Looking for info on "Bladder Molds".

Discussion in 'Boat Design' started by Squidly-Diddly, Sep 15, 2008.

  1. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    All the sites that come up make references but don't give any examples.

    I can't even tell if the material would go on the inside or outside of the mold, or bladder, or if it is a multiple step process like 'lost wax'.

    What shapes and objects is bladder molding used for?
  2. Chris Ostlind

    Chris Ostlind Previous Member

    Bladders are most frequently used for hollow body composite structures.

    Picture a two piece mold (tool) split down the center longitudinally. After the composite material (either wet lay-up or prepreg) is placed in the tool halves, the bladder is laid in place. The tool is closed and secured, the bladder is inflated, pushing the composite material against the interior walls of the tool and the tool is subjected to its appropriate cure cycle (heat or simple room temp cure). The bladder is deflated, the tool split open and the part is removed for trimming, etc to final form.

    Basically, the bladder serves as an interior tool element, guaranteeing that the part will be properly pressed against the tool walls to give an accurate final form.

    The process is used for anything where a final part, in one piece, is desired. Otherwise, the business of building two tools, each with the paired halves of the part can be employed with a post cure assembly process to reach the final form.

    It totally depends on the type of part, the engineered specs for the part and the economy of build and systems available in the fab shop.

    Yeah, I know, that sounds like a run around, but it is how these things get figured.

    If you picture the forks on a way trick road bike made from a carbon structure, you may see that the parting lines allow for the tool to be built so that a bladder process can be the most economical to produce. The tooling is fairly complex for this type of application, but it yields a build process that more than makes up for the up front complexity in its ability to produce clean parts quickly and economically.... if you are doing fairly big numbers of parts.

  3. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    Chris, sounds similar to 'vacuum bagging' on the other side

    of the lay up.

    How much pressure is used in a bladder(varies of course)?

    Would it be far less than a vacuum bag?

    Is a bladder just used to hold things in position during cure, while a vacuum sucks out excess resin?

    What are the bladders made of? I've seen quite thin plastic used on vacuums.
  4. Chris Ostlind

    Chris Ostlind Previous Member

    Depends, yes, on the structure of the tool and the complexity of the surface

    Very nearly the same... depending

    Yes, that's why it is typically used with pre-preg carbon so that the resin content is predetermined and accurate

    Typically rubber, much like an inner tube for a bike with high temp properties

  5. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    What Chris said plus:

    The max pressure obtainable in vacuum molding is one atmosphere or 14.7 psi at sea level. This is less at higher altitude which may be a limiting factor in some areas. Max pressure in a bladder system is limited by the strength of the mold and other equipment but one atmosphere is quite a lot.

    One principle difference in the two methods is that in a vacuum system, the mold has to withstand almost zero stress since the atmospheric pressure works against both sides of the mold and cancels out as if no vacuum was being drawn. In the bladder system any pressure applied to the bladder must be resisted by the mold, so it must be structurally very strong. At a pressure of +1 atmosphere (psig), one sq ft of the mold must be able to handle, without deformation, one ton of uniform pressure. That is why bladder molding is mostly a manufacturing operation and not done at home.
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