Flowability and Flowrate Tests

This is another rather vague term used, meaning any measure of the ease with which a bulk powder flows out of a hopper or down a chute38. Any of the non-aerated flow and handling properties could go under this category but this section only deals with those measures of flowability not covered elsewhere in this guide. 

There is, of course, a direct way of testing flowability by measuring the time for a standard amount of powder to be discharged from a specified funnel or hopper. This is sometimes referred to as free flow time , as for example specified by an ASTM standard method40 for testing metal powders the funnel used is the same as in the International Standard ISO 3923, Part 1. The ASTM method was used in measuring the flow time of sands in a study of particle morphology (see Ref. 10, Chapter 17). 

For less easily flowing powders, the funnel may be vibrated to assist the flow and the solid thus may even be tested in a consolidated state. This is done in the Durham Vibrating Cone apparatus41 in which a metal cone is vibrated in a horizontal plane. 

Another, this time indirect, method of assessing flowability was proposed by Carr36. This was based on measuring four properties of the powder (angles of repose and spatula, compressibility and uniformity coefficient, all as defined by Carr36) and, by awarding points (out of 25) for each, the sum total was then a measure of flowability and called Carr s Index . A commercial piece of equipment is available (Hosokawa Powder Characteristics Tester) for measuring the properties necessary for Carr s Index. 

Flowability is a difficult thing to measure as it depends very much on the design and characteristics of the delivery device as well as on the powder itself. The procedures for the free flow time are far from standard at the moment and improvement might be needed here. It seems, however, that it is far better to measure flowability by the failure properties which are functions of the powder only and for which theories exist to be used in equipment design and scale-up. 

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