(486a) Evolution of Powder Testers – Key Challenges and Opportunities

Authors: 
Freeman, T., Freeman Technology
Powders have been utilised by mankind for many thousands of years but methods for the measurement of powder flow characteristics have really only existed in recent decades. It was not until the 1960’s that engineers like Andrew Jenike, Jerry Johansson, Ralph J Carr Jr and Henry H Hauser started to develop more scientific and robust approaches to understanding and measuring powder flowability, with the development of several methodologies occurring within a few years. In 1964 Jenike released his ground breaking paper on the Storage and Flow of Solids, outlining his theory on the stresses generated in quasi-static and static regimes in hoppers and silos, and demonstrating the suitability of translational shear cells for measuring incipient flow during hopper discharge[1]. Carr [2] and Hasuner [3] published papers in in 1965 and 1967 respectively, describing the use of density changes as a function of consolidation to quantify powder flowability. Previously flow characterisation had been limited to techniques such as Angle of Repose (AOR), with Jenike stating that “it is only useful in the determination of the contour of a pile, and its popularity among engineers and investigators is due not to its usefulness but to the ease with which it is measured”, although Carr disagreed, suggesting that AOR provided an indirect measurement of size, shape, porosity, cohesion, fluidity, surface area and bulk density of the material [2]. It was during this period that Jerry Johanson was first entering the field, completing his PhD under the supervision of Jenike in 1962 and joining Jenike in the formation of Jenike and Johanson in 1966.

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 1990’s. The ATSM for measuring the flow properties of metallic powders using ball indentation was published[4] in 1990. Avalanching techniques were first proposed by the likes of Kaye in 1995[5] as a means of studying the rheological behaviour of powders. Meanwhile, Johanson and Schulze built on Jenike’s work for determining flow behaviour in hopers and silos. In 1991 Johanson patented the Johanson Indicizers (Hang-up, Hopper Flow and Flow Rate)[6] whilst Schulze introduced the Ring Shear Tester in 1994[7]. 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. 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 [8] to Malby and the POSTEC Uniaxial Tester [9] 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[10], 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 [11] and the Raining Bed Method (RBM), first proposed in 1967 but not developed fully until 2002 [12]. More recent developments based on the addition of air have included research by Tardos et al [13] 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 [14] 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 Jenike’s theory for quasi-static behaviour. The reality is this field is still very much in it’s infancy, and there remains plenty of opportunity for current and future scientists and engineers to advance our understanding of bulk solids behaviour.

1. Jenike, A.W., Storage and Flow of Solids. 1964: University of Utah.

2. Carr, R.L., Evaluating flow properties of solids. Chemical Engineering, Vol. 72 No.(2) 1965 pp. 163-168

3. Hausner, H., Friction conditions in a mass of metal powder. International Journal of Powder Metallurgy, Vol. 3 No.(7) 1967

4. American Society for Testing and Materials, Use of Automated Ball Indentation to Measure Flow Properties and Estimate Fracture Toughness in Metallic Materials, in ASTM 1092. 1990, ASTM: Philadelphia. p. 188-208.

5. Kaye, B.H., J. Gratton-Liimatainen, and N. Faddis, Studying the Avalanching Behaviour of a Powder in a Rotating Disc. Particle & Particle Systems Characterization, Vol. 12 No.(5) 1995 pp. 232-236

6. Fayed, M. and L. Otten, Handbook of Powder Science & Technology. 2013: Springer US.

7. Schulze, D., Entwicklung und Anwendung eines neuartigen Ringscherätes (Development and Application of a novel ring shear tester). Aufbereitungstechnik, Vol. 35 No.(10) 1994 pp. 524-535

8. Schwedes, J., Review on testers for measuring flow properties of bulk solids. Granular Matter, Vol. 5 No.(1) 2003 pp. 1-43

9. Maltby, L. and G. Enstad, Uniaxial tester for quality control and flow property characterization of powders. Bulk Solids Handling, Vol. 13) 1993 pp. 135-135

10. Freeman, R., Measuring the flow properties of consolidated, conditioned and aerated powders — A comparative study using a powder rheometer and a rotational shear cell. Powder Technology, Vol. 174 No.(1–2) 2007 pp. 25-33

11. Castellanos, M.A., R.A. Ramos, and M.J.M. Valverde, Dispositivo y procedimiento para medir la cohesion de medios granulares finos. 1999, Google Patents.

12. Formisani, B., et al. The bed support experiment in the analysis of the fluidization properties of fine solid. in 4th World Congress on Particle Techonology. 2002. Sydney (Australia).

13. Langroudi, M.K., et al., An investigation of frictional and collisional powder flows using a unified constitutive equation. Powder Technology, Vol. 197 No.(1–2) 2010 pp. 91-101

14. Calvert, G., et al., A new environmental bulk powder caking tester. Powder Technology, Vol. 249 No.(0) 2013 pp. 323-329