Preparing the carbon fibres
The autoclave: a very big oven!Throughout the laying-up stage of chassis production, in the composites clean room, the autoclaves have a number of different uses, coming into play for what are termed ‘de-bulks’ and also the ‘cure’ itself. “An autoclave is, essentially, a big pressurised oven in which we ‘cook’ the carbon fibre,” explains Composites Manager Colin Watts. “But of course, things are not quite a simple as that! Essentially, the construction of the chassis proceeds in stages, laying up the different cuts of carbon fibre, and the autoclaves have a vital role to play at each stage. We cook the parts at different temperatures and different pressures in a vacuum, to extract any air from the material.”
For every part that goes into the autoclaves, the process is the same: the carbon fibre laid up in the mould must be covered in a breathable plastic layer, to allow the air to escape; this is then covered in a breather fabric, before being placed in a nylon bag which goes into the oven and has vacuum hoses attached to it.
De-bulks and curesThe two principal processes during which the autoclaves come into play are the de-bulk and cure: “The first of these is used to compact and compress the material,” continues Colin Watts. “The key thing is not to go as far as with a cure, which is designed to produce the finished, ‘hard’ material. With the de-bulk, therefore, we use lower temperatures, which get the resin to the point where it flows and compacts the material down in the mould.”
For each ‘skin’ of the chassis, two or three de-bulks might be necessary before the plies are ready for the cure. “The cure is the process during which the carbon-fibre acquires its strength and stiffness,” explains Colin. “Typically, for the first cure of the chassis, we will put it in the autoclave for three to four hours, at up to 180°C under a pressure of around 100 psi.” Pressure is increased steadily as temperature rises: “the exact point at which we do this, though, is a closely-guarded secret,” smiles Colin. “It can bring an important competitive advantage.” Cures are run for the core and the inner skin as well, although at lower pressures.