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Epoxy meltdown problem

Discussion in 'Misc Electronics' started by Andre, Jul 30, 2003.

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  1. Andre

    Andre Guest

    Hi group .

    Anyone found a way to solve the epoxy "meltdown if you mix it in bulk"
    syndrome

    I found that thin layers poured at about 20 minute intervals seem to
    work , but is there a better method ?

    Thanks

    -A

    "Windows isn't a virus . Viruses DO something !"

    (unknown)
     
  2. Boris Mohar

    Boris Mohar Guest

    The rate of exothermal reaction is partially governed by the rate of the
    heat loss. If you mix in a bucket the surface area is minimal so the heat
    buildup at the "core" is high. Pour in a shallow container after mixing.
    If your potting volume is high maybe you have to keep it in the fridge
    while curing.

    --

    Regards,

    Boris Mohar
    Viatrack Printed Circuit Designs http://www3.sympatico.ca/borism/
    Aurora, Ontario
     
  3. Ian Stirling

    Ian Stirling Guest

    Find a slower curing epoxy.

    Ass filler to improve thermal conductivity, thermal mass and reduce total
    heat generation due to less epoxy. (maybe cheaper too)

    See if you can mix it in different ratios, and get a slower cure while
    not doing bad things.

    Cure in fridge.
     
  4. The other (snipped) suggestions make sense, especially adding a filler.

    I would not deviate from the specified mixing ratio until your epoxy is
    proven to behave well if you do this. Deviating from the specified mixing
    ratio can result in an incomplete cure.

    If the epoxy cures slowly at first and then suddenly with a nasty
    temperature rise at room temperature, it may take ages to cure if chilled.

    Another idea: If the layer is barely thick enough to get a nasty
    temperature rise, you may be able to head off the temperature rise by
    blowing a fan at it. But this will not work for potting something in a
    bucket.

    - Don Klipstein ()
     
  5. Andre

    Andre Guest

    Yes, I found that . :(

    Most of the epoxy worked except for two containers that partially
    melted .

    So much for using wax moulds . Works OK for small amounts though :)

    Idea - make an epoxy reflector by attaching a bowl to a motor, pouring
    in epoxy mixed with metal powder, and spinning to get the required
    curvature .
     
  6. CWatters

    CWatters Guest

    That's the normal trick but Epoxy doesn't always stick to itself very well
    so doing it in layers might not always work.

    If you are "potting" something in a container you might be able to stand the
    container in a bucket of water to extract the heat?

    Try cooling the epoxy in a fridge before you mix it? That will make mixing
    harder but should buy some time.
     
  7. Zak

    Zak Guest

    You need a slower curing hardener. Varing mixing rations is risky - the
    ingredients are quite reactive and you do not want too many of the
    reactive bits to remain.

    Slower curing hardeners exist especially because of this problem.

    Also, above some thickness it will be very difficult to achieve
    sufficient cooling.

    "epoxy retarder" shows some google hits in the direction I'd search.

    Note that for polyester resin this is more easily available in the form
    of 'clear cast' resins.


    Thomas
     
  8. Andre

    Andre Guest

    Well I think I fixed it :)

    Here is the preparation procedure .
    1) Obtain one or more (depending on the size of your casting base) CPU
    heatsinks and fans .
    2) Obtain some heatsink grease
    3) Affix fans to some hard surface (wooden grid maybe) so all the
    heatsink plates are aligned .
    4) Coat heatsink plates with heatsink grease
    5) Sit the mould on the heatsink plates . Be sure that the two are
    pressed together properly .
    6) Attach fans to 12V (or 5V) power supply.

    Put the object to be casted in the mould, and then pour in a maximum
    of 1.5cm of epoxy . (I haven't tested it with anything larger than
    this yet) .
    Put the completed assembly outside in fairly bright sunlight , so the
    epoxy gets a chance to set .

    The advantages of this method are that it ensures a reasonably clear
    cast as all the bubbles rise to the top, with no "bubbling" caused by
    overheated epoxy (plus you can re-use the mould) .

    Have fun :)

    -A
     
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