In Cold Isostatic Pressing (CIP) the powder is contained in a flexible mould commonly of polyurethane, which is then immersed in a liquid, usually water, which is pumped to a high pressure.
Advantages of Isostatic Pressing
The most attractive thing about isostatic pressing is that the powder is compacted with the same pressure in all directions, and, since no lubricant is needed, high and uniform density can be achieved. The process removes many of the constraints that limit the geometry of parts compacted unidirectionally in rigid dies.
CIP is also able to produce long thin-walled cylinders and parts with undercuts present no problem, geometries which would be extremely difficult to produce using conventional pressing techniques.
The process is being increasingly automated with consequent improvements in productivity, and production rates are in some cases comparable with die pressing.
Cold isostatic pressing is now firmly established as a production tool not only in powder metallurgy, but also in the manufacture of ceramics.
Hot Isostatic Pressing
Hot Isostatic Pressing (HIP) also finds extensive use for the compaction of powders.
In this case of HIP is not possible to use a liquid pressure medium and argon is normally used. Furthermore, the material used for the container cannot be an organic elastomer, and in general a metal container, referred to as a can, is used.
Since at the temperatures involved sintering takes place, the question of green strength does not arise, spherical powders which have the highest AD are favoured.
When is Hot Isostatic Pressing Used
The process is used in the production of billets of superalloys, high-speed steels, titanium, ceramics etc. where the integrity of the materials is a prime consideration.
With sintered metals a relative density of about 92% is sufficient to ensure that open porosity - i.e. surface-connected porosity has been eliminated and if vacuum sintering has been used so that there is no gas in the pores, such parts may be HIP’ed to full density without canning.
In a recently developed process, sintering followed by HIP’ing in the same vessel is achieved. The vessel is evacuated, raised to sintering temperature, and then, at a predetermined stage, high pressure argon is introduced. This process, called Sinter-HIP or Pressure Assisted Sintering (PAS) and is rapidly superseding the two stage process of vacuum sintering followed by HIP’ing in a separate apparatus for hardmetal cutting tools, and it can be expected to find increasing application more generally.