An autoclave is simply a large, heated pressure vessel. Autoclave moulding is similar to the vacuum bag process, except the lay-up is subjected to additional pressure, whilst heat is applied to cure the resin. Vacuum is usually applied during the initial stages of the cure cycle to remove volatiles and trapped air, but without causing excessive resin flow. Autoclave pressure may be maintained during the entire heating and cooling cycle.

The pressure exerted on the lay-up is normally within the range 50-100 psi. The autoclave moulding process produces laminates of high quality with minimum void content and control of laminate thickness is much better than that achieved by the vacuum bag moulding method. However, the capital equipment costs are high and the output relatively low, which restricts the use of the autoclave moulding process to higher value markets where high quality is essential. Many large primary structural components for aircraft, such as fins, wing spars and skins, fuselages and flying control surfaces, are manufactured by this method for guaranteed reproducibility.