Modelling of flowability measurement of cohesive powders using small quantities

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 these tests are not applicable because 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 sample size, penetration depth, indenter properties and strain rate 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 times the particle diameter exhibit fluctuations in powder flow stress measurements, which do not represent shear deformation. The penetration depth should be sufficiently large to cause notable bed shear deformation. It is found that this minimum penetration depth is approximately equal to 10% of the indenter radius. The hardness measurements are found to be independent of indenter stiffness within the wide range investigated.
The friction between the indenter and the particles slightly increases the hardness, although its influence on the internal stresses is negligible. Cubic and cylindrical indenters measure significantly larger hardness value compared to the spherical indenter. Increasing the inter-particle friction and cohesion results in higher hardness values and internal stresses, due to the increase in resistance to shear deformation. Simulations at a range of indenter velocities confirm that the ball indentation technique can be used to analyse powder flowability over a wide range of shear rates.