Isostatic presses are used for many applications ranging from casting repair to ceramic ball bearings. The process can use cold, warm or hot temperatures to accomplish the desired task. Cold isostatic presses are used mainly for pre-forming of powders prior to a further densification by sintering or hot isostatic pressing. Cold Isostatic Presses come in two types; wet bag, and dry bag. In a wet bag press the pressure vessel is full of water and the rubber bag (mold) is removed each cycle and refilled. This type of press is common when large, complex, or many different parts are required. Dry bag presses have the bag as an integral part to the vessel and are used when lots of the same style parts are made. Dry bag presses are limited to smaller sizes, and simple shapes that can be removed easily. Dry bag presses are typically automated from the powder filling to the part removal stage. The main benefit of isostatic pressing is uniform density, which results in predictable and repeatable shrinkage upon sintering. In small parts a uniaxial die press can produce acceptable parts, but in long tubes or large complex parts, die friction causes non-uniformity. This is where isostatic presses come into use. Most cold pressing is done with some form of binder to help hold the part shape. This binder is typically burnt out of the part in a furnace or during the sintering process.
Sintering is a process to consolidate a performed part. It can be done in vacuum or gas pressure. The gas type may be any of a wide variety to accomplish the desired reaction with the part material. Sintering is done at a temperature close to the melting point of the part and densification will occur as the powder particles diffuse together. Sintering has many variables and can produce parts of very high densities, typically 94 to 99 percent dense. For many applications this is all the part needs and further processing is not required.
Hot isostatic pressing is the application of heat and pressure simultaneously to a part causing it to shrink and densify uniformly. It can be used directly to consolidate a powder or supplementary to further densify a cold pressed, sintered, or cast part. The pressure medium is typically a gas (argon or nitrogen) but can be a liquid (glass) or molten metal. Heating is most always with an electric furnace. To densify a powder directly requires a can to transmit the pressure to the powder. This can (mold) can be mild steel, stainless, an exotic metal, or a glass capsule. This method is used when a simple shape is required. When a complex shape (such as a turbocharger blade) is required, it is common to cold press first because a rubber mold is much easier to make than a stainless steel mold. The cold pressed part (called a green body) can then be sintered to a high enough density to close any interconnecting porosity. This part can then be hot isostatically pressed without a can (mold) as the part itself will transmit the force to any internal porosity. This method is also used extensively on castings. It is hard to cast 100% dense castings, especially all the time, therefore hot isostatic pressing is used to fully densify castings after they are made. Many titanium castings are HIPped to ensure no internal porosity exists. HIPping can be used on just about any material from plastics to metals, to ceramics, to metal matrix composites. HIPping results in a very uniform grain structure and exceptional properties. Fatigue life is often better than achieved by any other processing method. HIPping can also be used to consolidate unusual powder combinations such as superconductors, ferrite’s, nuclear wastes, etc. It also is used to diffusion bond same and dissimilar materials that cannot be bonded otherwise. Many applications such as transparent ceramics cannot be manufactured any other way than HIPping.
There is much literature regarding both hot and cold pressing. Some sources include American Powder Metallurgy Institute, American Ceramics Society, APM, and Gorham Advanced Materials Research Institute. There are articles regularly in Ceramic Industry, Ceramic Bulletin, Industrial Heating, and Advanced Materials and Processes magazines. There is also information starting to appear on the Internet, including www.mpif.org. Many universities now have isostatic presses, and some specialize in isostatic pressing. Penn State and Michigan Technological University are leaders in powder metals, while Alfred and North Carolina are leaders in ceramics.
The major drawback to HIP has always been the high cost of the equipment and the process. This has started to change as higher material properties and parts value have increased to allow HIP to be considered. Many HIP units are custom designed to meet special requirements. These include Oxygen HIP units, Triaxial isostatic presses, Ultra fast HIPs, and Low Pressure Sintering HIP’s for silicon nitrides.