Such parts are difficult, or even nearly impossible, to make using conventional injection molding techniques, since a mold cavity of great length (e.g. greater than 10 meters) presents serious technical problems when it comes to filling it with injected plastics material, and requires injection tooling that is too expensive for short manufacturing runs.
That is why parts of large size, such as the blades of a wind turbine, for example, have until now been made by a molding method which consists in depositing successive layers of resin in a mold on successive layers of fiberglass cloth or the like, each layer of resin needing to be polymerized (cold, using a catalyst) before the following layer of resin is deposited, because of the heat given off by the resin polymerizing. By way of example, this can make it necessary to deposit and polymerize ten layers of resin each having a thickness of 5 mm in order to build up a wall having a thickness of 50 mm. Under such conditions, making a single wind turbine blade of great length (about 15 meters (m) to 35 m) can occupy one mold for three to four weeks.
In addition, if a low cost resin is used such as a polyester resin, it is necessary to protect personnel against the toxic vapor given off by the resin while the part is being made. Naturally, it is possible to use an epoxy resin instead of a polyester resin in order to eliminate the risks of pollution and toxicity, but to the detriment of cost price, since the cost of epoxy resins is about five times greater than the cost of polyester resins.
It is already known, e.g. from U.S. Pat. No. 5,037,599, to make a part out of composite material starting from a cloth made up of threads that are constituted by a mixture of thermoplastic fibers and of reinforcing fibers, the cloth being placed or draped in a mold and being thermoformed by applying heat and pressure, the heat causing the thermoplastic of the cloth to melt and the pressure pressing it against the wall of the mold. Nevertheless, that known method does not enable tubular parts to be made.