The question often arises, then, why not always use Pitch carbon fiber, since it is more rigid (i.e. high modulus). The answer is that, typically, stiffness and strength are inversely related for carbon fiber – as the modulus increases the strength decreases. For a given application, if higher strength is required, then the solution is almost always to use PAN carbon fibers. Pitch carbon fiber can be used to produce parts that are stiffer than steel, but strength will be sacrificed. The ideal applications for Pitch carbon fiber are usually those that will see low stresses, but require extremely small deflection. Robotic end-of-arm tooling is a common application that has these requirements. In addition, Pitch carbon fiber is usually more expensive than PAN options.

Carbon fibers alone are usually not very useful, since they cannot hold a compressive load on their own without another material holding them straight. This is the purpose of the matrix within the composite. The matrix stabilizes the carbon fibers so they can maintain their straightness while compressive loads are applied. It also transfers the internal stresses from fiber to fiber. One of the most common matrix materials is epoxy, which can be added to the carbon fibers by mixing two chemicals, a resin and hardener, and then applied with brushes, vacuum, presses, or any number of other methods. Some epoxy resin systems can be pre-added to the carbon fiber and only partially cured. This process usually requires the material to be kept below room temperature for storage and is called a prepreg carbon fiber material.

As an engineering material, carbon fiber reinforced plastics can be fabricated to have tailored mechanical properties along different axes by placing a greater percentage of the fibers in one direction, while sacrificing the percentage along an orthogonal axis.