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Bulk Properties of Powders ASM Handbook, Volume 7: Powder Metal Technologies and Applications Copyright © 1998 ASM International® P.W. Lee, Y. Trudel, R. Iacocca, R.M. German, B.L. Ferguson, W.B. Eisen, K. Moyer, All rights reserved. D. Madan, and H. Sanderow, editors, p 287-301 www.asminternational.org DOI: 10.1361/asmhba0001530 Bulk Properties of Powders John W. Carson and Brian H. Pittenger, Jenike & Johanson, Inc. THE P/M INDUSTRY has grown consider- Bulk Properties. One of the main reasons that and flow rate) result in a method that does not ably in the past decade. As a result of this powder flow problems are so prevalent is lack of measure either one very well. growth, more critical components in the automo- knowledge about the bulk properties of various Apparent Density or Tap Density. Neither of tive, aircraft, tooling, and industrial equipment powders. For many engineers, the name of a these parameters, nor their ratio (the Hausner industries are being considered for manufacture powder, such as atomized aluminum, is thought Ratio), is a direct indicator of powder flowabil- using this technology: This is placing increas- to connote some useful information about its ity. They do not, for example, assist in sizing ingly stringent quality requirements on the final handling characteristics. While this may be tme hopper outlets or calculating appropriate hopper P/M part. Variations in part density, mechanical in a general sense, it is not a reliable tool. Unfor- angles. properties including strength, wear, and fatigue tunately, major differences in flowability often Free-Flowing versus Nonfree-Flowing. Whether life, as well as in aesthetic appearance and di- occur between different grades and types of or not a metal powder is considered free-flowing mensional accuracy are no longer tolerated. As a powders with the same name. depends to a large extent on the size and shape of result, metal powder producers and P/M part For those who go beyond the generic name of the die cavity into which it is expected to flow. manufacturers must continually improve their a powder, one or more of the following four For example, a powder that flows through a Hall capabilities to ensure the delivery of a consistent, attributes are often relied on in trying to predict flowmeter might be considered free-flowing; uniform product. Research has demonstrated the behavior of metal powders and other bulk however, that same powder may have difficulty that these part qualifies are significantly affected solids. However, these attributes rarely provide completely filling a die cavity for a thin-wall by changes (variations) in the particle size distri- engineers with direct assistance during the de- bution, particle shape, and consequently the uni- part. Thus, these terms are relative and not abso- sign or specification of a bin, hopper, feeder, lute indicators of powder flowability. formity of powder blends (a combination of one chute, or conveyor. or more particle sizes of a single powder) and Flow Pattem Considerations. There are two Angle of Repose. Determining the angle of re- flow patterns that can develop in a bin: funnel mixes (a combination of one or more types of pose is relatively easy: simply form a pile of powders) (Ref 1, 2). flow and mass flow. Both patterns are shown in material and measure its slope. Knowing what to Fig. 1. do with the data is the difficult part. In funnel flow, an active flow channel forms For most materials, the angle of repose varies above the outlet with nonflowing material at the significantly, depending on how the pile was Powder Flow periphery. As the level of material in the hopper formed. Furthermore, the mechanics of pile for- decreases, layers of the nonflowing material may This article reviews the general factors of marion bear little resemblance to the formation or may not slide into the flowing channel, which powder flow, and the following properties are of an arch or rathole in a bin or hopper, uniform- can result in the formation of stable ratholes. In discussed, along with examples of their applica- ity of die fill, powder homogeneity, or to the addition, funnel flow can cause product caking, tions in equipment selection: other key parameters needed when designing a provide a first-in last-out flow sequence, and in- material handling system. In general, the angle crease the extent to which sifting segregation • Cohesive strength of repose of a material is not an accurate meas- impacts the discharging material. • Frictional properties ure of its flowability. • Bulk density Flow Rate. The Hall and Carney flowmeters In mass flow, all of the material is in motion • Permeability and flow rate (described later in this article) are widely used in whenever any is withdrawn from the hopper. • Sliding at impact points the P/M industry to characterize powder flow- Material from the center as well as the periphery moves toward the outlet. Mass flow hoppers pro- • Segregation tendency ability. However, there are two major flaws with vide a first-in first-out flow sequence, eliminate • Angle ofrepose this approach: stagnant material, reduce sifting segregation, provide a steady discharge with a consistent bulk The flow of metal powders in bins, hoppers, • If a powder will not flow through the funnel, feeders, chutes, and conveyors is not always reli- no information on its flowability can be deter- density and a flow that is uniform and well con- able or uniform. This often results in the press mined. trolled. Requirements for achieving mass flow having to operate at lower cycle times, wasted • Even if a powder does flow well, the value include sizing the outlet large enough to prevent product due to composition or apparent density obtained (s/50 g) cannot be extrapolated to arching and ensuring the hopper walls are suffi- variations, and operational nighanares. The pow- predict limiting press speed, limiting flow rate ciently smooth and steep enough to promote der may form a stable arch or rathole; particle through a feed hopper, or other rate-limiting flow at the walls. segregation may occur, resulting in unacceptable phenomena. Useful Bulk Flow Parameters. Armed with variations in the bulk density of the powder sup- information about the bulk properties of the plied to the feed shoe, or the powder may flood The attempt to combine measurements of two powder, engineers can optimize the selection of uncontrollably. material flow properties (minimum orifice size storage and handling equipment. These same 288 / Metal Powder Production and Characterization pxA 77777-/2, . .., "//////,/, "// (a) (b) Fig° 3 An idealized flow function test. (a) Consolidation. (b) Failure ff s(p)o.-" Mass flow t ,* Fig. 1 Two flow patterns that can occur in a bin: funnel flow and mass flow e~ n "0 gf= ,,... o o Ib/ft2 Fig. 4 Solids flow function (FF) and hopper flow factor (fO. See text for details. Cohesive strength can be measured as a func- tion of the applied consolidation pressure. This relation, which is of primary importance in the analysis of flow, can be described as follows: suppose that a quantity of a metal powder has been placed in a cylindrical mold of cross-sec- tional area A, with frictionless walls (Fig. 3), and (a) (b) consolidated under a force (p x A) applied to the Fig. 2 Examples of no-flow situations where the darkened areas represent material within the bin. (a) Cohesive arch at piston. Now suppose that, without disturbing it, the outlet of a bin. (b) Stable rathole formed within bin the consolidated cylinder of powder is removed from the mold and placed on a table, and a com- properties can be used to retrofit existing proc- enough strength to retain its shape once the hand pressive force is applied to it (Fig. 3). The force esses to correct flow problems. is opened. is increased from zero until the cylinder col- Discussed below are several bulk solids han- A similar range of conditions occurs inside lapses at some value of the force (fxA). The dling properties that are relevant to predicting bins, hoppers, and containers. Consolidation experiment is repeated for several values of p. flow behavior. The direct application of these pressures range from zero at the surface to rela- For each value of p, a corresponding value off is parameters has been proven over the last 30 tively large values at increasing depth within the obtained. Points (p, .1") are plotted in Fig. 4, and a years in numerous installations handling the full container. If a powder gains cohesive strength smooth line is drawn through them. This relation spectrum of powders used in the P/M industry, because of the pressures applied to it, an arch or is called the flow-function (FF) of the solid. including metal powders, fine chemical addi- rathole can form (Fig. 2). This compression test serves well as an illus- tives, polymers and waxes, and graphites/car- An arch (also called a bridge or dome) is a tration of the concept, but is not practical for a bons (Ref 3, 4). stable obstruction that forms over the point of number of reasons; for instance, a mold cannot narrowest cross section of the storage vessel be made frictionless, and it is difficult to obtain (usually the discharge outlet). The arch sup- uniform consolidation of a powder in a relatively Cohesive Strength ports the rest of the bin contents, preventing tall cylinder. In addition, most metal powders discharge. have such low cohesive strength that the com- Many metal powders and other bulk solids, A rathole is a stable pipe or vertical cavity that pacted cylinder of powder would fall apart when when poured from a box, flow like a liquid, empties out over the outlet.
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