Analysis of ball indentation on cohesive powder beds using distinct element modelling

The characterisation of cohesive powders for flowability is often required for reliable design and consistent operation of powder processes. This is commonly achieved by the unconfined compression test or shear test, but these techniques require a relatively large amount of powder and are limited to large pre-consolidation loads. There are a number of industrial cases where small amounts of powders have to be handled and processed, such as filling and dosing of small quantities of powder in capsules and dispersion in dry powder inhalers. In other cases, the availability of testing powders could be a limiting issue. It has been shown by Hassanpour and Ghadiri (2007) that under certain circumstances, indentation on a cohesive powder bed by a blunt indenter can give a measure of the resistance to powder flow which is related to flowability. However, the specification of the operation window in terms of indenter size and penetration depth has yet to be fully analysed. In the present work, the ball indentation process is analysed by numerical simulations using the Distinct Element Method (DEM). The flow resistance of the assembly, commonly termed hardness, is evaluated for a range of sample quantities and operation variables. It is shown that a minimum bed height of 20 particle diameters is required in order to achieve reliable measurements of hardness. A sensitivity analysis of indenter size reveals that small indenters with diameters less than 16 particle diameters exhibit fluctuations in powder flow stress measurements, which do not represent shear deformation. Larger indenters provide stable values for flow stress, subjected to the minimum sample criteria being met. The maximum indenter size is limited by the die size. If the indenter size is too close to the die size the walls provide confinement and increase the measured hardness.