Machinery's Handbook, 31st Edition
Characteristics of Metal Powders
1527
Fig. 4. Inter-Particle Friction as Indicated by the Angle of Repose The angle between the horizontal surface and the surface of the pile is known as the angle of repose and is related to the density, surface area, and coefficient of friction of the particles. Materials with a higher angle of repose have greater friction between par- ticles, while smaller angles suggest less friction. Smaller particle size generally indicates greater friction and steeper angles. Powder Flow: A simple definition of powder flowability is the ability of a powder to flow. Flowability is the result of a number of factors and for convenience they may be grouped under the following headings: • Physical properties of the powder particles, such as their size, shape, hardness, elas ticity, porosity, mass, interactions between particles, and so on. • Environmental factors that affect the powder’s bulk properties, such as air or mois ture content, external pressure, vibration, etc. These factors modify the physical dis tribution and arrangement of the particles in the powder mass. • Individual particle changes caused by factors such as attrition, chemical charge, etc. Powders will flow under gravity to fill containers or die cavities, so in this sense they behave like liquids. As particles in powders get smaller, they have a greater tendency to adhere to one another rather than to fall away under the influence of gravity. This is due to the fact that the surface area of the particles becomes large in proportion to the mass. Therefore, the surface forces of friction and cohesion begin to dominate gravitational forces as particle size decreases. Increasing interparticle friction and cohesion reduce the flowability of the powder. To reduce interparticle friction and facilitate flow during press ing, lubricants are added to the powders in small amounts. Particle shape also greatly affects the flow characteristics of powders. Depending on the fabrication method, particles can take on many different shapes, including spherical, rounded, and angular. The flowability of a powder can be quantified by measuring its gravity-driven flow rate through a calibrated orifice. The conventional method measures the time required for a 50-gram sample to flow through a standardized funnel. Packing, Density, and Porosity.— Packing, density and porosity are based on how much the metal powder will pack down when pressed. A packing of particles is an assemblage of particles and is widely encountered in many industries. Packing characteristics depend on two density measures: true density and bulk density.
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