Machine Design


Manufactured carbon comprises a large family of materials, available in hundreds of grades and shapes including plates, rods, tubes, and rings. Many of the grades have been developed for specific applications.

The parts are made from mixtures of coke and graphite powder bonded with carbon. The proportions and types of these materials are selected to obtain the lubricity and wear resistance required in the finished component. The carbon binder is derived from coal-tar pitch or synthetic resins chosen for their high coking yield. The mixture is then formed by compression molding or warm extrusion.

The formed shape is fired at temperatures as high as 1,300°C in an oxygen-free environment, which converts the binder to carbon. The resulting baked carbon part contains interconnected pores, which are usually impregnated with resins, fused salts, glasses, or molten metals. Impregnation enhances physical properties and can also reduce rubbing friction and wear rates.

During the firing operation, carbon parts shrink by 5 to 15%. Shrinkage of a given grade is predictable, however, and small parts having dimensions within a 1% tolerance band can be produced without machining. Some shapes are graphitized by a second baking to at least 2,600°F.

Most manufactured carbon parts require some machining to produce required tolerances. Only the hardest carbide or diamond-tipped tools can be used for some grades. Because of this, the larger U.S. carbon producers offer machining service for producing parts to customer specifications.

TAGS: Materials
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