Measuring the flow properties of consolidated, conditioned and aerated powders - A comparative study using a powder rheometer and a rotational shear cell

This paper compares powder flowability measurements using the two methodologies available from a Freeman FT4 Powder Rheometer. The twisted blade method is empirical and measures the energy needed to establish dynamic or three dimensional flow patterns at specific packing conditions. The other is an automated shear cell in which the powder sample is sheared across a single plane to determine its shear strength properties. Six different powders were evaluated to determine their flow performance when consolidated, conditioned and aerated or fluidised. Objectives were to correlate the data from the two methods and assess the sensitivity to some of the key variables that affect powder flow properties. The results showed that dynamic testing that produces shear without compacting the powder sample (upwards testing), provides data that correlates well with shear cell data. However the standard downward dynamic test that does compact, correlates less well but was highly differentiating. It is apparent that shear strength is only one component relating to flowability and that the measured flow energy is also dependent on the compressibility of the powder and the flow rate. Shear testing of conditioned powders at near zero normal stress used the position control mode rather than force control used for the standard shear tests. Aerated powders could not be evaluated with the shear cell, but were assessed using the dynamic methodology and showed very significant differences of flow, energy. In conclusion, both methodologies provide useful insights into flow behaviour with good repeatability of measurement, but dynamic data provides better differentiation between powders with similar theological properties in all packing states. (c) 2006 Elsevier B.V. All rights reserved.