Evaluating the Powder Testing Toolkit: Tapped Density

In a recent post I discussed the extent to which traditional test methods answer modern industrial requirements for powder characterisation. Today I’m focussing specifically on tapped density methods, considering their advantages and limitations when it comes to investigating powder flow properties.

Understanding tapped density methods

Tapped density methods are based on measurement of the increase in bulk density induced by tapping a powder sample. Bulk density of the sample is first measured in a “baseline” state, and then again after a defined tapping process. Carr’s Index and Hausner Ratio are alternative ways of representing the relationship between tapped and untapped density, and they enable classification of the powder according to a predefined scale: a Carr’s Index of less than 15, for example, indicates “good” flowability. 

Such techniques have a number of practical advantages. They are quick, relatively easy to carry out and the associated instrumentation is typically inexpensive. Density change as a result of vibration or unidirectional tapping is an important aspect of powder behaviour and happens routinely during transport and processing. However, when it comes to assessing powder flowability, tapped density methods only coarsely differentiate cohesive from free-flowing samples. Moreover, attempting to apply such data to predict the flowability of different samples within the processing environment quickly highlights some important limitations of the technique.

The limitations of inferring flowability from tapped density measurements

The use of tapped density measurements to assess powder flowability is based on the idea that the interactions that influence the packing or bulk properties of a powder are the same as those that control flow behaviour. I would argue that although this is broadly true it is not the whole story. The factors influencing bulk density and flowability are not exclusively matched nor do they impact both to the same extent, as the figure below shows.

In this simple experiment the change in bulk density induced by tapping is contrasted with the change in flowability measured directly by dynamic powder testing using a Powder Rheometer. Flowability changes by several orders of magnitude, while the change in bulk density is more modest.

This comparison highlights two important limitations of tapped density methods. Firstly, they are far less sensitive to changes in flowability than alternative techniques such as dynamic powder testing. And secondly, they may be misleading regarding the magnitude of the change in flowability following consolidation. Whilst such techniques may have a place in the modern testing toolkit, they are therefore less than ideal for detailed process design, optimisation, troubleshooting and QC to the standards now required for successful manufacture.

The flow energy data shown above also illustrates the limitations of tapped density methods. Here, two samples that exhibit almost identical flowability when in the low stress packing state, associated with a Basic Flowability Energy test demonstrate different flow energies when tapped (Consolidated Energy). The tapped density of the two materials are similar (data not shown) and yet they have different resulting flow energies, indicating that density changes aren’t necessarily proportionate to flowability changes. 

Further reading: Our e-book ‘Choosing a Powder Tester’ provides more information about the strengths and limitations of tapped density and many other types of powder testing instrumentation.