Monday 9 July 2018
Carbon fibre is the toughest construction material around. It has several times the tensile strength of steel yet at a fraction of its weight. Sometimes carbon fibre products are called graphite and it is true that both are made from pure carbon and both are black, but their internal structures make them very different materials.
Carbon fibre was first developed during the 1960s in the white heat of technology, so far ahead of its time that it was destined to spend many years as a solution looking for problems. The fibres themselves are fine and glossy looking like plastic but immensely tough as well as being moderately fireproof.
For practical purposes carbon fibres are usually loosely woven together as matting. The user stacks these layers and bonds them together with synthetic resin in the same way as fibreglass to form stiff moulded panels.
Over the decades carbon fibre has made its mark in high-performance jet engines, bullet-proof vests and Formula One car chassis as well as practically every kind of rigid sports equipment. In recent years some high-end furniture designers have featured carbon fibre for its fineness, lightness, strength and distinctive appearance.
In this article I will show you as a simple example how I integrated a carbon fibre top and bottom with walnut sides to make a case for a violin. I made a rudimentary mould and used a vacuum bag, familiar to furniture makers for veneering and laminating, to shape the carbon fibre sheets against it.
Handling carbon fibre
The raw carbon fibre material comes as a rolled-up sheet, woven from flat bundles of fine fibres. Looked at in close-up the fibres are straight, not spun like a cloth, allowing their glossy surfaces to slide over one another and easily unravel.
The woven pattern is described as twill because there is an offset in the weave, the weft passes over two warps and then under two warps. This is repeated but shifted along on each row which makes the matting more flexible as well as giving it the distinctive appearance of diagonal stripes.
Carbon fibre matting behaves and feels more like black woven straw than clot. Unless it is handled carefully it frays badly at the edges and easily pushes out of shape, losing the neatness of its pattern.
Cutting carbon fibre
The matting feels clean and dust free but as a precaution I used a dust mask for all cutting and sanding operations. Cutting a single thickness of the matting is not difficult, a sharp pair of scissors will crunch through the fine carbon fibres, but first you need to prepare the line of cut.
If you are going to cut squarely across the line of the weft, the easiest way is to pull out a fibre bundle from the edge of the matt, leaving a gap with a row of warps to cut across. For more complex shapes you need to protect the edges from fraying and distorting by first attaching masking tape. This should be pressed firmly into the matting. The position of the tape should allow for edge wastage and the pull of the material if it is to be shaped over a mould. Remember that, while the woven carbon fibre matting can be pressed into shape, the fibres themselves offer no stretch at all.
Mould for carbon fibre
The upper shell of the violin case needed to be shaped into an elongated dome to clear the high bridge which carries the violin strings over the arched belly of the instrument. The shape of this shell not only provides clearance but also gives it stiffness, flat sheets of resin-bonded carbon fibre being relatively flexible.
There were two options:
1. Make the single-sided mould to form the shell concave and press the carbon fibre sheeting into it
2. Make it convex with the sheeting stretched over it
I decided that, although pressing the outer face of the shell into a mould would allow its surface to be more accurately shaped, the mould would be much harder to make. Also I was concerned that the inside of the shell should be correctly shaped and dimensioned so I chose the second option to make a convex or positive-shaped mould.
The base of the mould was bandsawn from an 18mm-thick MDF board with one edge rounded over using a 12mm-radius router cutter.
To provide bulk to the centre of the mould I made a stiff inedible mixture from flour, salt and water, a recipe that makes play dough for children. Alternatively you could use modelling clay.
After pressing this to shape I covered it with several layers of Cling-film pulled tightly to make it smooth, thus removing the need to use any releasing agent.
Resin and its application
A number of different types of synthetic resin are suitable for casting carbon fibre, depending on the application. Epoxy resin, used by furniture makers as a gap-filling adhesive, is immensely strong and bonds well to carbon fibre. However, it is quite expensive and lacks clarity having a dull yellow colour. Acetate and polyester resins, although more brittle, have the advantages of clarity and lower cost and they polish more readily to a fine finish with abrasive paper.
A small measured volume of hardener is added to the resin and must be mixed thoroughly. The supplier will advise on proportions and handling precautions.
Depending on the stiffness and strength needed, you can use any number of layers of carbon fibre matting from two upwards. For a high-performance application an engineer might calculate the stress on the finished component and specify the number of layers accordingly, but for this type of application it is just a matter of making the finished shell thick enough so that it will not bend in use. After some tests on small samples I found three layers worked well for this job.
The resin mixture needs to be spread evenly, using a conventional paint brush with good, full bristles, that you are prepared to throw out afterwards. The resin must impregnate each layer of carbon fibre matting.
Avoid dry patches with no resin, but too much resin will squeeze through the matting and cause wrinkles under the vacuum bag. If in doubt use less resin as you can always coat more on afterwards. Alternatively you can buy sheets of absorbent material with a permeable release film to sandwich under the vacuum bag.
Vacuum bags exert the full atmospheric pressure of one kilogramme per square centimetre onto whatever shape is fitted inside the bag, pressing equally at every angle. At first I was worried that the pressure might distort the shape of the soft mould I had made from dough, but after thinking about it I realised that the pressure, being even to every part of the dough, would not affect the shape.
As the pump sucks a vacuum in the bag it presses onto the carbon fibre matting, firmly squeezing it against the shape of the dough-covered mould, excluding all air gaps but still retaining the intended shape.
Synthetic resins produce heat as they set from the exothermic reaction but they need a comfortable room temperature to start the reaction off and will cure more quickly in higher air temperatures.
To speed things up I warmed the area around the vacuum bag with a fan heater. The shell was moderately rigid in four hours and after withdrawing it from the vacuum bag, the Cling-film peeled easily away from the resin surface.
Carbon fibre finish
The outer surface of the shell still had some wrinkle lines on it from where excess resin had squeezed out of the carbon fibre matting and set, otherwise the surface was smooth.
I used a random orbit sander with good extraction to remove the wrinkles and generally flatten the surface, working through grades of abrasive disc and being careful not to cut into the carbon fibre.
Applying a gelcoat surface would be an option at this stage as it can be cut back to a high-gloss finish. Alternatively you can brush or spray on a water-based acrylic lacquer for a matt or satin finish. Either way the distinctive carbon fibre twill weave shows bright and clear through the finish.
Making a case
Before fitting the shells the rough edges need to be cut away, but with three layers of carbon fibre matting and cured resin it is no longer possible to use scissors so I used a pair of metal shears. Alternatively you might cut the component with a fine-toothed bandsaw blade.
To make the sides of the violin case I used walnut. I ripped a plank edgewise through the bandsaw to produce a 3mm strip then shaped this on the stove pipe of a log burner to match the outline of the MDF mould.
I bonded the carbon fibre shells to the top and bottom of the case sides with epoxy resin and glued further laminates of walnut into the sides to stiffen them, also bonding the edges of these to the carbon fibre before finishing and lining the case.