Cold Isostatic Pressing Technical Considerations
Compared with cold pressing, isostatic compaction applies pressure uniformly over the entire surface of the mold. Die-wall friction, which exerts a major influence on the density distribution of cold-pressed parts, is absent; so much more uniform densities are obtained. The elimination of die-wall lubricants also permits higher pressed densities and eliminates problems associated with lubricant removal prior to or during final sintering. Furthermore, if necessary, air can be evacuated from the loose powder before compaction. Consequently, isostatic compaction provides increased and more uniform density at a given compaction pressure and relative freedom from compact defects when applied to brittle or fine powders. Because of the uniform compaction pressure the cross section-to-height ratio of the part is not a limiting factor as it is with uniaxial pressing. In addition, cold isostatic pressing can be used to compact more complex shapes than possible with uniaxial pressing.
Generally, there is a distinct advantage in using isostatic pressing except for aluminum and iron compacted to high densities. At high densities both die and isostatic compaction produces similar green densities with iron and aluminum powders. For materials such as aluminum that have constant shear stress, the radial pressure becomes approximately equal to the axial pressure, i.e. approaches an isostatic pressure distribution. However, for materials like copper where yield stress is a function of the normal stress on the shear plane, the radial pressure remains less than the axial pressure. Although the pressure distribution within a cold pressed compact may become isostatic, presumably the pressure vs. density relationship should be identical with that of isostatic compacting only if the density distribution is equally uniform.
