Flowability Assessment of Weakly Consolidated Powders

will be measured to test the accuracy of their measurements.

measurements of both instruments at low stresses. The flow condition out of the hopper

not agree. A small hopper has been designed for gypsum powder based on the flow

the FT4 Powder Rheometer, however the measured values of unconfined yield stress do

cell and for the ball indentation method by sieve filling using an indentation attachment to

The reproducibility of flow measurement at low stresses is similar for the RST-XS.s shear

stresses as low as 100 Pa, however the variability increases for more free-flowing powders.

measurements with a coefficient of variation < 3% for the very cohesive titania at pre-shear

measurement in this range. The Schulze RST-XS.s provides unconfined yield stress

the intended applied stresses for titania DT51, and therefore does not provide a reliable

cell agree at moderate stresses, however below 2 kPa the FT4 shear cell does not achieve

Shear cell measurements using the FT4 shear cell and Schulze RST-XS.s low stress shear

than that of ball indentation and some shear cells.

the minimum consolidation stress that can yield a measurement with this method is greater

stresses the uniaxial compression test underestimates the unconfined yield stress, whilst

unconfined yield stress to be inferred from indentation measurements. However, at lower

moderate stresses, allowing constraint factor to be determined and ultimately for

Uniaxial compression measurements correlate with ball indentation measurements at

is followed.

packing state can be achieved by vertical consolidation when this bed preparation method

consistent across the radial direction of the powder bed. This indicates that a reproducible

excess powder heap away with an inclined blade, the flow resistance is found to be

consolidation stresses. Furthermore, by completely filling the powder bed and scraping the

indentation, whereas sieve-filling is able to produce a coefficient of variation < 3% at low

is found to provide a large coefficient of variation in the bed hardness measurement by

scraping away excess powder) prior to vertical consolidation. At low stresses, pre-shearing

and by blade conditioning, wire conditioning and sieve filling (with and without

approaches does not achieve this alone. Ball indentation measurements are assessed by preshearing

however the vertical consolidation applied in indentation and uniaxial compression

Shear cells pre-shear the sample in an effort to ensure a reproducible packing state,

shear cell methods.

measurement at low stresses is assessed here by ball indentation, uniaxial compression and

force measurement and (ii) the reproducibity of the loose packing state. Powder flow

measurement at low stresses is more challenging due to (i) the required resolution of the

the reliability and reproducibility of the measurement decreases. Powder flow

using shear cell measurements. As the consolidation stress applied to the powder is reduced,

with procedures developed for designing hoppers based on mass or funnel flow behaviour

The most widely established method for measuring powder flow is by shear cell analysis,

Measurement of powder flow behaviour is important for many powder handling operations.

xecutive Summary