What is carbon fibre ?
Carbon Fibre is a synthetic thread that is heated to such a degree that total carbonisation takes place. The very fine threads in carbon fibre are either left as long fibres along the roll length (known as uni directional fibre) with good strength in that axis, or they are woven together to form fabric cloth giving multi-directional strength with the classic woven appearance most people associate with carbon fibre parts.
There are many woven styles of cloth available including plain, twill (2×2 and 4×4) and satin weaves. The Carbon Fibre is then usually left dry to be used in resin infusion processes or wet laminating (wet lay), or it is also impregnated with an epoxy resin to make pre-preg (pre-impregnated).
Both these processes rely on the resin curing either via a catalyst or heat cured (oven or high pressure autoclave) The cured resin is formed into a mould to give the finished part its moulded shape, stiffness and rigidity.
What is wet lay carbon fibre ?
The manufacturer will most likely use the cheaper polyester resin material, a single layer of dry carbon cloth with the rest being glass fibre cloth. The mould may have a clear gel coat applied (brushed or sprayed) before brushing in polyester resin followed by placing the carbon cloth with more resin then the glass fibre and letting cure naturally.
The problem with this method is a resin rich part as there really is no control over the amount of resin used. The weave pattern is also vulnerable to being disturbed. If there are any contour changes there will be voids filled with resin. Generally on anything other than a flat part this method proves to be more unsuccessful, with the only real gain proving to be a carbon weave on show and a glossy finish offered by the clear gel coat.
What is pre-impregnated autoclaved carbon fibre ?
Carbon fibre cloth is pre-impregnated (aka pre-preg) with resin and is laminated into the mould, usually applying short bursts of heat assistance to help follow the contours of the mould. The mould is wrapped in a release film and then breather material before being placed into a vacuum bag and ‘debaulked’ by use of compressor withdrawing all of the air from inside the bag.
This ensures the carbon fibre is pushed down into the mould and does not leave any voids. This process is carried out after each ply of material is laminated into the mould. The mould is then placed into an autoclave (high pressure cooker) and the vacuum bag is connected to an air line located inside the autoclave. Pressures in excess of 100psi along with heat are then applied(both low and high temperatures can be used depending on the materials used).
The length of cure cycle is dictated by the type of pre-preg material used and the structural make up of the moulds themselves. This is the top end of carbon parts production as it uses autoclaves costing many tens of thousands of pounds.
Typically, pre-preg carbon offers a very low resin content in contrast to the volume of carbon fibre material, thus producing a stronger and lighter part, hence why high end motorsport circles lend their choice of methods to pre-preg and also why you will rarely find anything ‘wet lay’ about a carbon fibre part on a Formula 1 vehicle.