Correlating process performance with powder properties
Figures 2 and 3 show correlations between filling performance and four powder properties for configurations A and B. Data
are shown for all four formulations for configuration A, and for S1, S2, and S4 for configuration B. S3 was not intended to
be processed on configuration B during production, and data for this combination are therefore unavailable.
Figure 2: Correlations between filling-performance Cpk in configuration A and various powder properties.
The results indicated that for configuration A, less cohesive materials were more compatible than cohesive materials. High
basic flowability energy, aeration ratio, and flow function, along with low cohesion, were correlated to improved processing
performance. All of these features indicated low cohesiveness. The findings suggested that for this configuration, the powder's
ability to flow freely into the feed channel and pocket determined process performance. The small feed diameter of the configuration
made this design more susceptible to the powder arching and to the discontinuous flow that can occur with cohesive materials,
which could have a dramatic effect on processing performance.
Figure 3: Correlations between filling performance Cpk in configuration B and various powder properties.
The results for configuration B showed different behavior. For this configuration, cohesive formulations (i.e., those with
low basic flowability energy, aeration ratio, and flow function) performed better. This suggested that with the wider feed
channel, the ability of the powder to flow freely under gravity was less important. Further research would be required to
fully understand the mechanisms that gave rise to the observed behavior, but it is worth reiterating that the filling process
involves more than simply filling the pocket. Successful transfer of the extracted volume as the wheel rotates is also essential.
The wide feed channel may have encouraged successful pocket filling for all formulations, regardless of cohesiveness, but
as the wheel rotates, less cohesive materials may have been lost more easily, thus compromising dose weight. The types of
powders that were optimal for configuration A clearly were suboptimal for configuration B, and vice versa.
To succeed in matching filling-machine geometry to the demands of a specific formulation, it is critical to gain a good understanding
of the relationships between powder characteristics and process performance. The results presented in this article suggest
that the optimal powder property set depends on machine geometry. Although free-flowing formulations are preferable for certain
geometries, as might be expected, cohesive formulations perform better for other geometries.
The data indicated that certain types of geometry were better suited to cohesive materials, thus highlighting the importance
of studies such as these for personnel who develop filling solutions. Equally important, the data suggested that new, free-flowing
formulations will not necessarily perform better on an existing unit than previous products. Appropriate powder testing holds
the key to achieving a compatible powder–geometry match that delivers efficient operation over the long term.
Tim Freeman is director of operations at Freeman Technology, Boulters Farm Centre, Castlemorton Common, Welland, Worcestershire, WR13
6LE, UK, tel. +44 0 1684 310860, fax +44 0 1684 310236, firstname.lastname@example.org
1. S. Kotz and N.L. Johnson, Process Capability Indices (Chapman and Hall, London, 1993).
2. R. Freeman, Powder Technol.
174 (1–2), 25–33, (2007).