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jawaswag
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Hi all! I am intending to create an organic form sofa in CF. It's somewhat similar to the picture I've included below but a much smaller 2-seater version. My limitations are that I do not have space for a curing oven. Here are my concerns: 1) If I cannot cure an item in an oven, it means I will have to use room temperature cure resins and the means a hand layup method? 2) What can I make the mould out of? Is it a frame of wood with PU foam block for the the shape? If I do, how do I vacuum bag such a large mould? 3) Since the mould is likely to be a 'pool'-shaped form, can place a sheet over it and fill it up with water as a replacement over vacuum bagging? Would the high specific heat capacity of water affect the curing of the resin as it pulls heat away? .  Cheers and thanks in advance, Kai
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Steve Broad
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1. Yes, as far as I am aware.
2. As long as frame and PU foam block can withstand 14.7psi. A large bag :-)
3. Yes, as long as you can cover item with 406 inches of water (approx 10m) :-). 1 cubic inch of water = 0.036lbs and you need 14,7lbs per square inch. Heat the water to air temp.
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oekmont
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Room temperature cure doesn't mean wet layup. You could infuse the part as well. Using water isn't a good idea, as the water adds huge loads onto your mould, wich will likely deform it. A full vacuum bag, if done right, will not deform your mould.
For such large pieces with large tolerances for the final shape, I would use styrofoam , than cover it with an glass cloth and epoxy and than smooth everything up with body filler.
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jawaswag
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Thank you for the advice.
How do I vacuum bag such an item? In my head, the mould would be a similar size and shape of a bathtub. I’ve only seen people vacuum bag items that are more of a flat form.
Is the styrofoam you have in mind those that are granular cells that are solidified? That would be hard to carve the shapes, no? I haven’t seen any styrofoam that’s homogeneous type.
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oekmont
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You seem to have no experience at all in the composite techniques. So my most important advice is to try something much smaller and easier, and understand every step and than slowly build to such projects. The're is just to much wich can go wrong and easily waste several hundred dollars/euros/pounds.
The bagging technique is just the same as for flatter and smaller parts. With pleats you add extra bagging film length, wich allows you to get into the contours.
I would start with watching every easy composites YouTube video, as well as most videos of Matthieu libeert. After that binge watch, you should have a far more accurate understanding of what it takes to take on such a project.
Every remaining questions (and there will be a lot) will be answered happily here in this forum.
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Steve Broad
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+xRoom temperature cure doesn't mean wet layup. You could infuse the part as well. Using water isn't a good idea, as the water adds huge loads onto your mould, wich will likely deform it. A full vacuum bag, if done right, will not deform your mould. For such large pieces with large tolerances for the final shape, I would use styrofoam , than cover it with an glass cloth and epoxy and than smooth everything up with body filler. I have always assumed that wet layup also covered standard vacuum bagging and infusion as well as basic layup without vacuum, as they both are wet processes :-) The water idea was always a non starter IMO due to the pressures involved, but an interesting one nonetheless. However, I can't see how 10m of water adds any more load to the mould than a 14.7psi vacuum.
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oekmont
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No, infusion isn't a wet process, because you lay up the fibres dry. Hand lamination is the only wet process. In all it's countless variations.
It is quite simple: you have got a mould in the atmosphere. How much pressure is applied to the mould? About 10t/m^2. Equally from all sides. A solid body cannot be destroyed by so called isostatic pressure. now you bag the whole mould. How much pressure is applied to the mould. About 10t/m^2. The forces to the mould are exactly the same as before, as long as the bag doesn't bridge anywhere. Between the film and the mould is no atmospheric pressure, so both the mould and the film press against the laminate with 10t/m^2. Every increment of the mould/film shell is in balance of the two surface forces. So the mould has to take no structural forces apart from the weight of the mould, the laminate and the bagging materials.
Now the water scenario: on one side you got atmospheric pressure against the mold, on the other side you got atmospheric pressure on top of the water, as well as the hydrostatic pressure from the water. Every increment is obviously not in balance of the forces. And this disbalance has to be compensated by the mould. And a mould the size of a sofa easily takes a about several hundred kilos of water to fill. As long the mould isn't circular or spherical you will need a very strong mould to hold that without deformation.
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Steve Broad
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+xNo, infusion isn't a wet process, because you lay up the fibres dry. Hand lamination is the only wet process. In all it's countless variations. It is quite simple: you have got a mould in the atmosphere. How much pressure is applied to the mould? About 10t/m^2. Equally from all sides. A solid body cannot be destroyed by so called isostatic pressure. now you bag the whole mould. How much pressure is applied to the mould. About 10t/m^2. The forces to the mould are exactly the same as before, as long as the bag doesn't bridge anywhere. Between the film and the mould is no atmospheric pressure, so both the mould and the film press against the laminate with 10t/m^2. Every increment of the mould/film shell is in balance of the two surface forces. So the mould has to take no structural forces apart from the weight of the mould, the laminate and the bagging materials. Now the water scenario: on one side you got atmospheric pressure against the mold, on the other side you got atmospheric pressure on top of the water, as well as the hydrostatic pressure from the water. Every increment is obviously not in balance of the forces. And this disbalance has to be compensated by the mould. And a mould the size of a sofa easily takes a about several hundred kilos of water to fill. As long the mould isn't circular or spherical you will need a very strong mould to hold that without deformation. Fair enough on the infusion process. My thought was that the resin is very wet:-) If the mould is suspended under 10m of water the pressure around it would be approx 14.7psi, the same as a vacuum bag so I can't see the difference. Pressure is pressure.
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oekmont
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Also one important point: as long as you are just covering the wet laminat with a plastic film and the fill the plastic film in the mold with water, you got an open system. The resin in the laminate and the water in the mould are in the same pressure system, connected through the air at one atmospheric pressure. There is no force against the laminate or the bagging film, as you got hydrostatic+ atmospheric pressure on both sides of the fibres/film.
Think about it that way: you fill the mould partially with water (resin) cover it up with a film and than add additional water. What you get is just a plastic film floating in water. It isn't pressed against the mould, it can go wherever it wants, as long as the're is no transfer if water between both sides.
All you achieve is a compression of the air bubbles.
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Steve Broad
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+xThank you for the advice. How do I vacuum bag such an item? In my head, the mould would be a similar size and shape of a bathtub. I’ve only seen people vacuum bag items that are more of a flat form. Is the styrofoam you have in mind those that are granular cells that are solidified? That would be hard to carve the shapes, no? I haven’t seen any styrofoam that’s homogeneous type. This needed a large bag.  
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