”Cooking” carbon fiber using Autoclaves
”Cooking” carbon fiber using Autoclaves
Autoclaves play an important role in manufacturing the chassis in the composites department, and later followed by final machine stages in assembly. According to Composites Manager Colin Watts, an autoclave is a big pressurized oven, in which the carbon fibre is molded. During the initial laying-up stages of the chassis autoclaves have a number of different uses, which come into play. These stages are termed as de-bulks’ and ‘cure’.
The complex process of the construction of chassis takes place in stages, laying up the different cuts of carbon fibre, and the autoclaves have a vital role to play at each stage. These parts are cooked at different temperatures and different pressures in a vacuum, to extract any air from the material.
Every part that enters the autoclave undergoes the same process. 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.
The two principal processes during which the autoclaves come into play are de-bulk and cure. De- bulk process is used to compact and compress the material under lower temperatures, which get the resin to the point where it flows and compacts the material down in the mould. The key thing is not to use high temperatures as in cure, which is designed to produce the finished, hard material.
For each skin of the chassis, undergoes two or three de-bulks before the plies are ready for the cure. The cure is the process during which the carbon-fibre acquires its strength and stiffness. Typically, for the first cure of the chassis, it is put 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 cure is done is considered as an important competitive advantage, a trade secret of the respective manufacturing companies. Cures are run for the core and the inner skin as well, although at lower pressures
Then comes the final machining stage. Once the final cure has been completed, and the mould is 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:
Thus the chassis 01 is almost complete after being baked number of times in the autoclaves and processed under jigs and large job machines.
For more information about Eschmann SES Little Sister Autoclaves