I am thinking of making a Carbon Fibre Harp.
I am having a problem with how to make the neck and column.
If you don't know, this is shaped like a curly number seven. It needs to be very strong as it has about 500 Kg of pull across it from the strings. I would like to make it in one piece. The sound box fits between the ends of the seven but this is relatively easy.

I was thinking of making it in a mould made of sheets of MDF laid flat with the cavity cut out of the middle sections.

I would make the piece by wrapping layers of Carbon Fibre around sections of expanded polystyrene so that it would end up with an internal box section support structure.
I would also include a bicycle inner tube so that when the top is bolted on to the mould I could inflate this and force the piece into the corners.
I was wondering if I could laminate using infusion resin and include a length of spiral tube between layers leading out of the mould so surplus resin would be forced out. I think the laminating resin would be too thick to do this.

Does anyone think this would work?

Brian.

Hi Brian,

Thanks for the post. You've got some interesting ideas there and some that maybe I'm not quite following.

Taking the process in order, it would be perfectly possible to make a 'one-off' mould using sheets of thick MDF and profile out your shape by cutting some of the layers to form the cavity. You could use a ball router to radius the inside corners or you could cut the sheet straight through (using a jigsaw) and then, when the bottom 'closing' sheet it in place you could use bodyfiller or something similar to radius off the inside corner. You would need to do this to avoid any awkward 90' corners. To make the MDF more sturdy and therefor cabable of taking the moulding pressures I would thinkt hat you'd need to coat the whole surface in some lightweight glass and epoxy and then flat that back. Finally, I would suggest a 2 pack high build filler primer as the surface layer which again you would flat and then polish. As you've planned, make this in two halves that bold together. Pu the the split line dead centre (rather than just making it so that you remove the top sheet to remove the part) as this will make it much easier to demould and also mean that you can put the radiuses in the top and bottom corners.

For your pressure moulding process, I don't understand exactly how the polystyrene foam is going to work along side the bike inner tube? - Do you intend having the inner tube inside the polystyrene core? If so I've not gont any experience of such a process but I can see some merit in it (specifically that it avoids the neccessity to have something like an inflatable silicone bladder) and that it gives you a good positive shape to laminate around prior to putting the laminate into the mould. If this process was to work it might be worthwhile wrapping the foam core with release film (first) then breather, then perforated release film. The release film would stop any reson from sticking to the foam core, the breather would be there to facilitate airflow out of the laminate and absorb excess resin and the perforated release film would be there to allow excess resin to flow from the laminate into the breather.

As projects go it's adventerous but exciting too and I can see that good results could be acheived if you put the work in with to make a decent mould (it's always the way!).

All the best, Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Hi Matt,

Thank you for your reply. Things can be difficult to explain but you have the right idea for the mould.
Looking at it as flat sheets with the "7" laying down, the top and bottom surfaces would be flat. I am sure that I could be more adventurous with curves but I am copying a wooden harp to get the sizes right. The cavity would be 24mm thick. I don't think I can explain my polystyrene and inner tube idea without pictures and I have not got that far yet. But see if this gives you the idea.

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f  ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
f  .............. f .............. f iiiiiiiiii f.............. f .............. f
f  .............. f .............. f iiiiiiiiii f.............. f .............. f
f  .............. f .............. f iiiiiiiiii f.............. f .............. f
ffffffffffffffffffffffffffffffffffff iiiiiiiiii ffffffffffffffffffffffffffffffffff
f  .............. f .............. f iiiiiiiiii f.............. f .............. f
f  .............. f .............. f iiiiiiiiii f.............. f .............. f
f  .............. f .............. f iiiiiiiiii f.............. f .............. f
f  ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff
ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff


f   is fibre in various thicknesses.
.   is expanded polystyrene.
i   is the inner tube

I was thinking of working with the top solid sheet of the mould removed and not splitting it in the middle as you suggested.
The first piece of fibre would be put across the bottom and up the left side then laid aside across the top of the mould to be folded over later.
The second piece of fibre would be put across the bottom and up the right side then laid aside across the top of the mould to be folded over later. This is the outside shell.
I would then put in some thicker, say 650gm layers across the bottom, (Not shown in the diagram.) and up the left side. These would be separate pieces if it will not bend into the corners. Then a strip of 10mm expanded polystyrene followed by more vertical strips of fibre. Then the same again for the next "Cavity"
Then another horizontal layer of thicker fibre and the 2 "Cavity" sections above that.
Same again for the right hand side leaving a slot in the middle for the inner tube which would be in a polythene tube so I can pull the tube out from the end when it is finished.
Then finishing off with a horizontal layer of thick fibre and wrapping the right hand piece that was laid aside over the top and the left hand piece folded over that.
This sandwich would be slightly thicker than the mould cavity so it would be squashed down when the top is bolted on. Then the inner tube inflated which would only be able to push things left and right, and tightly into the sides of the mould.
It all sounds very complicated but it needs to be strong and light.

The mould would be separated vertically right from left. ( or inside of the "seven" from the outside.)

Brian 

Hi Brian,

I understand your plan and it's an interesting one. I did not realise that you wanted internal shear layers of reinforcement, I imagined just a foam core and then the reinforcement like a 'box section' around it. Having the shear layers will add lots of strength but certainly complicates your construction so that you end up needing to make it the way you plan. If you did without these intermediate layers then you could simply make the foam core an almost perfect fit for the MDF mould and then wrap your carbon around the foam core (which would be easy to do with the core out of the mould) and laminate it through with resin before squeezing it down into the mould, putting your end cap on and inflating the inner tube.

I've got to say that I think you'll have problems removing the part from the mould if you have parallel sides and hope to lift the part out somehow by simply removing the 'top' sheet of MDF - you'll have nothing to grip on, the parallel sides will lock in the part and the none radiused corners will also lock the part. This is fair enough if you plan on dismantling the MDF mould completely to get the part out but just don't expect to be able to get it out without doing this.

I'm also worried about these none-radiused 90 degree corners. The fabric is likely to bridge around these corners leaving them looking far from perfect. Radiused corners (even if you only do it with plasticine or filleting wax) would help a lot but you can't do it unless you had a centre split line (when you could also make the sides taper slightly too!).

Good look with it - I'll be watching with interest.

Best regards, Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

i have experience by 2 sides mold by presing each other and trust me part comes very hard but problem is that air not go out and part is bad cometic look also some places change color. from 10 parts good been with straight pattern and around to good look like 2 not more, so i suggest vacuuming or infusion or do pre preg with autoclave self made.

Thank you for the comments.
I have done a lot of reading and thinking since I posted this.
I have changed the way I plan to make it. I will (When I get the time to make it!) take lots of photos and and tell you all how I get on.

Brian.