Under the skin: what exactly is carbonfibre?

Carbonfibre is one of those things that gets taken for granted as the super-lightweight material that costs a lot and few people can afford. But what is this stuff? 

The answer to that is pretty much what it sounds like: fibres made of a graphite-like carbon. Those fibres are made from stuff called polyacrylonitrile (or PAN, a plastic), which in turn is made from acrylonitrile. The PAN fibres are heated at around 300deg C to burn off anything that isn’t carbon and then superheated to produce the carbon fibres. 

That takes time and a fair amount of energy, which is why carbonfibre isn’t the most sustainable of materials – that and the fact that it can’t be recycled into high-grade material in the same way as aluminium can. 

Wound onto large bobbins like some kind of elongated 21st-century cotton reel, the fibres are then woven into matting that feels much like smooth, rudimentary sacking but looks black and shiny. The weave can vary for different uses but the main types are called twill and plain weave. 

At that point, how the carbonfibre is used varies. The fibre can already be impregnated with uncured resin and in this form is, not surprisingly, called prepreg. It must be cold-stored and, to use, it’s cut into the appropriate shapes much like a tailor would cut cloth for clothing. Then it’s ‘laid up’ in the mould for the component being made, the direction of weave in each layer decided by the engineers who designed the component. In a car’s main structure, like a Formula 1 or supercar tub, the direction of weave depends on which direction forces are working. 

Next, the mould goes into a silicon-treated bag and the air is sucked out to compress the layers of carbon fibres together. Then the carbonfibre is ready for curing ideally in an autoclave (a pressurised oven) at 120-180deg C and 2-6 bar. That’s the way F1 car tubs are made. Where a megabucks autoclave isn’t available, then a bagged carbonfibre component can be cured in an oven (while maintaining the vacuum) at 60-100deg C. 

A more expensive method but one used for the production of some car components and, in certain cases, complete car monocoques is a heated press. The moulds, or ‘dies’, are made of precision steel and similar to those used for pressing steel or aluminium body panels. Expensive steel tooling such as this is also used for resin transfer moulding (RTM), where the dry carbonfibre is laid up in the mould, which clamps together and is injected with resin at 10-20 bar and then heated. Once part-cured, the job can be fully cured in an oven later. RTM is fast and can be used to mass-produce several thousand parts a year. 

One of the latest developments in carbonfibre is forged composite, where fibres are cut into short lengths of about 5cm, mixed with resin, injected into a mould and pressed (forged). More of that in part two next week, plus a look at who is doing what with carbonfibre and on which cars.

BMW’s high-fibre diet

Looking like Dumbledore’s spare wig, polyacrylonitrile, the stuff used to make carbon fibres, is known as a ‘precursor’ because it’s a stage before the finished material. Used for BMW i production, polyacrylonitrile ‘tows’ consisting of 100,000 fibres each are made at the Moses Lake plant in Washington, the US.