Do you completely understand all aspects of your powder process, formulating, specifying and manufacturing product in an optimal manner on the basis of underlying knowledge? Or do you, like many other producers, rely heavily on the experience of formulators, supervisors and operators running sub-optimally but well enough to get by?
Here we discuss how your experience can be fully exploited by correlating process observations with measurable powder parameters. This approach brings improved understanding, which in turn leads to cost reductions and a lowering of the risks associated with change.
Fundamental Information on Powder Behaviour
For powder processors, fundamental information has always been elusive because of the complexities of powder behavior.
How to determine the combined effects of attrition, segregation, moisture adsorption, consolidation during storage and many other variables amounts to a serious challenge. Consequently the development of new formulations, process design and process operation, have all tended to rely heavily on previous experience.
Stumbling on Solutions Using A Trial and Error Approach
A trial and error approach to problem-solving is common and although time-consuming can be successful. Different solutions are assessed; one proves optimal and processing or formulation development continues. The limitation of such an approach is that the reasons for the success, or indeed the original problem, often remain unknown. A ‘fix’ has been developed but understanding has not necessarily increased.
The experience base that develops from working in this way is highly valuable but can only improve operation within a well-mapped window, unless more widely applicable process knowledge can be extracted from it. This is possible if aspects of experience are correlated with specific powder characteristics.
Understanding which aspects of a powder make it behave, in a certain way, provides the underlying knowledge necessary to make changes away from the established operating regime. We can extrapolate outside the box, rather than simply interpolating within it.
For example, tests have shown that the propensity of a material to flood from a hopper can be predicted from dynamic studies of basic flow energy and aeration behavior. In an analogous way consolidation behavior during storage can be assessed through systematic consolidation studies. Many such correlations exist; the key is identifying those of relevance for a particular process or product.
A Knowledge Based Approach
Quantifying and understanding experience is a valuable way of exploiting the experience base within a company. It is also an excellent starting point for a shift from experienced-based to knowledge-based operation. This paper outlines how to achieve this goal, beginning with an examination of the drivers for changes.
Drivers for Change
Continuous improvement is now a ubiquitous feature of manufacturing life, the alternative being a drift towards uncompetitiveness; change is inevitable. There are two key ways in which processors can improve their competitive position; they can reduce variable costs or improve product performance. Variable cost reduction goals may include:
- increasing throughput – by increasing reliability/reducing the frequency of stoppages or by processing at a faster rate
- reducing waste – by, for example, ensuring that more of the product meets the required specification without re-work
- switching to alternative, lower cost feed materials
- automating process control
In order to pursue these goals, processors need to understand what changes to make and/or the impact of these changes on the process and product.
For example, effective troubleshooting requires identification of the root cause of the problem; adoption of an alternative feed demands careful assessment of the potential impact on all aspects of the process; automation requires the identification of critical controlling parameters and an understanding of how these influence process behavior and product quality.
Introduction of New products or Formulations to Address Performance Issues
Companies may introduce new products or formulations to address specific performance issues. Potentially this is even more demanding, particularly if it involves the introduction of a new powder product. In this case the following questions need to be answered:
- Will the new formulation process well in the existing equipment?
- If the existing equipment is unsuitable can it be modified or is a new unit needed?
- Are the existing storage facilities suitable for the new product? Is it particularly sensitive to pressure, temperature, humidity, vibration etc?
- Will the new product give the customer the required performance? Will it be adversely affected by storage and handling procedures at the customer site? Will it process well in their equipment?
The aim is often to develop a blend that will process well in existing or identical equipment. Successfully introducing a new material with the minimum of pre-production trials has enormous benefits, reducing both time-to-market and development costs.
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The Benefit of Experience
Companies that process powders have, typically through a combination of knowledge, experimentation, and trial and error, learned how to manufacture products that meet the required specification.
Gathering this experience will have incurred significant, unquantified costs. Where a range of products is manufactured, some powders may be particularly tricky to handle, requiring constant monitoring or the attention of an expert operator. Others may process easily, even at high rates, giving excellent product quality. Relatively frequent unplanned stoppages may be viewed as an inevitable feature of day-to-day operation.
Historically, sensitive and reliable powder characterisation methods have not been available and reliance on subjective human assessment therefore remains widespread. It is unusual to find QC specifications for flow properties of a material, or even bulk properties. This is despite the fact that tests have shown that dynamic measurements can differentiate between samples of the same specification, identifying those that will process poorly [See ref].
Batch-to-batch variability, of either feed or product, is also common. This can give rise to a wide range of processing problems – bridging in a hopper, blockage in a feeder, excessive aeration, or variability in product quality, dose weight or composition.
The problems may be clearly linked with a change in material, but the underlying reasons for the poor performance are not understood. This is primarily because so many factors can be responsible for a change in powder behavior. Moisture content, fines, particle size, shape and texture, air entrainment, adhesion and a wide range of other parameters are all influential.
On the basis of process experience it is possible to classify materials using ‘Processability Rankings (PR)’. For example, a PR of 2 may describe a trouble-free powder that processes consistently and easily at high rates. A PR of 9, on the other hand, may indicate a difficult powder that needs constant monitoring and processes to a variable product. This is a useful way of starting to quantify experience.
Valuable information can also be generated by:
- recording the symptoms and circumstances of any unplanned shutdowns or lost batches
- correlating performance with supplier, if more than one is used
- correlating in-house batch variability with customer feedback
- comparing powder performance on parallel production lines
- identifying periods when the plant performs poorly; for example is performance worse on a Monday morning after a weekend shutdown?
Investigating and analyzing experience in these ways highlights powders that are optimal in terms of processing and product performance. Relevant powder characterization gives insight into why these powders behave as they do.
Relevant Powder Characterization
Powders are complex materials and can be difficult to characterize well. There are many measurement techniques, all of which give some insight into powder behavior. Historically, however, many have been unreliable and overly simplistic.
Options for process relevant characterization improved greatly with the development of powder rheometers and dynamic test methodologies. More recently these instruments have evolved into fully automated universal powder testers with a range of complementary measurement methods, making them an ideal tool for powder processors. It is now relatively easy to measure powder properties that correlate directly with processing performance.
The FT4 Powder Rheometer® from Freeman Technology
The FT4 Powder Rheometer from Freeman Technology provides automated test programs for the measurement of shear, bulk and dynamic properties.
Samples may be analyzed when consolidated, conditioned, aerated or even fluidized, allowing the impact of air, a critically important variable, to be fully explored. Conditioning, - gentle agitation of the powder prior to measurement - ensures that samples are always measured in the same baseline state, giving the instrument exceptional reproducibility and repeatability.
The Freeman FT4 Power Rheometer
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Powder Properties that can be Determined by Powder Rheometry
Using instruments such as the FT4 Powder Rheometer from Freeman Technology, databases of powder properties can be established. Measured data may include:
- Flowability parameters - Basic Flow Energy (BFE), Specific Energy (SE) etc [See ref]
- Shear properties – yield locus, unconfined yield stress (compression strength), cohesion, internal angle of friction
- Bulk properties – bulk density, compressibility , permeability
Other Factors that can be Assessed Using a Rheometer
In addition, many other factors (de-aeration behavior, de-blending/segregation, caking, effect of moisture and attrition) may be investigated to generate the most complete picture of the powder. The resulting database provides the information for rationalizing process experience. Close correlation of powder properties and processing experience provides the key to confident product and process development.
Where Knowledge of Powder Properties Can be Useful
For example, consider a filling process in which blend A processes well at high rates (PR=2) while blend B is problematic (PR=6). One goal may be to improve the processability of B by slightly changing the blend. Alternatively, completely new formulations may be in development, the aim being to manufacture a third product on the same line. In either case it would be highly advantageous to understand the properties of A that give it such excellent performance.
By measuring an array of powder properties for blends A and B it is possible to determine differences between the two samples, thereby identifying parameters that need to be controlled to achieve the required performance. Intuitively, for filling operations, it is likely that variability in flow energy, de-aeration, compressibility and/or shear strength characteristics will provide the key. Identifying which of these are critical gives a blueprint for the type of powder that will process well on the line; other powders meeting this specification are also likely to be a good match.
Effective Powder Management
By correlating experience with powder properties, processors can operate in a much more effective way. It becomes possible to:
- Establish effective QC criteria for both feed and product
- Understand and address the causes of batch-to-batch variability
- Assess in a process-relevant way, differences in alternative suppliers’ materials
- Define the characteristics of materials that will process well on a given line
- More effectively match powder and processing equipment
- Reduce the risk associated with introducing new formulations
- Improve the performance of marginal formulations.
Decisions and actions are based on understanding and knowledge gained from quantifying experience and therefore have a greater likelihood of success.
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Powders are complex, and characterization methods have historically been unreliable. Manufacturers handling powders have therefore learned to rely heavily on human experience and subjective assessment. This has created a wealth of valuable knowledge that enables operation within a defined window. The drivers for process and product improvement are however strong, making change inevitable. Extracting information from past experience, in a form that can be used to move forward, is the key to confident renewal.
With state-of-the-art powder testers such as the FT4 Powder Rheometer from Freeman Technology, shear, bulk and dynamic powder properties can be measured easily. Powders can be characterized in a consolidated, conditioned, aerated or even fluidized state. These instruments deliver the fullest insight into powder behavior allowing identification of the key parameters influencing process performance.
This is not a first principles analysis. Defining powder flow properties in terms of particle size, shape, hardness and many other variables remains beyond our current capabilities. It does, however, allow formulators and manufacturers to rationalise behavior in terms of parameters that can be measured reproducibly, and to differentiate between alternative materials in a process-relevant way. This is a significant step forward towards optimised operation that is based on product and process knowledge.
Freeman, R F (2007). Measuring the flow properties of consolidated, conditioned and aerated powders : a comparative study using a powder rheometer. Powder Technology 174, 25-33
This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
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