GKN Aerospace is working toward out-of-autoclave processing of carbon-epoxy laminated structures upward of 23 feet long and 275 pounds in weight and has already manufactured components up to 8.2 feet long and weighing 175 pounds in an R&D environment.
According to John Cornforth, the company’s head of technology, 10 years has passed since GKN’s R&D team recognized the potential benefits of out-of-autoclave processing, especially when coupled with other technology advances. It has been working since then to advance the manufacturing readiness of the process to the point where it can be considered a viable alternative to autoclave processing for series manufacture of both primary and secondary structures.
Carbon-epoxy composites for aerospace application are usually cured in an autoclave under a pressure of 100 pounds per square inch–equivalent to the pressure an African bull elephant would exert if it stood on a 12-inch square. Out-of-autoclave processing uses a different resin formulation which enables the component to be cured under vacuum pressure alone.
Doing without an autoclave has many benefits. For one, the tooling used to cure the component does not have to be so substantial, so composites tooling becomes viable. A composite tool is both better and cheaper than a conventional invar tool, costing just half as much and offering almost zero coefficient of thermal expansion, which minimizes the likelihood of component distortion during processing and its return to ambient temperature.
The technique can also reduce energy use, Cornforth told a recent conference at the Royal Aeronautical Society in London. “An autoclave has an enormous thermal inertia, which is compounded by the use of heavy metallic curing tools mounted on transportation trolleys.” The trolleys have to support the heavy tools and ensure minimal distortion even when located on imperfect surfaces.
In the out-of-autoclave process the heat is applied locally, directly from the tool itself, with the external surfaces of the component well insulated, so heat consumption is much less and spin-off environmental benefits result.
At the same time, autoclave capacity no longer limits component size, and for a green-field facility, the elimination of the need for autoclave capacity makes for
a considerable reduction in capital cost. With the process also come what Cornforth terms holistic benefits. “Visit any factory manufacturing composite components using traditional autoclave processing and you will always see large tooling sets being wheeled around,” he explained. “They are queued ready to take their slot in the autoclave schedule, then wheeled back after cure to the work zone where the product will be de-molded.”
The out-of-autoclave process, on the other hand, can support a cellular-based lean operation where all the processing on a component–layup, cure and de-mold–is done in one location. There is no longer a need for substantial trolleys, and time is not wasted in moving parts around the factory. “Most importantly,” he commented, “there is no longer a bottleneck that goes by the name of autoclave.”
It can also avoid one of the disappointments of autoclave manufacture, when a complete large component is built only to be rendered useless by a problem within the autoclave during the cure cycle, such as vacuum loss due to bag failure or hose leakage. Out-of-autoclave curing provides much greater access to the component during this critical stage and offers opportunity for remedial action in time to save it.
One complementary technology is self-heated tooling, where the tool itself has incorporated an integrated heating system. The amount of heat is zone controlled and varied according to the laminate thickness, so the rate of heating can be closely controlled. And heat emanating from the tool surface rather than the external surface minimizes the risk of trapping air and volatiles within the laminate during cure.
GKN Aerospace has developed a fiberoptic thermometer applying the Bragg Grating principle, Cornforth added. This replaces the need for multiple disposable thermocouples to be added to the edges of the pre-cured laminate stack in order to monitor temperature during cure, thereby saving both time and money. And embedded fiberoptic thermometers can monitor temperatures throughout the component rather than just around the edges.
“The components manufactured so far are of excellent quality,” Cornforth added. “Some of the advantages on offer definitely make this a target technology for a factory of the future.”