Powder Flow

This article reviews the general factors of powder flow, and the following properties are discussed, along with examples of their applications in equipment selection:

• Cohesive strength

• Frictional properties

• Permeability and flow rate

• Sliding at impact points

• Segregation tendency

The flow of metal powders in bins, hoppers, feeders, chutes, and conveyors is not always reliable or uniform. This often results in the press having to operate at lower cycle times, wasted product due to composition or apparent density variations, and operational nightmares. The powder may form a stable arch or rathole; particle segregation may occur, resulting in unacceptable variations in the bulk density of the powder supplied to the feed shoe, or the powder may flood uncontrollably.

Bulk Properties. One of the main reasons that powder flow problems are so prevalent is lack of knowledge about the bulk properties of various powders. For many engineers, the name of a powder, such as atomized aluminum, is thought to connote some useful information about its handling characteristics. While this may be true in a general sense, it is not a reliable tool. Unfortunately, major differences in flowability often occur between different grades and types of powders with the same name.

For those who go beyond the generic name of a powder, one or more of the following four attributes are often relied on in trying to predict the behavior of metal powders and other bulk solids. However, these attributes rarely provide engineers with direct assistance during the design or specification of a bin, hopper, feeder, chute, or conveyor.

Angle of Repose. Determining the angle of repose is relatively easy: simply form a pile of material and measure its slope. Knowing what to do with the data is the difficult part.

For most materials, the angle of repose varies significantly, depending on how the pile was formed. Furthermore, the mechanics of pile formation bear little resemblance to the formation of an arch or rathole in a bin or hopper, uniformity of die fill, powder homogeneity, or to the other key parameters needed when designing a material handling system. In general, the angle of repose of a material is not an accurate measure of its flowability.

Flow Rate. The Hall and Carney flowmeters (described later in this article) are widely used in the P/M industry to characterize powder flowability. However, there are two major flaws with this approach:

• If a powder will not flow through the funnel, no information on its flowability can be determined.

• Even if a powder does flow well, the value obtained (s/5O g) cannot be extrapolated to predict limiting press speed, limiting flow rate through a feed hopper, or other rate-limiting phenomena.

The attempt to combine measurements of two material flow properties (minimum orifice size and flow rate) result in a method that does not measure either one very well.

Apparent Density or Tap Density. Neither of these parameters, nor their ratio (the Hausner Ratio), is a direct indicator of powder flowability. They do not, for example, assist in sizing hopper outlets or calculating appropriate hopper angles.

Free-Flowing versus Nonfree-Flowing. Whether or not a metal powder is considered free-flowing depends to a large extent on the size and shape of the die cavity into which it is expected to flow. For example, a powder that flows through a Hall flowmeter might be considered free-flowing; however, that same powder may have difficulty completely filling a die cavity for a thin-wall part. Thus, these terms are relative and not absolute indicators of powder flowability.

Flow Pattern Considerations. There are two flow patterns that can develop in a bin: funnel flow and mass flow. Both patterns are shown in Fig. 1.

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