Why is it important to measure powder flow?

What Mechanisms Influence Powder Flow?

Powders consist of particles, typically combined with air, and often appreciable levels of moisture too. Powder flow occurs when its constituent particles move relative to one another.

Powder flow is influenced by particle properties, such as size, shape, surface roughness and charge, and by external variables, for example, aeration or consolidation of the sample, or the ingress of moisture. The flow properties of powders are industrially critical but complex, and not yet amenable to mathematical modelling.

Figure 1 – Variables that influence powder flow
 

It’s a common misconception that powder behaviour can be described by just understanding its flowability, and equally that flowability is a discrete property that can be quantified with a single number. Unfortunately, neither of these assumptions are correct, which explains why a fundamental understanding of powder behaviour remains elusive. Consider a loosely packed powder in a glass jar and visualise its response when the jar is tumbled. Then consider how differently it would behave if the jar was first tapped several times on a hard surface. Differences in behaviour between the loosely packed state and the tapped, consolidated state would be due to the bulk characteristics of the powder. If the powder were dry sand, then it would behave in a similar way before and after tapping. However, if the powder were flour, for example, it would flow very differently after it had been tapped. A single flow parameter, or knowledge of physical properties such as particle size or shape alone, would not inform on this varying behaviour.

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Why Measure Powder Flow?

Powders are integral to a huge range of industrial processes, contributing to an estimated 80% of all manufactured goods. Despite being commonplace throughout many applications, powders continue to present challenges during product development, manufacturing, and in quality assurance. Powders may be subjected to storage, transportation and handling, before and during processing. In storage, they can consolidate or cake and become more resistant to flow. When aerated, for example by discharge from a hopper, separation of the particles by air reduces resistance to flow. Typically, a consolidated powder can require around 100 times more energy to make it flow than the same powder aerated; for a fluidizable powder this may be more than 1000 times. 

It is therefore essential to generate relevant data by testing powders under conditions that simulate the environment and specific stresses associated with a process or application.

Figure 2 – Powders are used in a wide array of industrial processes, from food to spray coating
 

How to Measure Powder Flow?

Dynamic powder testing with a Powder Rheometer^® directly quantifies powder flow properties across a broad range of stress regimes by measuring the energy associated with bulk powder movement. Powder rheology has seen considerable advances in the last decade and our understanding of how to test powders to support industrial processing and use has continued to progress. Studies can now be implemented effectively and precisely to derive valuable correlations for the optimisation of both process and product performance.

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Further Reading

• An Introduction to Powders - explains how and why powders behave the way they do, and how this impacts powder flow.
• Modern Tools for Hopper Design - examines the issues surrounding hopper design and shows, using a worked example, how the FT4 Powder Rheometer software simplifies the whole process – from evaluation of feedstocks through to specification.
• Developing a Design Space for a Die Filling Operations - gain an understanding of the relationships between powder characteristics and process performance to match filling-machine geometry to specific formulations.
• Profitable Powder Flow Processing - a whitepaper which summarises ways in which our technology has helped users to work more effectively. Experience has shown that by boosting efficiency, from R&D to manufacture, the FT4 can contribute significantly to a better understanding of powder flow and ultimately greater profitability.
• Working with Powders - a presentation which examines how comprehensive powder flow characterisation can optimise your process.

• Measurement and quantification of caking in excipients and food products with emphasis on the non-homogeneous interaction with ambient moisture - a peer-reviewed paper which highlights the unique value of dynamic powder flow testing for the elucidation of caking mechanisms, focusing on non-homogeneous caking or crusting.

Further information on powder flow and powder behaviour for different industries is also available via our applications pages - freemantech.co.uk/applications