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Why are powders challenging?

As many as a hundred different parameters can influence powder behaviour. The critical nature of air and moisture content is widely recognized in the process environment, but many other parameters are also important. Primary factors include particle size and distribution, shape, porosity, hardness and surface texture. Temperature, vibration, flow rate and storage time are all good examples of secondary or system variables.

Unfortunately, the corollary of this is that powder behaviour is not easily controlled. Maintaining consistent behaviour can be very difficult when it demands close control of so many variables. For example, in the initial stages of a process, a powder may have sharp, needle-like particles, but if they are not strong then the size and shape distribution downstream will be markedly different. Flow properties are likely to be transformed. Alternatively, or indeed additionally, sensitivity to moisture content may be an issue. Variations in moisture content nearly always affect powder flowability, but the impact is particularly marked for very hygroscopic materials. A powder that flows freely when relatively dry may become much more cohesive if stored in damp conditions or processed in a humid environment.

Relevant data are required for optimal formulation, process design and operation, but it is not possible to define powder properties in terms of the many variables that influence them. The conventional response to this challenge has been to rely heavily on experiencebased development and operation; an approach that, by its nature, must be relatively inefficient.

Optimizing the value of experience

In general, experience is gathered through trial and error experimentation. For example, with time an operator may learn that material A processes easily at high rates while material B is more challenging, requiring careful control of specific variables and a much lower throughput. A third blend may prove almost impossible to process on the same line, but may be easy to handle in a different piece of equipment. Such experience is not easy to transfer, as it relates only to a specific powder or processing line. Often, little is known of the underlying reasons for the variability in performance.

Identifying the properties responsible for the excellent performance of powder A and the poor performance of B is the key to more fundamental process understanding; for example, powder A may perform well because it is freeflowing while B is more cohesive. If this is the case, then measuring terms that define these aspects of behaviour (e.g., specific energy (SE), basic flow energy (BFE), shear strength etc.) will facilitate the development of a specification for materials that process well.

Most companies have an extensive experience base, information and observations gathered at the formulation and process development stages as well as during daytoday operation. Correlating this experience with measured powder properties provides insight into the practical relevance of specific parameters, optimizing value. Such analysis narrows down the number of variables that could be influencing process behaviour to the few that actually are. When the factors influencing behaviour, or causing a problem, are understood, steps to change a process or formulation can be made on the basis of knowledge. Adoption of a knowledge-based approach is the key to Quality by Design (QbD).