Epoxies Improve Aircraft Skin Process
The project was undertaken at the Boeing Tooling Center in St. Louis. The LCTC process uses an aluminum honeycomb substructure in combination with RenPaste RP 4040 syntactic epoxy patties from RenShape Solutions, the tooling arm of Vantico Inc. Adhesive and Tooling Div. (East Lansing, MI). This provides a cost-effective rapid tooling system. The resulting models can be machined directly from computer-aided design (CAD) data and can withstand temperatures of up to 193 C for the initial supported cure of prepreg parts.
Boeing Rotorcraft and Vantico created the LCTC process. The objective of the research was to develop a method for building economical, precision models for producing up to 10 composite parts. The project called for a sag-resistant tooling paste that did not require freezing storage, exhibited a coefficient of thermal expansion comparable to aluminum, a heat-up rate similar to composite tooling and the ability to perform during intermediate-temperature autoclave cycling. RP 4040 epoxy patties fit the requirements for fabricating models and tools directly from CAD data.
"LCTC models are ideal for prototype lay-up tools because they can be fabricated quickly in the shop without requiring the special handling and clean rooms needed for more expensive prepreg tooling," explains Jay Byington, senior specialist for tool design engineering at Boeing.
To fabricate each of the eight lay-up tools, a base substructure was made from 1-inch-thick aluminum plate. Selection and thickness of the base material is critical with large tools, because the plate must be stiff enough to prevent deformation from the sheer weight of the model during handling. This was especially important with these tools. They had very low contoursone being almost flat. A rough-shaped tool core was then built using lightweight, commercial grade aluminum honeycomb bonded to the base surface with a temperature-resistant RTV silicone adhesive. Successive layers of honeycomb were bonded using a foaming adhesive until a machine-able block was formed. The core was then machined with HSS cutters on a five-axis high-speed mill to produce a model surface 0.5 inch smaller than the final contour.
To form the tool face, RP 4040 epoxy patties were applied to the undersized core in 66- by 30-inch sections with a 1- to 1.5-inch gap left between each section to minimize tool stresses. Within each section of the model surface, patties were kneaded to slowly push edges together and then rolled to form a uniform contour without closing the spaces between sections. It was also important to avoid pushing excess material into the honeycomb cells to prevent paste from dropping below the intended height for machining the LCTC tool face.
The sectioned model was then cured using a stepped ramp and soak procedure provided by Vantico with the patties. Once cooled, spaces between patty sections were filled with additional RP 4040 paste, and the tool was cured again.
To complete the model, carbide ball-end mills were used to machine the cured RP 4040 tool face with roughing and finishing cut speeds and feeds to prevent chip-out and excessive dust. Before building parts, the tools were checked to verify pressure and vacuum integrity.
"With the LCTC lay-up tools, we saved time and money in producing BMI prepreg prototype skins without sacrificing dimensional accuracy," says Byington.
For more information on epoxies, call 517-351-5900 or visit www.vantico.com.