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High Tech On The Kitchen Table

Quite a lot has been written over the years about using composites to build our shiny (or not so shiny in this event) toys. Using either low cost glass or the more expensive but much stronger carbon fibre. But always using the same ‘wet lay-up’ process: that is where you buy in the fibre as a dry fabric and then brush on the resin as you put the material into the mould. 
 
The advantages of this system is its relatively low cost and not needing much in the way of specialised equipment and it is a process that I and HQ Fibre who do most of my composite bits (the shiny ones) have used for many years to good effect. 
 
But for some while now people, usually called Simon Sanderson, have been telling me how much better ‘pre-pregs’ are. A pre-preg, for those not familiar with composite jargon is material that has had the resin applied in just the right amount via a bath of resin and lots of rollers in a very big machine. It is then supplied to you on a roll ready to use. The resin having been applied already cannot have any hardener added so it is heat sensitive, needing 80-180°C to cure it in a reasonable time. But of course it is slowly curing all the time and to slow this down it is kept in a deep freeze where it should be good for a couple of years.
 
So already getting complicated but many advantages: firstly, when you start cutting the material up it does not fray of go fluffy, even in very small pieces. So you can add extra layers where they are needed very easily. Then there is the complete absence of sticky resin that seems to get everywhere. Instead the material itself is slightly tacky and sticks best of all to itself, which is very good and makes the building up of layers very easy, and as it is heat cured you have many hours to do it.
 
For professional use you would be laying the material up into a mould and then vacuum bagging it down to consolidate the layers, ideally at high pressure in an autoclave (pressure cooker). But for those of us just wanting a one off you can do what Simon has been doing for years and I have finally got around to for the new Ratty: that is to lay up the material onto a foam core that you have carved and sanded to shape. 
 
This can be Styrofoam but you will need to by very careful about temperature (ask Lee!) or PVC which is available in several grades, or ideally ‘Rohacell’ which is the basis in a lot of F1 to aerospace composite structures and will take 180°C and high pressure. It is also very nice to carve and sand! But we now have a stockist in Norwich Emkay Plastics Ltd who will sell you odds and ends. This was how the main frame for Genesis was produced and for a first effort really was quite satisfactory. There was a distinct lack of shinyness, but strength and weight were both far better than earlier wet lay-up versions.
 
So very much a process of two halves: the laying up is very Blue Peter, but everything else is rocket science, i.e. deep freeze, vacuum pump, large temperature controllable oven, and of course a roll of very expensive carbon pre-preg.
 
My suggestion would be if you are keen to try out the idea is to find a company that does this sort of thing commercially and see if they will let you pay them for the carbon and vacuum bagging but let you do the laying up bit, which after all is the labour intensive bit.
 
You are unlikely to get anything as shiny as Simons bikes first time but we all have to start somewhere and this process does have a very enjoyable learning curve.
 
Mike Burrows

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