Pre-preg is "pre-impregnated" composite fibers where a thermoset polymer matrix material, such as epoxy, is already present. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The thermoset matrix is only partially cured to allow easy handling; this B-Stage material requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure.
Pre-preg allows one to impregnate the fibers on a flat workable surface, or rather in an industrial process, and then later form the impregnated fibers to a shape which could prove to be problematic for the hot injection process. Pre-preg also allows one to impregnate a bulk amount of fiber and then store it in a cooled area (below 20°C) for an extended period of time to cure later. Unfortunately the process can also be time consuming in comparison to the hot injection process and the added value for pre-preg preparation is at the stage of the material supplier.
Material advances
Recent advances in Out-of-Autoclave (OOA) processes hold promise for improving performance and lowering costs for composite structures. Using vacuum-bag-only (VBO) for atmospheric pressures, the new OOA processes promise to deliver the less than 1 percent void content required for aerospace primary structures. Led by material scientists at Air Force Research Lab, the technique would save the costs of constructing and installing large structure autoclaves ($100M saved at NASA) and making small production runs of 100 aircraft economically viable.