question is how much vacuum is required ??

Discussion in 'Fiberglass and Composite Boat Building' started by tunnels, Jul 1, 2010.

  1. tunnels

    tunnels Previous Member

    Sucking a core into wet glass how much vacuum is needed ?
    Sucking a core into core bond how much vacuum is required ?
    How much is enough??
    Can the vacuum ever be to high ?
    and if so what happens if it is ??
    Should the core be wet out first ? and if so how wet should it be?
    Should the resin have kickoff (Gelled) before the core is put down into the wet glass or the core bond ?

    :confused:
    FOOT NOTE TO THIS IS IT HAS NOTHING TO DO WITH RESIN INFUSION !!
     
    1 person likes this.
  2. hoytedow
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    hoytedow Wood Butcher

    It takes very little. As long as you maintain less ap inside than out you have succeeded. The atmosphere will do all the rest of the work.
     
  3. tunnels

    tunnels Previous Member

    Very little as in ?? can you put a figure to that ? like % of ultimate vacuum on a scale from 1% to 100%:confused:
     
  4. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    As long as the pressure inside the bag is less than 1 atmosphere, the pressure outside will keep the collapse ongoing unless the bag is something rigid like titanium.

    http://en.wikipedia.org/wiki/Bar_(unit)
     
  5. tunnels

    tunnels Previous Member

    Dosent any one use a vacuum gauge ?? why not ? its all just guess work ?
    Come on !! :confused:
     
  6. hoytedow
    Joined: Sep 2009
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    hoytedow Wood Butcher

    When there is nothing left in the bag but solid and liquid with no gas of any kind you have succeeded and no pressure gauge is necessary. Solids don't compress. Liquids don't compress. Only gas compresses and properly done the gas is sucked from the bag.
     
  7. rwatson
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    rwatson Senior Member

    Tunnels !!!! Of all people who is an expert in foam coring work *you* have to ask such a basic question. ????

    By now you will have guessed that hoytodow doesnt have the foggiest idea,but its not that hard to find out luckily.

    The answer is - it depends, and it is improratnt that you have the right amount - in fact you may have to control the vacuum separately over different parts of the mold. The vaccum rate controls the speed of the infusion, and some bits may want a slower rate to avoid dry spots. In some parts of the mold, you may need to increase the vacuum to "force" difficult spots.


    a ten second Google search brings up sites like -

    http://www.performancecomposites.com/tech_article2.html

    which quotes " the air is evacuated from the part using a vacuum pump (approximately 27 in Hg)." (Hg is mercury, 27 is mm)
    One standard atmosphere equals 14.7 psi (29.92 in.-Hg)

    Also, if you do a bit of basic searching in this site, you will find you can get enough vacuum with an old dairy pump, for example - its not a huge bit of machinery to find thank goodness.

    I cant find my copy at short notice, but duck down to your local FG shop and get a copy of Goegeon Bros "Resin Infusion" booklet. Its well laid out, and easy to read, with lots of usefull hints.
     
  8. lewisboats
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    lewisboats Obsessed Member

    small correction... in(inches) of mercury is in inches...not mm
     
  9. rwatson
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    rwatson Senior Member

    whoops - thanks for that :)
     
  10. apex1

    apex1 Guest

    Next correction: our "expert" asks for bagging, not infusion!

    Richard
     
  11. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Yes, a good point - the word INTO should perhaps be better expressed as ONTO do you think ? Made me think of infusion straight away.

    Thats going to require even less pressure - some people have used Vacuum cleaners for small parts, but the motors burn out when the airflow stops.
     
  12. tunnels

    tunnels Previous Member

    Correct !!To suck down a core !:p Infusion is something differant !

    So no one uses a gauge , why not ??How do you know when you have enough suck ??How much is enough , thats the question im asking ?
     
  13. apex1

    apex1 Guest

  14. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    You are going to have to test Tunnels as its dependant on the viscosity of your resin.
    Do a test on a flat bench and then cut the job so you can measure the area and weigh it to determine your resin to cloth mix
    I buggered stuff with too much when using very thin epoxy but never have a problem with vinylesters which also set up in 20 minutes so that for sure helps not to pull the resin out of stuff.
    A guy I ask advice of in the states does a ramped up vac from about 5-6in to 20 over time with epoxy.
    He has custom made hardeners ( hence custom curing times)as he has both feet in the door of some chemical companies.
    My experience is less is more..
    more disasters with too much vacuum
    I only do repairs....
     

  15. tunnels

    tunnels Previous Member

    Believe it or not but you have almost answered a part of the question i have asked .:D
    There is no one simple answer to what i asked ! I am amazed out of all the proffesional people here that no one has a proffessional answer to the simple questions . :confused:
    Repair work you can learn so much from other peoples mistakes !! look and learn and try to understand why things break the way they do !!:p
     
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