Carbon fiber may also be engineered for strength. Carbon fiber variants differ in flexibility, electrical conductivity, thermal and chemical resistance. Altering the production method allows carbon fiber to be made with the stiffness and high strength needed for reinforcement of plastic composites, or the softness and flexibility necessary for conversion into textile materials. The primary factors governing the physical properties are degree of carbonization (carbon content, usually greater than 92% by weight) and orientation of the layered carbon planes. Fibers are produced commercially with a wide range of crystalline and amorphous contents.
Because carbon cannot readily be shaped into fiber form, commercial carbon fibers are made by extrusion of some precursor material into filaments, followed by a carbonization process to convert the filaments into carbon. Different precursors and carbonization processes are used, depending on the desired product properties. Precursor fibers can be specially purified rayon (used in fabrication of the space shuttle), pitch (for reinforcement and other applications) or acrylics (for varied end uses). Since carbon fiber may be difficult to process, the precursor fiber may be converted into fabric form, which is then carbonized to produce the end product. The following materials are common precursors for carbon fiber