Carbonization of molded graphite: carbonizing (also known as baking) the green shape is the next step. Carbonization takes place in a furnace in an inert or reducing atmosphere. The process may last from a few days to several weeks depending on the constituents, and the size and geometry of the part. The temperature is raised slowly to 600C, at which stage the binder softens, volatiles are released and the material begins to shrink and harden. Typical shrinkage is 6%. The parts must be supported by a packing material to prevent sagging.
The temperature is then raised to 760 to 980C. This can be done faster than the first temperature step, since most of the volatiles have by now been removed, the material is already hard, and sagging is no longer a problem.
Impregnation: After the carbonization stage, the material has a high degree of porosity. To further densify it, it is necessary to impregnate it with coal-tar pitch or a polymer such as phenolic. Impregnation is usually carried out in a high-pressure autoclave and the carbonization process is repeated. In special, limited-use applications, non-carbon impregnating materials such as silver and lithium fluoride impart specific characteristics, particularly increased electrical conductivity.
Graphitization: during graphitization, the parts are heated up to 3000C. The temperature cycle is shorter than the carbonization cycle and varies depending on the size of the parts, lasting from as short as a few hours to as long as three weeks. It is usually performed in a resistance furnace or in a medium-frequency induction furnace.
Graphitization increases the resistance of the material to thermal shock and chemical attack. It also increases its thermal and electrical conductivities.
Puffing: puffing is an irreversible expansion of molded graphite which occurs during graphitization when volatile species, such as sulfur from the coke, are released. Puffing is detrimental as it causes cracks and other structural defects. It can be eliminated by proper isothermal heating and by the addition of metals or metal compounds with a high affinity for sulfur.
Purification: for those applications that require high purity such as semiconductor components and some nuclear graphites, the material is heat-treated in a halogen atmosphere. This treatment can remove impurities such as aluminum, boron, calcium, iron, silicon, vanadium and titanium to less than 0.5 ppm. The halogen reacts with the metal to form a volatile halide which diffuses out of the graphite. The duration of the treatment increases with increasing cross section of the graphite part.
Machining: the graphitized material can now be machined to the final shape. Since it is essentially all graphite, machining is relatively easy and is best performed dry to avoid water contamination. Common cutting tool materials are tungsten carbide, ceramic and diamond. A good ventilation system is necessary to control and collect powdery dust.
The details of material formulation, processing steps, and equipment in the graphite-molding process can vary considerably from one manufacturer to another. In many cases, these details are jealously guarded and most companies consider their proprietary processes essential to their economic survival.