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Thread: Carbon Fiber Fabrication help

  1. #1
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    Carbon Fiber Fabrication help

    I'm making a carbon fiber piece that will hopefully be around a 4 inch by 4 inch by 5 feet tube that is a triple layer of 2x2 twill 6k tow carbon fiber (black picture) then 4 harness satin weave kevlar (yellow picture) then another layer of carbon fiber.

    When I'm done this will be 5.62 pounds of material and epoxy resin at a 50:50 ratio.

    Now it will actually have a pneumatic arm below it that will be lifting this carbon fiber arm and up to 5 pounds of piece for a total of 10.62 pounds being lifted.

    So here are my questions:

    When using wet lay up technique to make this carbon fiber piece, is it better structurally to make two halves from a mold and epoxy them together along a seem or to combine them along an overlapping?

    Or is it better to use foam and lay the carbon fiber around the mold in a single piece?

    Also, what is the best way to lay the carbon fiber if the force is downward? Is it that I lay the carbon fiber to where the fibers try and run down the part best they can will that give me the best strength?

    Sorry for all the questions, but I'm confident that this forum is the smartest on this kind of stuff!

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  2. #2
    Constant Bitrate
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    Does the tube need to be hollow? Do you want round or square(ish)?
    What does the surface finish need to be? Relatively smooth or glass smooth.

    You mention laying it up on a piece of foam. Would you then need to remove the foam?
    The foam can add a great deal of strength to a structure. Think of it as the center web of an I-beam. The foam will help resist buckling of the skin.
    If the tube is to be round, just laying the fabric over it and wetting it out might work. If there are to be any sharper corners you will need to wrap the layup with something to keep the fabric in contact with the foam and each other. You should wrap it no mater what to compress the layers. They make a specialised shrink tape that you wrap, then hit with a heat gun. You could use electrical tape, or that plastic cling wrap stuff they use to wrap pallets. Or place it in a bag and draw a vacuum.

    If you wanted to use a female mold and it's to be round, a length of PVC pipe split in half could work. Use car wax as a release.
    Lay the fabric so that one edge is just below the edge of the mold and the other is above by 1/2". Do the same thing for the other half of the mold, then when you put the two halves together the fabric sticking up laps over the other side forming the seam. Make the layers the same width which will stagger both edges. Staggering the edges of the layers (first sticks up 1/2", second sticks up 5/8", third stick up 3/4") will make it easier to not have large voids in the surface along the seam.
    After wetting out all the fabric in the mold, add extra epoxy along the seam and on the fabric sticking up. Brig the two halves together carefully so you don't catch the fabric and clamp the mold together. Then use a stick to work the fabric that was sticking up. This laps over the other fabric edge forming the seam.

    Orientation of the fabric should probably be balanced. One layer at 0-90, the next at 45-45 and so on. What is the reason for the Kevlar? Kevlar will help keep the thing together should it suffer an impact, but won't add much otherwise (my opinion, may be wrong).

    Use a laminating resin such as West System, ProSet or MGS. If you try to use a 30minute epoxy glue you will end up with a mess. West System is probably the easiest to find. http://www.westsystem.com/ss/where-to-buy/
    West System is also a lower performing resin compared to ProSet or MGS. ProSet is better and MGS the best (of the three listed). West will still yield a good part, but the flexural modules, heat deflection temp and other numbers are lower than the other two.

    You can use a spray adhesive to help keep the fabrics in place somewhat. A light mist of 3M77 over the fabric (while it's flat on the table) can help keep the fabric together once cut.
    Use gloves (no latex). Epoxy can cause allergic reactions in some, and you can build up a sensitivity to it over time. Some I know can no longer use epoxy their reaction is so immediate and severe.

    Good luck
    davidk

  3. #3
    Variable Bitrate djvillar's Avatar
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    I'm curious, why go with kelvar if you're going to cover it? Also even at 5' you should be under 5.5lbs if you sufficiently get the resin out of there.

    I would do two part mold:
    1. Go to home depot and get a steel channel.
    2. On then add a lip to the channel.
    3. Make your negatives 2x
    4. Now make your positives 2x
    5. put some resin in the lips and connect the two positives.
    6. wet outside seem with resin, lay strip of fiber, and one more coat of resin... seem is gone. Enjoy new tube.

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    If you do the mold really well, you might not see the seem, but it would give it the best structural integrity to lay a strip of fiber. Keep us posted. Also you want an epoxy resin... not a polyester resin.

  4. #4
    Variable Bitrate djvillar's Avatar
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    Check this out:
    Tube tutorial

  5. #5
    Constant Bitrate
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    Speaking from experience, making a mold for a one-off part is a bit overboard. Try to find something that is already made that will work.
    Using the method described in the last two posts, where the halves are cured then joined, is also a bit more work and potentially weaker on the seam if not done properly. Joining the two halves then adding another strip of fabric to the outside defeats the purpose of a mold with a nice finish. Adding a strip to the inside would make more sense.

    A wet seam as I described, when using PVC pipe as a mold, yields a seam that is very strong. The seam is done while the entire layup is still wet and therefore you don't need to sand or prep anything. Joining two cured parts requires cleaning and sanding the areas to be joined.
    Here is an example of some of my latest work.
    http://www.rcgroups.com/forums/showp...25&postcount=6
    http://www.rcgroups.com/forums/showp...85&postcount=5

    If I were doing this project, with only the information the OP provided, I would use the foam core method. I'd mount the foam like it was in a lathe so I could wrap it using this http://www.uline.com/BL_5401/Goodwrappers
    If I didn't have a motor to turn the foam during wrapping I'd have some help that could do the turning.
    Once the fabric is wetted out it becomes a slippery mess if you try to wrap it using any sort of tension. And having everything wet is not the time to find out you need more hands.
    If going for ultimate strength, after the resin has cured for 8 to 12 hours it should go into a hot box. Heating after initial cure will raise the properties of better resins like ProSet or MGS. West resin does not benefit much from a post cure.

    If this is your first time doing composites you may want to practice on a small part first. The amount of carbon your talking about isn't cheap at about $40/yrd.

  6. #6
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    There are many ways to tackle this type of project and some are cheap some are not. I would point out that 50/50 resin content is not ideal for strength you want to weigh your fiberglass or carbon and mix the equivalent amount of resin and bleed out as much as possible 27% would be ideal. There is a water soluble hold making material thats like cement you can make any shape you want and pull vacuum on it and stick it under the tape and wash it away. In school we wrapped carbon on fluorescent light tubes pulled vacuum cured and broke the tubes. and also used pvc pipe with a 1/4 cut down the side, and a 1/4 piece to fill it once cured pop out the 1/4 strip and it flexed enough to pop out. Hope that helps

  7. #7
    Constant Bitrate
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    they make carbon fiber and other materials in sleeves. like a chinese finger trap or what looks like braided nylon. put it over a form such as foam or pvc, wet it out with west systems resin and then vacuum bag it. this will yeild the best results you are looking for. no seams or bonds to worry about.

    http://www.fibreglast.com/product/Br...FYub7QodjUsIKA

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