Measurement of powder flow behaviour is important for many powder handling operations.
The most widely established method for measuring powder flow is by shear cell analysis,
with procedures developed for designing hoppers based on mass or funnel flow behaviour
using shear cell measurements. As the consolidation stress applied to the powder is reduced,
the reliability and reproducibility of the measurement decreases. Powder flow
measurement at low stresses is more challenging due to (i) the required resolution of the
force measurement and (ii) the reproducibity of the loose packing state. Powder flow
measurement at low stresses is assessed here by ball indentation, uniaxial compression and
shear cell methods.
Shear cells pre-shear the sample in an effort to ensure a reproducible packing state,
however the vertical consolidation applied in indentation and uniaxial compression
approaches does not achieve this alone. Ball indentation measurements are assessed by preshearing
and by blade conditioning, wire conditioning and sieve filling (with and without
scraping away excess powder) prior to vertical consolidation. At low stresses, pre-shearing
is found to provide a large coefficient of variation in the bed hardness measurement by
indentation, whereas sieve-filling is able to produce a coefficient of variation < 3% at low
consolidation stresses. Furthermore, by completely filling the powder bed and scraping the
excess powder heap away with an inclined blade, the flow resistance is found to be
consistent across the radial direction of the powder bed. This indicates that a reproducible
packing state can be achieved by vertical consolidation when this bed preparation method
Uniaxial compression measurements correlate with ball indentation measurements at
moderate stresses, allowing constraint factor to be determined and ultimately for
unconfined yield stress to be inferred from indentation measurements. However, at lower
stresses the uniaxial compression test underestimates the unconfined yield stress, whilst
the minimum consolidation stress that can yield a measurement with this method is greater
than that of ball indentation and some shear cells.
Shear cell measurements using the FT4 shear cell and Schulze RST-XS.s low stress shear
cell agree at moderate stresses, however below 2 kPa the FT4 shear cell does not achieve
the intended applied stresses for titania DT51, and therefore does not provide a reliable
measurement in this range. The Schulze RST-XS.s provides unconfined yield stress
measurements with a coefficient of variation < 3% for the very cohesive titania at pre-shear
stresses as low as 100 Pa, however the variability increases for more free-flowing powders.
The reproducibility of flow measurement at low stresses is similar for the RST-XS.s shear
cell and for the ball indentation method by sieve filling using an indentation attachment to
the FT4 Powder Rheometer, however the measured values of unconfined yield stress do
not agree. A small hopper has been designed for gypsum powder based on the flow
measurements of both instruments at low stresses. The flow condition out of the hopper
will be measured to test the accuracy of their measurements.