Eaton developing carbon-reinforced PEKK to replace aluminum in aircraft air ducts

A duct for air handling may seem to be a straightforward component, but when that duct is part of the environmental control system on an aircraft, there are potentially a number of requirements it needs to meet.

“There’s a very extensive list,” says Si Chen, a senior specialist engineer with a focus on polymer nanocomposites who is part of the Advanced Materials and Processes Group with Eaton. “There is a V0 flame rating: A flame must be extinguished within 10 seconds on a vertical surface. There are things like chemical resistance, smoke and toxicity — in addition to application-specific requirements like vibration handling and the specific strength requirements of the part.” Another notable requirement is for duct material to be electrostatic dissipative (ESD). And all these needs must be satisfied for a part that is also apt to be geometrically complex enough to wend around and through other components of the aircraft. Formed aluminum has been the material of choice to make aircraft ductwork that meets all these needs, but Chen is part of a team at Eaton that has now realized a potentially faster, cheaper and more flexible method of manufacturing aircraft ducts. 3D printing in polymer composite offers a way to produce ducts on demand, without tooling and with greater design freedom.

Key to this solution was developing a 3D printable material satisfying all the requirements of the aircraft application as well or better than aluminum. Eaton’s material, the product of development work performed over years, is a carbon-fiber-reinforced PEKK tailored to the ductwork role.

Javed Mapkar is senior technical manager of the same group at Eaton. The material is ready — proven within Eaton’s testing internally — and is now proceeding through third-party testing so it can be applied as a replacement for aluminum in aircraft production. That work is now being done through the National Institute for Aviation Research (NIAR) at Wichita State University. He says, “For this ESD PEKK material, we are trying to qualify it for B-basis allowables. We need to make multiple batches of the material, print on multiple different printers, and show repeatability.”

A 3D printing platform capable of using pellet material is part of this solution as well, and part of the process now being qualified. Material development was done on, and parts have been made on, a Titan pellet-fed extrusion 3D printer from 3D Systems. Eaton could have pursued a solution using material in the filament form more common to extrusion 3D printing, but getting to produce the material in pellet form simplified and sped material development. Pellet stock also offers the more practical way to deploy enough material for an unattended build of multiple large aircraft ducts. The Titan machine used, with a build volume of 50 × 50 × 72 inches, makes it possible to 3D print ducts up to 6 feet high in multiples of four to eight ducts per build — which is what production will ultimately entail.

A production build like this might take on the order of 12 to 15 hours, Chen and Mapkar say, depending on the size and quantity of ducts. Importantly, the ducts could also be all different in this build. Aircraft production quantities are small enough, and duct designs in a plane varied enough, that duct manufacturing is inherently high-mix, low-volume production — yet ducts made of formed aluminum are beholden to hard tooling. The chance to do away with this tooling simplifies manufacturing and brings new freedom to design, and also makes it possible to develop, test and modify new designs quickly.

After qualification of the material and process, Mapkar says Eaton is considering various scenarios for how to bring this new option to aircraft makers. The company already has a precedent in the form of an additive manufacturing production facility for metal aircraft parts, so one possibility might be a toolingless factory similar to this metal part site for on-demand production 3D printed composite ducts.