Parts produced out-of-autoclave supply aerospace, automotive and industrial markets.

Quality was the big driver when aerospace manufacturers began incorporating composite parts into their aircraft 25 years ago. With the basic resin systems then available, autoclave was the best way to compress the laminate’s plies and produce a part with very low voids. While autoclaves have worked well, the equipment is expensive and it limits parts size.

Out-of-autoclave (OOA) technology is solving these problems for some applications. “OOA is considered to be a cost effective way to produce a part, and it also avoids the financial investment in purchasing autoclaves and freezer storage,” explains Henri Girardy, Hexcel’s business development manager for new processes. “Other advantages of OOA technologies are the potential for a higher degree of part integration: for example, the co-molding of skin, stiffeners and spars in the same operation, which leads to an overall reduction in the total process cycle, with increased production rates.”

In the broadest sense, OOA includes any composite production technology that doesn’t use an autoclave. That definition encompasses thermosets and thermoplastics and processes such as resin transfer molding.

But some companies in the aerospace industry prefer a narrower definition. “We’re talking about thermoset prepreg under vacuum bag only (VBO) curing,” says Michael Cichon, director of product marketing at TenCate. There are variations within OOA processing technology that could be considered out of autoclave, including the use of hydrostatic pressure, resin film infusion, bladders for pressure intensifications and SQRTM (same qualified resin transfer molding). In SQRTM, for example, the prepreg is placed in a closed mold, and the same resin used in the prepreg is injected into the mold.

An Evolution in Prepreg OOA

OOA prepreg has gained widespread acceptance in the aerospace industry because it produces quality parts at reduced cost.

“OOA properties are now equivalent with autoclave materials and are no longer a deciding factor,” says Gary Bond, Boeing research & technology technical fellow. “There are a number of factors that go into deciding which material and process are used to fabricate Boeing composite parts, including the part geometry, thickness, production rate, number of parts required, tooling budget, etc.”