(486a) Evolution of Powder Testers – Key Challenges and Opportunities
As powder handling processes increased in complexity and manufactures moved to reduce costs and increase production output, the requirements for improved powder characterisation resulted in a flurry of further developments in the 1990âs. The ATSM for measuring the flow properties of metallic powders using ball indentation was published in 1990. Avalanching techniques were first proposed by the likes of Kaye in 1995 as a means of studying the rheological behaviour of powders.Â Meanwhile, Johanson and Schulze built on Jenikeâs work for determining flow behaviour in hopers and silos. In 1991 Johanson patented the Johanson Indicizers (Hang-up, Hopper Flow and Flow Rate) whilst Schulze introduced the Ring Shear Tester in 1994. Like the translational shear cell developed by Jenike, these methodologies derive, amongst other parameters, the Unconfined Yield Strength (UYS) of a powder as a function of the Major Principle Stress (MPS), however the Jenike Shear Cell required a highly skilled operator and took several hours to generate a Flow Function (FF), whilst the Johanson Indicizers and Schulze Ring Shear tester were much easier to use and reduced test times to a matter of minutes rather than hours.Â Other attempts had been made previously and during the same period to develop a Uniaxial Shear Tester, from Williams in 1970 who utilised a split mould  to Malby and the POSTEC Uniaxial Tester  in 1993, however these devices were never established commercially.
Since this period, advances in powder flow characterisation have remained steady. As engineers and scientists have better understood the need to measure powder flowability at the stress and strain rates found in modern processing equipment, methodologies have been developed which measure the powder under more dynamic conditions. In 2000 Freeman Technology developed their first Powder Rheometer, with Stable Micro Systems releasing a similar instrument around the same period. Freeman Technology further progressed the concept of stress/strain rate relevant measurements with the addition of the capability to introduce air, generating low stress aerated environments, and a shear cell for higher stress environments. Around this period, the measurement of the air pressure differential across the powder bed resulted in the introduction of the Permeability test on the Freeman FT4 Powder Rheometer, the Sevilla Powder Tester, patented in 1999  and the Raining Bed Method (RBM), first proposed in 1967 but not developed fully until 2002 . More recent developments based on the addition of air have included research by Tardos et al  investigating the fluidisation behaviour of powders in a Couette Device, and by Anton Paar who developed a Powder Cell which uses air to condition the powder prior to measurement. In terms of understanding flow in hoppers and silos the early work of Jenike is still as valid today as it was 55 years ago, however the desire to directly measure the UYS at lower stresses using simpler and lower costs technologies is evident, with techniques such as the Particulate Systems SSSpin Tester, developed by Kerry Johanson, the University of Leeds Environmental Caking Rig  and the Freeman Technology Uniaxial Powder Tester as recent additions to the field.
Going forwards, powder processing continues to grow in complexity, especially as industries such as the pharmaceutical sector move towards continuous manufacturing and emerging technologies such as 3D printing become established. Recognising the need to understand and measure powder behaviour under dynamic conditions, at low stress regimes, is driving research in terms of modelling and characterisation methods. Rationalising the physics of such environments is the key to a more fundamental understanding of these complex materials and is the next step in moving beyond Jenikeâs theory for quasi-static behaviour. The reality is this field is still very much in itâs infancy, and there remains plenty of opportunity for current and future scientists and engineers to advance our understanding of bulk solids behaviour.
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