Pre-preg is a term for “pre-impregnated” composite fibres. These usually take the form of a weave or are uni-directional. They already contain an amount of the matrix material used to bond them together and to other components during manufacture. The pre-preg are mostly stored in cooled areas since activation is most commonly done by heat. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure out.

There are several advantages and disadvantages of the pre-preg process in comparison to the hot injection process. Pre-preg allows one to impregnate the fibres on a flat workable surface, or rather in an industrial process, and then later form the impregnated fibres to a shape which could prove to be problematic for the hot injection process. Prepreg also allows one to impregnate a bulk amount of fibre and then store it in a cooled area for an extended period of time to cure later. Pre-preg on the other hand can be time consuming in comparison to the hot injection process.

Recent advances in Out-of-Autoclave processes hold promise for improving performance and lowering costs for composite structures. Using vacuum bag only 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.