Comparison of uniaxial compaction tester and shear tester for characterization of powder flowability

The complete characterization of powder flow properties with shear cells is a long and time consuming process that requires specially trained operators or costly automated instruments. For these reasons, in industrial practice, the use of simpler and less extensive measurement by uniaxial compaction testers is often preferred. However, previous studies in the literature indicate that the results of the two techniques are not directly comparable due to the different stress state conditions achieved in the two testers. In this study, an experimental campaign to measure the flow function of five different powders with a ring shear tester (RST) and a uniaxial compaction tester (UCT) was performed. Different flowability results that arose for the more cohesive powders are explained by the wall friction effect in the UCT. Re-evaluation of the results accounting for the wall friction gave substantial agreement between the two experimental techniques for a calcium carbonate powder and only at low consolidation levels for the other four food powders. Phenomena other than wall friction seem to appear within these powders tested at high consolidation levels. The comparison between the results of the two techniques suggests that straightforward extrapolation of the UCT flow functions to a low consolidation condition can lead to an underestimation of powder cohesion.