Vacuum bag moulding

In the simplest form of vacuum bagging, a flexible film (PVA, nylon, mylar, or polyethylene) is placed over the wet lay up, the edges are sealed, and a vacuum is drawn. A more advanced form of vacuum bagging places a release film over the laminate, followed by a bleeder ply of fiberglass cloth, non-woven nylon, polyester cloth, or other material that absorbs excess resin from the laminate. A breather ply of a non woven fabric is placed over the bleeder ply, and the vacuum bag is mounted over the entire assembly. Pulling a vacuum from within the bag uses atmospheric pressure to eliminate voids and force excess resin from the laminate. The addition of pressure further results in high fiber concentration and provides better adhesion between layers of sandwich construction. When laying non-contoured sheets of PVC foam of balsa into a female mold, vacuum bagging is the technique of choice to ensure proper secondary bonding of the core to the outer laminate.

Vacuum injected molding

Typically, this process uses a low viscosity polyester or vinyl ester resin along with fiberglass fibers where resin is injected into fibers using vacuum. Flexible film is placed over laminate, similar to vacuum bag moulding. Main difference is that in this process laid laminate is dry while resin is injected into laminate using a series of tubes. After dry laminate is negatively pressurized, resin is flown into laminate from outside tank where pressure difference guides resin into laminate. Normally the process is capable of producing composites with a fiber volume fraction between 40 and 50%. The resin to fiber ratio is important for determining the overall strength and performance of the final part, with mechanical strength being most influenced by the type of fiber reinforcement. The type of resin used will primarily determine the corrosion resistance, heat distortion temperature, and surface finish. Resins used in this process must have low viscosities due to the limited pressure differential provided by the vacuum pump. High performance fibers, such as carbon fiber, can also be used. However, their usage is less common and is mainly for the fabrication of high end parts.

Wet hand layup

Wet hand layup forming combines fibre reinforcement and the matrix as they are placed on the forming tool. Reinforcing fibre layers are placed in an open mould and then saturated with a wet resin by pouring it over the fabric and working it into the fabric. The mould is then left so that the resin will cure, usually at room temperature, though heat is sometimes used to ensure a proper cure.