It is well known that the packing of particulates is one of the most fundamental and important powder-handling processes, and various properties of the particulate assembly are basically determined by the geometrical arrangement of individual particles. Void fraction or voidage is a most popular and simple expression of packing characteristics of a powder bed. In an actual powder system, void fraction e is defined by

e ρ

ρ ρ 5 2 5 21 1b



where rb, rp, M, and V are apparent density, particle density, and mass and volume of powder bed, respectively. Particle volume fraction or packing fraction defined as f 5 12e is frequently used also to express the packing structure in place of the void fraction.