Removable mandrel for double tapered filament-winding spar?? Does it exist??

Discussion in 'Fiberglass and Composite Boat Building' started by Erwan, Jul 27, 2018.

  1. Erwan
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    Erwan Senior Member

    Hi,
    Most of my problem is in the title of this thread.
    My purpose is to investigate if there is any "mandrel technology" which make possible a spar, made seamless (filament winding) while being tapered both ends.
    Max diameter in the middle should be around 90 mm and around 60 mm at the tips.

    So if anybody has any idea of existing such technology, or any idea on how to make one with lost mandrel while maintaining good mechanical properties, using epoxy prepeg carbon and autoclave cooking.
    Do not hesitate to drop a few words.

    Thanks in advance

    EK
     
  2. SamSam
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    SamSam Senior Member

    If it was a long oval I would say 'probably', if it's round you want, I would say 'why you be so difficult?'
    How long is this spar to be? Are you making one or many?
    I have no idea of mandrel technology, but a thought that comes to mind is a straight metal mandrel of the smaller dimension with a layer of foam of the correct dimensions on the outside. The layup done, the metal mandrel would be pulled out leaving the foam inside. The foam could be left inside or possibly scraped out or dissolved somehow.

    Found this...
    Carbon Fiber Spar - Page 1 | Bateau2 http://bateau2.com/howto/cf_spar.php
     
  3. JosephT
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    JosephT Senior Member

    Hi Erwan, similar shapes have been filament wound like this before. It is achieved by doing the filament winding over a disposable foam mandrel that is mounted on a on a removable spindle.

    Surface finishing options:

    a) After winding, for best finish consider a 2-piece female mold (clam shell) and cook it in an autoclave to smooth out the surface.

    or

    b) After winding just put it in the autoclave and cook it. Later, manually sand the surface to a smooth finish. Note: a lot of sanding required if you go this route as the surface will not be as smooth.

    Foam mandrel removal: Upon cure remove the disposable foam core using either chemical dissolving or physical removal (e.g. from ends, access panels, etc.). Using the mechanical method the foam core should be soft enough to be scraped away with non-metallic abrasion tools that will not harm the structure. Capture liquid/solid foam waste pieces & dispose of properly.

    Good luck!
     
  4. Erwan
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    Erwan Senior Member

    Thank You SamSam, Thank You Joseph,

    Definitly, it seems that the foam shape on an aluminium spindle is the way to go.

    I don't really care about the finish, the spar will be inside a "leading edge".
    I was worrying about the foam mandrel which could not bear the high pressure (5 to 6 atmospheres) and hight temperature (120°c) of an autoclave.

    When your mandrel is aluminium, the high temperature create dilatation of the mandrel which in turn increases the tension of the carbon filament, all good for the structural result.

    If the mandrel is made of foam, The foam must be strong enought, to resist the high pressure and may be some foam will expand a little bit with temperature, just like aluminium.

    The point is to find the good foam, I am going to look for it, as it is by far the best cost-effective solution for such a spar.

    Thanks again and Happy week-end.

    EK
     
  5. rxcomposite
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    rxcomposite Senior Member

    I seem to remember in filament winding method they use salt as a mould/mandrel for negative reliefs. Once laminated and cured, the salt is broken or dissolved.
     
  6. Erwan
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    Erwan Senior Member

    Thank you RX, that is the first time I heard about this technic, it deserves some investigation.

    Have a nice week

    EK
     
  7. rxcomposite
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    rxcomposite Senior Member

    Erwan- It is sand + PVA or sand + sodium silicate for dissolvable mold. it is only a page or two so borrow from your library the book Filament Winding Composite Structure Fabrication by S T Peters and others.
     
  8. JosephT
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    JosephT Senior Member

    This foam is rated up to 400F/204C. Generally, the higher temp pre-preg is used for aerospace. A lower temp pre-preg + autoclave should be suitable for a mast.

    Machinable Foam - Tooling Foam Board https://www.generalplastics.com/markets/tooling-and-molds

    I have not read about the salt/sand method that rxcomposite mentions. Sounds very interesting.
     
  9. JosephT
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    JosephT Senior Member


  10. Erwan
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    Erwan Senior Member

    Thank you very much for the precisions Rx and for these great links Joseph.

    That a great reading, before I was a bit obssessed by autoclave, as I though the difference between autoclave vs vaccum+owen was around 20%.
    But with 3% difference in structural properties, it is a "no-brainer" especially for a one item production.
    Great information about the good resin to be used too.
    Thanks again
    EK
     
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