Flowability Assessment of Weakly Consolidated Powders

Author Last Name: 
Colin Hare, Azza Mahmoud, Alex Stavrou, Ali Hassanpour
Report Type: 
Research Area: 
Powder Flow
Publication Year: 
Publication Month: 
United Kingdom

Executive Summary
Measurement of powder flowability under low stress conditions is often unreliable or
inconsistent. The ball indentation, uniaxial compression and shear cell methods have been
used to measure flowability of titania under a range of stresses. The bed preparation method
has been assessed in ball indentation, and shear cell measurements have been carried out
for several grades of titania with two different shear cells. Furthermore, the influence of
size distribution and interface energy on constraint factor have been assessed
experimentally, and the influence of a broader range of particle properties has been
investigated using the Discrete Element Method (DEM).
Indentation hardness measurements have been shown to be dependent on the loading
condition, with critically consolidated beds providing a greater hardness than vertically
consolidated beds. It is shown that at lower stresses it is challenging to apply indentation
to critically consolidated beds since an insufficient fraction of the exposed bed is located
in the shear plane. Reducing the vane height of the shear cell lid allows indentation
measurements to be made at lower stresses, though the optimal lid design is unlikely to be
universal for all powders.
The flow behaviour of various titania grades is found to be largely similar between the FT4
shear cell and the Schulze RST.XS.s shear cell. At lower pre-shear stresses the FT4 has
been shown to apply normal stresses noticeably greater than the target stresses for these
titania powders. A direct comparison could not be made with the Schulze shear cell due to
normal stress data being unavailable. Repeated testing in the Schulze shear cell shows that
at lower stresses the variability in measured yield data increases; this appears to be largely
driven by variability in the pre-shear shear stress, thus implying the conditioning is
insufficient to overcome the influence of prior handling. The precise normal stresses
applied in a shear test are shown to have limited influence on the generated yield locus, so
long as they cover a sufficiently wide range.
The constraint factor of bi-disperse, cohesive glass beads has been shown to decrease with
the addition of coarse material and decrease with the addition of fines. DEM simulations
suggest that the constraint factor remains constant across all stress levels, and that
constraint factor increases with an increase in rolling friction coefficient or interface energy.