By Robert G. McGregor, General Manager – Global Marketing, Brookfield Engineering Laboratories, Inc.
Tablet production depends on steady flow of powder from a feed hopper to the tablet machine. Hang ups are unwelcome because they interrupt production and necessitate a costly restart. Testing the powder for flowability before manufacturing start up is a must in order to prevent this type of problem.
R&D has always used a popular method like the Tap Test because it’s quick, easy to perform, and the data may give a picture of potential flow problems. However, this particular test is more an indicator of compaction density for the powder and not flowability in gravity discharge from a hopper.
Shear cells (see Figure 1), a popular tool for characterizing powder flow behavior, actually create a density curve as part of the standard Flow Function test. This standard test involves compaction of a powder sample in the shear cell and measuring the sliding friction between particles when the trough rotates relative to the lid. At each compaction pressure, the reduced volume of powder sample is measured and a corresponding density value is calculated. The graph in Figure 2 shows an example of density vs. consolidating stress. This particular piece of information has given R&D the ability to convince manufacturing that shear cells actually do a better all around job. Interestingly enough, the pharmaceutical industry has become a major practioner of the shear cell method.