An introduction to the industry-spanning powder testing methods used to quantify flow behaviour.

Ensuring effective powder flow is a widely shared industrial challenge. Whether you’re making metal components by additive manufacturing, packaging detergent, ensuring the stability of food ingredients under storage or filling tablet dies, knowing how easily a powder moves under different conditions can be the key to success. As a result, many industries rely on bulk powder testing in research and development, for process optimisation, the assessment of alternative feedstocks, QA and QC.

This reliance explains the multitude of pharmacopeial specifications, ASTM and ISO standards relating to the assessment of powder flow and to bulk density, a complementary metric.

Many of these standards reference four core powder testing methods: Angle of Repose, Flow Through an Orifice, Tapped Density (Hausner Ratio and Compressibility Index), and Shear Cell. If you’re working with powders, these are the methods you need to know. Today we’ll take you through the basics of each, explaining the method and how to interpret the results.

Angle of Repose

Angle of Repose

 

What parameter is measured?

The angle, relative to the horizontal, that a powder assumes when it comes to rest after flowing freely from a funnel.

How?

Measuring Angle of Repose involves:

  • Loading a known mass of powder into a closed-ended funnel
  • Opening the funnel to release the powder onto a flat surface of fixed diameter
  • Measuring the height of the resulting conical pile and dividing it by the radius of the base to quantify the tangent of the Angle of Repose
  • Taking the inverse tangent to determine the Angle of Repose
What do the results indicate?

Angle of Repose is indicative of the friction between particles and the extent to which the powder resists movement. A low angle between 25 to 30o is indicative of little friction and excellent flowability. Conversely, powders with an Angle of Repose greater than 45o are unlikely to flow sufficiently well to be suitable for efficient manufacturing (see table below).

Flow Property

Angle of Repose (degrees)


Excellent

25-30

Good

31-35

Fair – aid not needed

36-40

Passable – may hang up

41-45

Poor – must agitate, vibrate

46-55

Very poor

56-65

Very, very poor

>66

Flow through an Orifice

Flow through an Orifice

 

What parameter is measured?

The time taken for a known mass (or volume) of powder to flow through an orifice of defined geometry. Alternatively, the mass of powder that flows through an orifice of defined geometry in a defined time.

How?

Powders may be held in a hopper or cylinder and the orifice may vary with respect to both size and shape, depending on the application of interest. However, with all apparatuses measurement involves:

  • Loading a known mass or volume of powder into a closed-ended funnel or cylinder
  • Initiating powder flow by opening the shutter covering the orifice
  • Measuring the mass or volume of powder that flows into the collection vessel, continuously, as a function of time as the funnel or cylinder empties. The alternative is a discrete measurement of mass over a defined period
What do the results indicate?

Flow Through an Orifice methods are best-suited to powders that are relatively free-flowing, though testers vary with respect to their limits of usability. For example, a Carney or Gustavsson funnel is recommended for metal powders that fail to flow through a Hall funnel. Both particle- and process-related factors influence the results but there is scope to vary equipment parameters to maximise relevance for a specific application. For example, a funnel made from pharmaceutical grade 316 stainless steel can be used to simulate a pharmaceutical production hopper.

Tapped Density

Tapped Density

 

What parameter is measured?

The volume of a powder sample of known mass before and after subjecting it to a defined period of tapping (or dropping).

How?

Tapped Density measurements assess the change in density associated with vibrational consolidation from readings of:

  • The mass of a defined volume of powder, as poured into a measuring cylinder, to determine untapped bulk density
  • The volume of the same powder sample after tapping a defined number of times, or to the point of constant volume, to determine Tapped Density
What do the results indicate?

Tapped and untapped or bulk density are useful parameters in their own right but can also be used to determine the Hausner Ratio and Compressibility Index. These two closely related parameters are indicative of flowability, where:

                                                                              Hausner Ratio = ρtappeduntapped

                                                                Compressibility Index = [(ρtapped– ρuntapped)/ ρtapped] *10

Particle properties such as size, shape, density, surface area and cohesiveness have a defining influence on the impact of vibrational consolidation. Tapped Density measurements are therefore most useful for assessing changes in flow behaviour attributable to these same variables. A Hausner Ratio of 1 – 1.11 or a Compressibility Index of 1 – 10% is indicative of excellent flow behaviour. Conversely, poorly flowing powders will produce a Hausner Ratio in excess of 1.35 and a corresponding Compressibility Index > 26% (see table below).

Compressibility Index (%)

Flow Character

Hausner Ratio

1-10 (USP 1-May-2024)

Excellent

1.00-1.11

11-15

Good


1.12-1.18

16-20

Fair

1.19-1.25

21-25

Passable

1.26-1.34

26-31

Poor

1.35-1.45

32-37

Very poor

1.46-1.59

>38

Very, very poor

>1.60

Shear Cell

Shear Cell

 

What parameter is measured?

Shear Cell testing involves measurement of the force required to shear one consolidated powder plane relative to another.

How?

Shear Cell measurements are more complex than those of the preceding techniques. Furthermore, there are multiple Shear Cell designs and methodologies vary accordingly. A full description lies beyond the scope of this blog but in simple terms analysis typically involves:

  • Consolidation of the sample through the application of normal pressure
  • Rotational or translational shearing of the sample
  • Determination of the force associated with the shearing process
  • Repeat measurements at multiple values of applied normal stress
What do the results indicate?

Shear Cell analysis quantifies the ease with which a consolidated powder begins to flow and is indicative of the strength of cohesive interparticular forces. It was developed specifically to support the design of hoppers and while measured metrics may be taken as an indicator of relative flowability the technique retains most value for this purpose. That said, the consolidated flow regime that Shear Cell analysis induces is relevant to other processes. Test data are therefore used more generally to investigate and compare powders for processing under conditions of moderate stress.

Take away tips

By assessing how easily powders flow under specific conditions, manufacturers can predict how their powder will behave during key processes such as mixing, packaging and blending. Testing for flowability allows for the optimisation of equipment settings such as hopper designs and feed rates, to avoid issues such as clogging, inconsistent product quality or downtime. It also aids in maintaining consistent batch to batch production, improves product uniformity, reduces waste, and can also help manufacturers create a design space during product development. Overall, powder flowability testing helps enhance operational efficiency, reduces costs, and ensures high-quality final products.

Choosing the right powder testing method is all about the intended application. Look to industry standards and established practices to narrow down the options. If you’re interested in learning more about our solutions for powder testing, then take a look at our range. We have options for all the methods discussed so whatever powder you’re testing we’re sure to have a solution that can help.

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