Characterising powder formulations, mixes and blends for flowability is a necessity that is sometimes elusive and frequently done with archaic methods. The reason? There has really been no cost effective way to test and characterise powder flow properties. In some cases this is merely guesswork. Some common tests, such as ‘smear’ tests, angle of repose, and flow through a funnel have one commonality: They are subjective and do not provide a number or value that correlates consistently with behaviour on the production floor.
Another common method, the ‘tap’ test, calculates values known as the Hausner Ratio and Carr Index; these tests use the measurements for the freely settled bulk density and tapped bulk density to determine flowability. These indices have been criticised as not having a strong theoretical basis. Legacy types of tests like these can be fraught with errors due to the operator procedure and sample preparation technique, thus making the data that results seem subjective as well.
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| Figure 1: Brookfield’s Powder Flow Tester | Figure 2: Shear Lid used for Flow Function Test |
Sophisticated powder flow measurement tools have been in existence for years, but can be pricey and difficult to use. So this leaves the powder manufacturer with a choice of a subjective test which may cause more problems than it is worth, an expensive time consuming flow instrument that requires a trained operator, or finally the option to send a sample to a powder analysis house in return for a costly analysis document. After six years of development in conjunction with the prestigious Wolfson Centre at the University of Greenwich, UK, Brookfield Engineering introduced a low cost, easy to use Powder Flow Tester (see Figure 1 and 2) based on shear cell methodology initially established by the Jenike shear cell from the 1960’s.
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| Figure 3a: Flow Function Graph |
The Powder Flow Tester is an easy to use instrument; an operator can be trained on its use in minutes, not days. Totally automated and software driven, the Powder Flow Tester gives comprehensive powder flow data such as powder flowability, bulk density, internal friction angle, arching dimension and rat hole diameter. All of these parameters can be realised in one 45-minute test. The user can use any or all of these to characterise their powders. For a Flow Function Test (see Figure 3a), after prepping a sample, the user merely inserts the trough of sample powder into the instrument, enters identity data for the sample, and then presses a software button to begin the test. The user is then free to move on to other tasks while the test runs. In 45 minutes, a comprehensive data set is derived on a small sample of powder (either 263cc or 43cc depending on size of shear cell that is selected). (See Figure 3b)
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Figure 3b: Bulk Density Curve Graph
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Powder manufacturers, formulators, and developers now have a way to easily analyse their products for flow behaviour during characterisation, formulation and blending. The Brookfield Powder Flow Tester allows fast comparison for up to eight data sets. Thus, formulations, for example, can be verified against known good “controls” to ensure the correct procedure and methods have been ascribed to. But Brookfield takes powder flow analysis a step further.
What if you have two or more separate powders with separate bulk densities? You need to blend them, but how are they going to flow?
What if you have two or more separate powders with separate bulk densities? You need to blend them, but how are they going to flow? You can test them individually, but that doesn’t give a true picture of how these powders will flow when mixed. What if they jam and cause expensive downtime? Trying to determine the flow problem can waste even more time and money. Why not characterise these powders ahead of time for optimal flow characteristics…but how?
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Figure 4: Normalized Flow Function Graph
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The answer is to use a Normalised Flow Function (see Figure 4), a key feature in Brookfield’s latest release of its powerful software, Powder Flow Pro. This is a flow function that has been normalised for changes in bulk density at increasing levels of consolidating stress, which essentially accounts for the compression of powder particles in the hopper as the fill level in the bin rises: This approach incorporates the bulk density information from groups of materials, such as formulations of multiple powders. It is the main function that should be used for comparing and ranking the relative flowability of groups of materials.
Consider that the flow function represents the strength of the material (its resistance to flow). The bulk density or self-weight of the material provides the force to overcome the internal strength. Thus to obtain a valid comparison of different classes of bulk solids, where there could be large differences in the bulk density, it is necessary to normalise the flow function. The normalised flow function is calculated by dividing the unconfined failure strength by the powder’s bulk density at each consolidation stress and gravity to give units of length (i.e. the arching potential).
The normalised flow function is interpreted as follows. A free flowing material will have a normalised flow function that is close to horizontal and lies approximately on the consolidation stress axis (i.e. an outlet dimension of zero at all consolidation stresses). A non-flowing material will have a steeply sloped line. Poor flowing materials can be rated by the magnitude of the outlet dimension and also by the rate at which the outlet dimension increases with increasing consolidation stress.
The Brookfield Powder Flow Tester gives fast, comprehensive data for all facets of major flow characteristics at a very low price point. For the first time in the science of powder flow, there is an instrument that is easy to use, cost effective and gives easy to understand definitive data.
