Inventor's Corner: Technique molds large, hollow, monolithic composites
A unique manufacturing technique from Marc Quenneville combines vacuum infusion with rotational molding, which he says no one has thought of doing before. The process completely eliminates the seam typically found in large composite structures such as aircraft fuselages, seaplanes, and submarines. These seams must be hand-ground and, worse yet, they weaken composite structures. In contrast, the new technique molds large monolithic composite structures using almost any moldable material such as thermosets, ceramic composites, and carbon-fiber reinforced plastics. Quenneville’s patented prototype is intended to provide the basic concept for fabricators to build larger and more-elaborate versions of the production machines. He made the prototype from scrap plywood, I-beams, and miscellaneous equipment lying around his boatyard.
The outer shell of the prototype is a 4 × 4 × 8-ft I-beam-reinforced vacuum chamber made from several layers of plywood. Inside, a two-piece mold assembly (usually made from fiberglass) bolts together and rotates about an axis.
To mold an aircraft fuselage, for example, the first step is to make what is called a “plug” of the fuselage. This entails using a CNC to carve out the shape from special polyester to within 0.0001 in. The next step is to hand lay up the plug with fiberglass and slit the resulting shape down the middle. This produces a mold that is the negative of the original shape and smooth on the inside. A rubber bladder shaped like a fuselage (or any other structure) is covered with carbon-fiber preform and set inside the mold. The bladder could be made from the same size plug, or a slightly smaller plug.
Sealant tape is then applied between the two mold halves and they are bolted together. Then a tiny hole is drilled in the mold. A vacuum pump sucks the air out of the chamber. Users pour resin into the resin reservoir and open the cocks. The resin travels into the mold, due to atmosphere pressure, through tubes in the rotating shaft that hold the mold. But the material’s viscosity keeps it from escaping out of the tiny hole. A motor rotates the mold to distribute the resin evenly. The bladder contains air and expands as the vacuum is created. This helps push the carbon fiber, now impregnated with resin, tightly against the inside of the mold for a clamping effect.
After the carbon-fiber fuselage sets up, it is removed from the mold. Users cut out, say, the door, and remove the bladder.
The process could change how composite structures are manufactured because the biggest problem until now has always been the seam, says Quenneville. “Composite is basically plastic and carbon fiber and bolts can crush the fabric, he says. “Every time a craft flies, the low outside pressure and the higher inside pressure makes all the fasteners try to leak. Most aerospace companies just keep making composite pieces bigger and bigger. No one has thought to make a large structure all in one shot.”
Airline companies are scrambling to get the planes because they use significantly less fuel, adds Quenneville. Carbon fiber is about seven times stronger than aluminum, pound for pound, and it is lighter. Carbon fiber is also about three times stronger than steel and, unlike aluminum, it does not fatigue.
Another idea: The technique could be used to contain nuclear waste. This would entail hydroforming lead spheres, putting the nuclear waste inside them, and then electron-beam welding the sphere. It would be wrapped with a thick layer or fiberglass and used as the bladder. The objects could be used, for example, as fill for highway overpasses. This would be better than putting nuclear waste in steel drums, says Quenneville.
Quenneville’s patents are #7,722,348 B1 and #8,029,263 B1. He can be reached at [email protected].