Boeing is developing a new technology to improve molding of large carbon composite structures for aircraft, including large fuselage sections with integral stringers and co-bonded frames.
A newly awarded patent details methods and equipment to reduce thickness gradients in molded composite parts caused by gravity-induced settling of the resin during curing.
In the Boeing invention, a composite fiber structure is formed by injecting resin into composite fiber layup, and then rotated to reduce gravity-induced migration of the resin through the layup. The goal is to reduce the vertical gradient to less than about 10 percent. Rotation continues until resin flow stops.
It appears Boeing is trying to speed up production of composite parts improving the molding process. The patent states: “Vacuum-assisted resin transfer molding (VARTM) is being used more frequently to mold large composite structures, such as sections of aircraft.”
VARTM is preferred to conventional RTM because it eliminates matched-metal tooling costs, reduces volatile emissions and allows for low injection pressures. In VARTM process, vacuum pressure is used to force liquid resin into dry composite reinforcements (preforms) that have been laid in a sealed mold.
The “mold” can be just a soft bag enclosing the entire structure to be molded. Resin is pulled into the mold through vacuum pressure.
The patent further states: “When molding relatively large structures, such as an aircraft fuselage, gravitational effects on resin flow behavior can create undesirable thickness gradients in the finished structure. These gradients, which may approach 25 percent or more, result from the fact that the force imposed by gravity tends to draw the flowing resin downwardly toward the bottom of the molded structure during the curing process.”
Bottom wall thickness can be significantly greater than wall thickness near the top of the structure, reducing the integrity of the molded structure, and adding unwanted weight.
Boeing famously pioneered use of carbon composites on the Dreamliner 787, but had to develop much of the technology itself, including the large autoclaves used to cure fuselage and wing sections. More than half of the airframe is composites, eliminating the need for 1,500 sheets of aluminum and up to 50,000 fasteners.