abstract The packing arrangement of the particles in a tablet die, before pressing, has no hitherto been considered as a variable in the tabletting process. Because of the complexity of this subject, the present investigation has been restricted to monosized and binary mixtures of spheres in a cylindrical container. The container could be revolved at speeds in excess of 1,000 rpm so that the thickness of an annular layer of water could be measured. From this, the voidage at any radial position could be found. The plot of voidage against distance from the cylindrical wall is a wave form which is damped out after about 5 particle diameters. For packings of monosized spheres the oscillations are regular. There appear to be two kinds of binary mixtures, those in which the particle sizes are similar, and those where the diameter ratio is greater than 0·4. The voidages calculated by suitably weighting the values for each component are in agreement with the measured voidages of the first kind but not with those of the second. Because there is an initial distribution of voidage, it is concluded that there must be some radial movement of the particles during the compacting operation.