Many commercial products are handled as powders at some point in their manufacture or during end use. As a result, efficient powder handling underpins the success of many industrial processes, from bulk solids transport to additive manufacturing and pharmaceutical tablets production.
However, powder processing is widely recognized as being challenging, with unit operations often exhibiting lower reliability and greater variability than those only involving liquids and gases.
Different grades of powder are routinely specified as either feed or product in terms of composition and particle size alone. A common problem with this approach is that these specifications fail to reflect how a powder performs in a process and especially how it flows. Therefore, a change in processing performance can occur when all aspects of the process are apparently constant. This raises the question of which tools engineers can usefully apply to find the insight needed to directly address such issues.
This article examines the potential of a new powder testing technique, uniaxial testing, within this context. It looks at what parameters it generates, how these compare with other metrics used to assess powder flowability and the value of this method within the processing environment.
Focusing on flowability
The flowability of a powder often directly impacts manufacturing efficiency and the value of an end product. In process, it can affect plant downtime and the consistency of exiting material. For a finished product, flowability may correlate directly with, for example, the hardness and content uniformity of a pharmaceutical tablet, or the strength of a moulded or printed metal component. These relationships make flow testing a valuable tool for powder processors that can be used to answer questions such as:
- Can I take advantage of this lower raw material supply without compromising manufacturing efficiency or product quality?
- Why do we have variability in fill weight, discharge rate or blockage frequency when we can detect no variability in raw materials or operating conditions?
- Can we process this new formulation in our existing equipment, or do we need to modify the plant to accommodate it?
- If our quality control is robust then why do certain batches of product trigger an increase in customer complaints?
To respond to the industrial need voiced by the questions above, a powder testing technique must be relevant, meaning it must produce data that can be correlated with process and product performance. Equally importantly, it must be highly repeatable and reproducible because uncertainty in data accuracy erodes its ability to robustly detect important but sometimes subtle differences. A third issue is that of practicality. Measurement times, ease of training, manual input requirements and the cost of equipment all directly impact the value of any proposed solution for industrial powder testing.
Options for powder testing
Powder processors have long recognized the potential merits of measuring flowability, and many methods have evolved to tackle the problem. Simple techniques include the angle of repose, flow through an orifice and tapped density methods such as Carr’s index. These meet the need for low cost testing equipment and low levels of manual input, but in many circumstances they fail to provide the insight needed to robustly tackle process-related issues. These techniques tend to suffer from relatively low reproducibility and can also be misleading in terms of the data generated. For example, changes in bulk density induced by tapping for materials such as colloidal silica can indicate different flow properties than what is exhibited.
Biaxial shear cell testing is commonly used to support the hopper design methodologies advanced by Jenike in the 1960s. It remains in widespread use for this application and is more generally applicable for measuring and comparing the cohesivity of powders under the moderate to high stresses that exist in storage and other processing environments. In fact, shear cell testing is one of the most established techniques for industrial powder testing.
A well designed tester used appropriately offers high repeatability and enables closer control of the applied test conditions than other simple methods. Shear cell testing can, therefore, be used to directly assess the impact of certain process parameters on flow, such as consolidation stress. However, the practicalities of analysis can be limiting regarding the need for low cost and responsive measurement. Shear cell analysis is far less relevant for investigating processes where powders are under low stress, aerated or even fluidized.
In terms of process relevance, shear cell analysis is ideally complemented by dynamic powder testing, a technique specifically developed to address the requirement for powder testing under conditions that simulate a range of process environments. With dynamic testing, powders can be measured in a consolidated, moderate stress, aerated or even fluidized state to gain a fuller understanding of how they will behave under all the conditions likely to apply during processing. Dynamic testing enables processors to systematically investigate the impact of aeration on powder behavior up to and including the point of fluidization. It can also examine the effect of variables such as flow (or strain) rate, moisture content, storage time and electrostatic charge.
On the other hand, the recently commercialized technique of uniaxial testing has much to offer to address the practical limitations of shear cell analysis.
Introducing uniaxial testing
In simple terms uniaxial testing involves measuring the stress required to break or fail a freestanding, consolidated column of powder. This is the unconfined yield strength (UYS) of a material through extrapolation of the measured data, and it is the same term that is derived in biaxial shear testing. This ability to directly measure a powder characteristic already in widespread use is a key attraction of uniaxial testing for industrial application. Furthermore, eliminating extrapolation steps associated with uniaxial shear cell testing minimizes sources of error and helps make uniaxial testing highly repeatable. Equally importantly, from an industrial perspective measurement times are short, and equipment costs are low.
Uniaxial testing is not a new concept, but the practicalities of measurement have only become tractable as powder testing technology has matured. In recent years, powder testers have become progressively more precise and automated, offering an enhanced foundation for the development of new testing protocols. In collaboration with researchers at the University of Edinburgh¹ and an industrial partner, Freeman Technology has used modern capabilities to develop and commercialize a uniaxial tester, making the technique widely accessible to the industrial community.
The recently introduced uniaxial powder tester includes manual and advanced versions (see Image 1) and incorporates features such as double-ended compression to ensure that the constructed column is homogeneously consolidated ahead of fracture.
The testing procedure itself is simple and quick. Powder is loaded into a sleeve and then consolidated, either manually or by a motor-driven vented piston, which automatically controls the applied normal stress. Once consolidation is complete, the consolidation stress is reduced to zero and the sleeve is removed — before the piston is then moved down again at a constant speed to fracture the free-standing column. The UYS of the powder is determined directly from the measured force/displacement data.
The tester can measure a wide range of different powders, from those that are relatively free-flowing, such as microcrystalline cellulose (MCC), to limestone and other more cohesive powders. Figure 1 shows how closely UYS data measured using the tester agrees with biaxial shear cell data for MCC and talc samples. In absolute terms, biaxial shear cell UYS values tend to be slightly higher than those measured with the tester because of differences in the measurement conditions applied, but the techniques are normally interchangeable in terms of the trends reported.
Evaluating the potential benefits
The introduction of this new uniaxial tester brings relevant, repeatable powder testing down to a new price point, offering an opportunity to extend its application. In addition, faster measurement times increase the attractiveness of testing within the processing environment to solve process-related problems. This raises the questions: For whom is uniaxial testing likely to prove most useful? And for which applications?
One area where uniaxial testing is likely to add value is in quality assurance and control (QA/QC). Here rapid, repeatable measurements are required either for raw material acceptance or product release. For those applying or considering biaxial shear cell analysis within this context, uniaxial testing is a less expensive alternative that offers shorter measurement times, lower manual input and reduced training requirements. However, in many circumstances, uniaxial testing can improve the relevance and precision of QA/QC acceptance, enhancing manufacturing efficiency and/or product quality.
The attributes of uniaxial testing also make it well-suited to process troubleshooting, especially where techniques such as tapped density or flow through an orifice have failed to provide the insight needed to solve the problem or where no powder testing is in place. In many cases, uniaxial testing could prove particularly helpful for solving hopper discharge problems, a perennial problem in powder processing plants for assessing the adequacy of storage environment control or for rationalizing issues such as variable fill weight.
The commercial introduction of a uniaxial powder tester opens up new opportunities for powder processors looking to improve process efficiency and product performance. Making powder testing more accessible in terms of cost, time and training requirements facilitates its wider application to directly address manufacturing problems. Over the long term, uniaxial testing has the potential to deliver substantial economic benefit through better QA/QC, more effective troubleshooting and more broadly making powder testing a routine aspect of powder processing.
1. Bell, T.; Catalano, E.; Zhong, Z.; Ooi, J.; & Rotter, J. (2007) Evaluation of the Edinburgh powder tester. Proceedings of the PARTEC, 2-6.
Tim Freeman is the managing director of Freeman Technology Ltd.