DENSITY

Most properties of a PM part are closely related to its final density. This density is the mass per unit volume of the part expressed in grams per cubic centimeter (g/cm³). Normally, the density of structural parts is reported on a dry, un-impregnated basis, while the density of bearings is reported on a fully oil-impregnated basis. Density is most commonly determined using the method given in MPIF Standard 42.

Density is also expressed as relative density, which is defined as the ratio of a PM part’s density to that of its pore-free equivalent. In practice, PM parts less than 75% of relative density considered to be low density; those above 90% are high density; and those in between these two points are considered as medium density. In general, structural parts have relative densities ranging from 80% to above 95%. Forgings and HIP products often exceed 99%. Many self-lubricating bearings have relative densities on the order of 75% and filters usually relative densities of 50%.

POROSITY

Porosity is the percentage of void volume in a part. It is the converse of density. A part which has 85% relative density will have 15% porosity. Porosity in PM parts can be present as a network of interconnected pores that extend to the surface like a sponge or as a number of closed holes within the part. Interconnected porosity is important to the performance of self-lubricating bearings and is part of the specification for these types of materials.

Porosity is a controllable function of the raw material and processing techniques. Parts can be produced either with uniform porosity or with variations in porosity (and density) from one section to another to provide different properties. For example, parts can be made self-lubricating in one area and dense and strong in other areas.

PERMEABILITY

The ability to pass fluids or gas as, for example, in filters is another unique property that can be designed into PM products. Depending on the forming and sintering techniques, a PM part can provide permeability ranging from highly restricted to open flow. The part can be produced with permeabilities that will separate materials selectively; diffuse the flow of gases or liquids; regulate flow or pressure drop in supply lines; or act as flame arrestors by cooling gases below combustion temperatures. Filters can be produced in almost any configuration, including sheets and tubes.