The process of
''cooking'' carbon fibre using autoclaves
As published by Racing Live on Dec 14th, 2003...
Throughout the laying-up of the chassis in the composites department,
the autoclaves have a crucial role to play. Then come the final
machining stages before the chassis is ready for assembly…
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
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 cures
The 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.
Once the final cure has been completed, and the mould ‘cracked’
to reveal the final part, the chassis upper and lower must undergo
final machining. The halves of the chassis are mounted in purpose
built jigs on the Huron machine, and holes are machined through
the carbon fibre and into the various metal inserts for suspension
pick-ups or engine mounts. Further work on a large JOBS machine
allows detailing such as the obligatory camera mounting position,
or areas around the fuel filler, to be completed, as well as the
internal profiles of the chassis. Once this has been done, the two
halves are ready to be bonded together: chassis 01 is almost complete!