Machinery's Handbook, 31st Edition
1528 Characteristics of Metal Powders True Density: True density of the material would be density of the mass if the powder particles were melted and formed into a single piece. During the compression stage the particles are reshuffled, becoming generally closer together as inter-particular pores are eliminated. As the powder is compressed, its density increases. In order to process to its true density, all porosities, open and closed, must be eliminated. True density of a solid material is defined as the ratio of its mass to its true volume; the true density value is given in many tables as one property of engineering materials. The true density value is calculated as: (6) where r t = true density, lb/in 3 (g/cm 3 ) M s = mass of solid material, lb (g) V t = true volume, in 3 (cm 3 ) Bulk Density: Bulk density is the density of the powder in its loose uncompressed form. A container of known volume is filled with the powder. The powder is then weighed and the density is determined as: (7) where r b = bulk density, lb/in 3 (g/cm 3 ) M p = powder mass, lb (g) V p = volume of powder in uncompressed form, in 3 (cm 3 ) Major factors determining bulk density of a powder are particle size, particle size distri bution, particle shape, and cohesiveness. Bulk density is an important property in packag ing and powder handling. Packing Factor : The packing factor value is determined as: (8) Typical packing factor values are 0.5 to 0.7, meaning that the material will compact 30 to 50 percent. The packing factor depends on the particle shape, the distribution of the particle sizes, and external pressure applied during compacting of the particle. Vibration of the powders can also increase packing. Porosity : Porosity is the ratio of volume of the pores (empty spaces) in the powder to the bulk volume. This ratio can be expressed as: (9) where j = porosity; V p = volume of pores, in 3 (cm 3 ); and V b = bulk volume, in 3 (cm 3 ). Pores, or spaces within the powder, are in large part determined by particle shape and size, since the shape dictates how particles will contact each other. Spaces that exist be- tween particles of a powder and are open to the outside are called open pores. Those spaces expose the external surfaces of powder particles. During compaction of powder, these spaces are eliminated. If any amount of this space remains after pressing, it will result in porosity in the manufactured part. These open pores are permeable to the atmo- sphere, and they are permeable to fluids such as liquid lubricants and water. The other type of empty spaces that exist in a powder material are called closed pores. These pores are not open to the outside atmosphere. Closed pores can develop during pressing and sintering if the open-pore region becomes closed off. Another type of closed pore exists within the material of powder particles themselves. PF ρ b ρ t = --- ϕ V p V b = --- ρ t M s V t = ---- ρ b M p V p = ----
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