female mould vacuum bagging

Discussion in 'Fiberglass and Composite Boat Building' started by stu large, Sep 19, 2009.

  1. stu large
    Joined: Sep 2009
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    stu large Junior Member

    Hi Guys,
    I've been reading about the problems people have about vacuum bagging inside female moulds, when I had what might be a silly thought, female moulds are the opposite of male moulds, so why not use the opposite technique, instead of vacuuming a bag, why not try inflating one inside the mould.
    Probably this has been tried before, if so can anybody comment on this.
     
  2. apex1

    apex1 Guest

    Oh yes, anyone can.

    Try blowing the same pressure into a closed bag than evacuating a few ccm gives!?!

    Regards
    Richard
     
  3. Itchy&Scratchy
    Joined: Jul 2008
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    Itchy&Scratchy Senior Member

    Its called bladder moulding,thats how they make carbon fibre mountain and road bikes.:)
    J
     
  4. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    The vacuum is in the item itself hence gets pressurised from both sides, while pressure is going to be on a large volume. Your stuff will have to be able to handle the pressure.

    I actually had the same idea, pressure is much easier to make than proper vacuum, and you can get multiple times the pressure from pressure than from vacuum.

    You get molds theat has a male and female and works in a press, you get a very dense, light and super strong item, but how to make it big enough for a boat ? It would be very very nice though.

    If you can come up with something practical I would be interested for sure.
     
  5. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    Where might I find the source material relating to the problems of vacuum bagging in female moulds?
     
  6. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Derek Kelsall uses pressure bags in his catamaran designs.

    Thanks for the thred. I think I will consider this "positive" technique for some jobs on my next project.
     
  7. wardd
    Joined: Apr 2009
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    wardd Senior Member

    i used to work for martin Mariette on lay up tooling made from aluminum and carbon fiber and sometimes the tape seal would fail, cleanliness of the surface that the tape adhered to was of utmost importance ( that can not be stressed too much ) any oil dirt would cause it to fail

    our standard was not to lose more than a couple pounds over half an hour and that was after a vacuum was pulled and taken off, the bagging and tool had to hold it

    like i said clean check and clean some more

    acetone and denatured alcohol was used as it left no residue

    and did i say make sure it was clean?

    and we pulled vacuum from several places on the tool

    the tools were put in an autoclave under heat and pressure
     

  8. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    Thanks wardd,you have listed the reasons why bags fail to achieve the desired result regardless of whether the tool is male or female.The reminder to be scrupulous about cleanliness is apt.The fact remains that a vacuum bag, in an autoclave ideally,is normally a better idea than a pressure bag.Because no new loads are applied.
    I had some experience with pressure bags in the seventies and they worked reasonably well for consolidation.The problem came in the form of distortion of the mould.We were using rigid supports,approximately an inch inside the laminate,and using the support to provide a reaction force to the pressure bag.The bag itself was a lot like a flexible water tank and was inflated using a small manual pump bought from a camping supplier and applying a pressure of about 4psi-as near as we could guess.If you do a rough calculation of the total force applied to a sixteen foot hull mould,it is no great surprise that the hulls were not quite the shape the designer intended.The situation gets worse if you have measurement rules to consider.I remain convinced that a vacuum system is a better idea and a much more flexible solution.
     
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