Measurement of the flow properties of powders with special reference to die fill

The flow behaviour of four pharmaceutical powders was investigated using a model shoe-die-filling system. The variation of mass delivered to the die as a function of shoe velocity provides a measure of flowability. The paper discusses the concept of critical velocity, above which incomplete filling is observed, in the context of pharmaceutical powders. The filling process was recorded using a high-speed video system, which allowed the different flow patterns to be observed, and how this influences the critical velocity to be evaluated. The influence of humidity, which was investigated in detail for one of the powders, was found to be small. The initial conditioning of the material, the die opening and if die filling takes place in air or in vacuum, however, were found to change the flow behaviour significantly.