Further improvements to our Powder Flow Analysis capabilities

Users of the Powder Flow Analyser (PFA) will now be interested to know that we now have split vessels available for the measurement of bulk density, as well as a new probe which attaches to the PFA motor for the measurement of compressibility.

Using the split vessel, excess powder can now be discarded after the usual two conditioning cycles using the standard PFA blade. This leaves behind a precise volume of powder. Knowing this exact volume, along with the weight of the powder (that the PFA load cell measures), conditioned bulk density can be measured accurately and automatically.

The split vessel adds an extra benefit of a flat pre-compression surface once the extra powder has been discarded and the subsequent testing of volumes of sample that are exactly the same. At this point in the test project, the user is instructed to remove the PFA blade and attach the compressibility probe. Once the probe initially reaches the surface of the powder and registers a force of 10g, the “initial bulk density” is recorded.

The probe continues to push on the powder until it reaches a force of 5000g. The more compressible the powder, the further the probe will travel during this time. Here, “compressibility” is defined as the ratio between the final and initial bulk densities:

This is equivalent to the Carr Index often used for powder analysis. In general, a more free flowing powder is less compressible (as the powder particles have already flowed into a more close-packed state). This corresponds to a small increase in bulk density with stress and a low compressibility.

A more cohesive powder tends to show the opposite behaviour and a high compressibility. Holes are present in the compression probe to allow for air pockets to be released during compression from the underlying powder column. Along with the measurement of Compressibility and Bulk Density additional parameters can also be measured within this test after it has been split into three sections: Stiffness, Relaxation and Elastic Modulus. Current users of the PFA projects will notice that projects in Exponent Connect have now been included to measure compressibility and existing projects updated to include the measurement of bulk density when using the split vessel.

To optimise 'walk away time' from the PFA, you may wish to use a new project that ties all three standard PFA tests together (cohesion, Powder Flow Speed Dependence (PFSD) and caking) and runs them sequentially on the sample – this new project is called ‘Three PFA Tests in One’.

This is now included in the sample PFA projects; it is not advisable on a sample that is likely to have a high degree of segregation or attrition, but may otherwise be found useful on many samples if sample quantity and test time are limited. It is highly recommended that the help file for each test is studied before performing the sequential test, so there is a good understanding of what each test does.

The project is set up to perform tests in order with cohesion first, followed by PFSD then caking. This order has been chosen so the least destructive tests are performed before the most destructive (caking) so the tests are most representative of using a new sample each time.