Ball indentation on powder beds for assessing powder flowability: Analysis of operation window

Abstract

The characterisation of bulk behaviour of cohesive powders is very important in processing of particulate solids, e.g. for reliable powder flow out of storage vessels. For filling and dosing of small quantities of powders in capsules and for dispersion in dry powder inhalers, the interest is on the behaviour of loosely-compacted powders in small quantities and under very low applied loads. Furthermore at the early stages of drug development, the quantity of the powder available is often very small and the traditional bulk testing methods are neither possible nor applicable. In this work we evaluate a method to infer powder flowability by ball indentation. This technique provides a measure of flow resistance which can be related to the unconfined yield stress. It can be applied at very low loads and requires only a small sample quantity, typically a few mm3. The operational window in the ball indentation method in terms of minimum sample size, penetration depth and indenter properties (such as size, shape, friction and Young's modulus) has been analysed and reported here.