Carbon Fiber Composite Material
The composite material in carbon fiber is obtained by a series of fibers (carbon, naturally) immersed in a resin matrix. The resin is the "glue", which has the duel purpose of giving the desired shape to the carbon fiber and at the same time allow the loads to be correctly transmitted between the various fibers. The carbon fiber gives the desired mechanical characteristics to the material produced. The Alfa 4C carbon monocoque uses a specific fiber called PREPREG.
Prepreg is simply a carbon fiber pre-impregnated with resin. The Prepreg fiber is stored in flexible rolls. On the outside they look like fabric rolls used in the textile industry. The first step of the carbon monocoque production process consists of making the specific cuts by means of numerically controlled machines. Then the entire surface of the die (4) of the monocoque is covered with the cuts obtained (3). Once the entire die is covered with the fiber, a sealed bag (1) is placed on it.
The purpose of the bag is to create an isolated chamber (2) with respect to the outside. Air at ambient pressure is present in this chamber. A suitable valve on the bag is used to suck up the air, in order to maintain a slight vacuum in the chamber (2) with respect to the outside, bonding the bag (1) to the fiber. Now the die, prepared in this way, is ready to be placed in an autoclave, to undergo the thermal cycle.
The so called "polymerization" of the resin takes place inside the autoclave. Basically, a thermal cycle is performed inside the autoclave in which temperature and pressure are kept under control. Temperature is increased until a maximum of about 150-170° C (302-338° F) and the area inside the autoclave is kept at a pressure of about 5-6 bar (73-87 psi) during the entire cycle. During the period in the autoclave, the resin in the carbon fibers melts, becomes liquid, and due to external pressure, and being in a chamber with a slight vacuum, can spread and fill the various empty areas between one fiber and another, thus creating an actual mesh. In the final stages of the thermal cycle, the temperature is gradually reduced, in order to allow the resin to become stiffer and give the fiber the shape of the die. The instant in which resin polymerization takes place is the key moment of the cycle: this is when the component assumes the desired mechanical characteristics.