What are the best materials to Build a rtm mould

Discussion in 'Fiberglass and Composite Boat Building' started by Nicholas Clark, Aug 5, 2012.

  1. Nicholas Clark
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    Nicholas Clark Junior Member

    :confused:Hi i have built a wooden plug to build a boat mould. I was going to coat this with epoxy ressin but is the best material to coat in before i coat in a release agent tooling ressin and gelcoat. Plus after i have used the sheet wax what materials are best forfor the clear top mould and where can i purchase these.
     
  2. rwatson
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    rwatson Senior Member

  3. Herman
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    Herman Senior Member

    The wooden plug can be finished with epoxy, then polished or painted. Keep in mind that heat will slightly distort it, and seams, nails and other details might show through, even during the mould making process. To prevent this a thick and heat resistant coating is better than a thin coating. Glass fabric might help with maintaining a constant thickness.

    After making your mould (usually polyester rapid tooling system) spray the mould with PVA, then apply the sheet wax, and detail. If you want a really smooth finish spray the wax with paint, then rework. (or just polish the top mould after manufacture).

    The top mould usually is either a high temperature and somewhat flexible vinylester resin, or epoxy resin, or sprayable silicone.

    I suggest contacting Composite Integration in the UK for more details and help with making your mould.
     
  4. Nicholas Clark
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    Nicholas Clark Junior Member

    Thanks for your help. Unfortunately my material suppler has told me that i cannot make a twelve foot long boat using rtm. He said the core of the Closed Mold Matting becomes soaked in water when it penetrates the gel coat. Is this true or can i use a different method. Thanks Nicholas!:confused:
     
  5. Herman
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    Herman Senior Member

    Perhaps his matting is not suitable, but making a 12ft boat in (VA)RTM is not the problem.

    What kind of boat is it? Your laminate will definately need some engineering, but very probably nothing undoable.
     
  6. SamSam
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    SamSam Senior Member

    Why would the mat penetrate the gelcoat?
    Why couldn't you paint the finished product instead of gelcoat?
    Why do you want to make an RTM mold to begin with?
     
  7. Nicholas Clark
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    Nicholas Clark Junior Member

    The Matt does not penetrate the gel coat. The water penetrates the gel coat if the boat is left in the water. Everything is what you call pervious like a balloon lets air threw the rubber after time because it is under pressure and deflates slowly. Apparently the felt core in the closed mold Matt becomes saturated in water after time.
     
  8. Nicholas Clark
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    Nicholas Clark Junior Member

    I am only building a sciff which is not going to be in the water all the time i suppose. Do you think i can add extra layers of Matt to the outside skin before i put the closed mold Matt in the rtm mold. The sales man said that one layer is not enough to stop the water penetrating.
     
  9. SamSam
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    SamSam Senior Member

    Oh. Well, that's just the regular fiberglass problem then and wouldn't seem to rule out RTM. If mat can get soaked with water, then any type of fiberglass can get soaked with water. I don't think it can anyway. It's glass. Window glass doesn't get saturated with water. Wine doesn't leak out of bottles after sitting around for decades. What happens is if water penetrates the resin and gets to a foam or balsa type core, that will get saturated.

    Crappy lay-ups that leave air around the glass fibers will let the spaces get filled with water, but the fiberglass itself won't absorb the water. If there are microscopic spaces within the fiberglass bundles (which wouldn't happen if filled with resin as it's supposed to be), water can be wicked along there, so in practice, a crappy layup in mat will be more waterproof than a crappy layup in cloth, as the wicking is stopped or hampered at the end of each chopped strand, whereas it can keep traveling along a continuous bundle/strand for a long ways.

    Are you sure you want to make an RTM mold? From what I ever heard of them it was a sort of expensive, involved process for highly technical parts such as for aircraft. My understanding of an RTM mold is you make a female mold, line it with sheet wax of whatever thickness you want the finished product to be, then make another mold over that. Take it apart, take out the wax, buff, finish and prepare both the molds, line them with the fiberglass, clamp them together and fill with resin using pressure and vacuum.

    What is this 'felt core' you referred to?
     
  10. Herman
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    Herman Senior Member

    The matting for (VA)RTM can be for instance Rovicore or Polymat, which has a polypropylene "core" inside, which will fill with resin (it is more a distance-keeping fabric than a core)

    There are others, like "Lancore" from Lantor with expancel in the middle.

    The outer layers are usually CSM.

    There is no problem adding more CSM to the outside, or other fibers, such as Lantors D7760 surface veil. This will help block water.

    The resulting might or might not be on the heavy side, it is just what you expect from it. I have no idea what the laminate should be on a skiff. If it is a rowing skiff, it will be fairly thin, and for racing sculls RTM might be too heavy.

    In general a production run of 50 will justify the expense of making a top mould.
     
  11. SamSam
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    SamSam Senior Member

    I was under the impression both molds were built the same, as in solid/rigid. If the top mold can be 'flexible' as you said earlier, that would seem to be a kind of permanent infusion process without messing with the one use disposable bag and vacuum and resin tubes.
    If the resin supply and vacuum tubes are eliminated, how does the resin get into the laminate? If the top is flexible, wouldn't that eliminate any pressure feed of the resin?
    I guess if the pressure feed and vacuum were balanced, the laminate could be saturated without the top mold bulging too much, and then the resin supply shut off and the vacuum would suck out the excess and compress the top mold into the bottom one.

    OK, I went back and read the directions you supplied and my questions were answered.
    http://www.composite-integration.co.uk/about-rtm.php

    To the OP, if the skiff is only in the water part time it shouldn't be a bother. Think of a fiberglass shower/tub enclosure, consistently the worst layup in the world but they never have any problems soaking up water. Actually, a lot of boats are made from cheap materials (Polyester resin and mat) and sit in the water for years with no problems.
    More mat layers on the outside will help retard moisture absorption.
    What is this 'felt core' stuff?
     
  12. Herman
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    Herman Senior Member

    felt core: Lantor Coremat, or any variant you can think off. (including infusion compatible ones.)
     

  13. SamSam
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    SamSam Senior Member

    I used some Coremat in the early '80s, I think it had just come out and I used it to bulk up a mold. It seemed unevenly lumpy and seemed to soak up more resin than a bath towel. I can imagine it would soak up water. I never used it again and still have most of the roll.
     
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