Packing density
Packing density refers to the apparent density of a particle collective, i.e. the mass of particles per volume including the voids between the particles. The more efficiently the particles fill the available space, the higher the packing density. A high packing density is often observed in powders with a wide particle size distribution, as smaller particles can fill the spaces between larger particles.
The packing density ρPack is often described using the bulk density:
ρPack = m · VPowder
Here, m is the mass of the powder and VPowder is the volume of the loosely packed particle collective. In addition, the packing density can be characterised by the dimensionless packing factor ϕ:
Φ = VSolid / VPowder
VSolid is the actual solid volume of the particles, VPowder is the total volume including voids in the powder.
After a mixing process in a precision mixer, the packing density is ideally distributed homogeneously throughout the space. A homogeneous packing density is an important prerequisite for consistent product properties such as bulk density, tabletability or sintering behaviour. Mechanical stresses after mixing, for example vibration, sloping, pneumatic conveying or fluidisation, can impair the mixing quality. They can activate segregation mechanisms. At the same time, the packing density can increase locally due to settling and rearrangement of the particles.
In powder metallurgy and high-performance ceramics, the combination of high packing density and good homogeneity is particularly important. When pressing and sintering powder collectives, the initial packing density significantly determines the achievable density and mechanical properties of the end product. The aim is to increase the packing density without causing segregation.
Powder systems are not usually present in a vacuum, but in ambient air. Dry powders can therefore be regarded as two-phase disperse systems consisting of solid particles and a surrounding gas phase. When liquids are added, a three-phase system of solid, liquid and gas is created. Once the particles are completely enclosed by liquid and the gas space is displaced, a suspension is formed, which can again be described as a two-phase system (solid in liquid).