Carbon fibre, as the name suggests is made from fine strands of carbon, the base element of many things including diamonds. The individual strands are layered into a wide flat thread, the threads are then woven into a mat. This allows for many variations, the main mat is referred to by its "twill". Twill is the way it has been woven for example a 2 by 2 twill has 2 vertical strands passing over and under 2 horizontal strands making the commonly seen grey and black pattern adorning many a road car.
From its make up,
Carbon fibres can be woven in any combination with 2 by 1 and 2 by 2 being the most common. By changing the weave the properties of the mat can be changed. The mat is then usually layered in several applications to make the part. Similarly by changing the layers either by the direction they flow or by the type of twill further properties can be added to the finished parts strength. It requires a particular understanding to make carbon fibre work in the desired way however it is not as hard as you might imagine.
Through to its application
This deep understanding of material application required, is in many ways similar to any woven fabric, even your jeans, can demonstrate the properties of carbon. Denim will stretch in certain directions (around the waist!) whilst remaining unchanged in another direction, and if a single thread is broken in the middle it can all unravel. Carbon Fibre will behave in the same way with strength and flexation, if during the manufacturing process a thread is torn it's a weak spot, which is why F1 teams will X-Ray parts.
Mould considerations for Carbon Fibre parts
When designing a part in Carbon Fibre great consideration has to be given to the mould, or moulds, required to manufacture your part. From which method of "fusion" of the carbon fibres - see opposite column for details- to the curing process dealt with here. Carbon Fibre to maximise its strength, and to be honest this applies to any resin and material process, requires the correct ration of resin to material. Too little and the part becomes brittle and fractures easily, too much and the part becomes resin rich therefore its core strength is in the type of resin properties and not in the Carbon layers. finding a balance is not hard with todays processes, but often over looked is the moulds construction. As most methods require heat to cure the part expands and contracts, if the mould does not expand and contract at the same rate on its surface contact area then it will add a new property to the finished part, producing fine cracks in the outer layer. Usually the reason for finite failure is to be found in the mould cavity, not only does the mould surface need to expand and contract matching the part but the entire mould needs to expand and contract exponentially to prevent the part from becoming "bound" by the mould in extreme cases the part can literally break in the mould.
Production options, PrePreg-Wet-R.I.M.
Because of the understanding needed for the woven raw materials, be it Carbon, Kevlar, or Glassfibre we have found our abilities in Leather, Suede and Alcantara share much in common. Similarities in covering a chair in leather as to a bonnet in carbon make for these two processes working within one workshop with each bringing a deeper understanding.
Pre Preg, or Pre-impregnated to give it its full name. This is a mat supplied with the resin already infused into it. Often requiring to be kept in a freezer to prevent part curing taking place. This is the age old way of making Carbon parts it gives good control of resin to mat ratio's but can be messy to deal with, particularly in cutting as the resin starts to flow from the minute its removed from the cold store.
Wet lay - using a dry mat then applying, normally by hand, the resin. Its main problem is having a varying degree of resin to mat ratio within the same part, and being very messy.
R.I.M. or Resin Infusion Manufacturing. This is our preferred method, you use a dry mat and place it into your mould, trim it and fit it exactly as required. Then using various methods the mould is sealed, then a vacuum is applied to remove all the air. The next stage is to bleed the resin into the mould using the vacuum to pull it through. This provides a very accurate ratio and is the only way to guarantee no air pockets exist.