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Potting in thermally insulating foam

Discussion in 'Electronic Design' started by John Devereux, Mar 14, 2013.

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  1. Hi,

    Looking to prevent air currents and provide thermal insulation around
    some awkwardly shaped parts.

    Anyone know any products that would let me pot things in something like
    expanded polystyrene foam? Or maybe some small insulating beads since I
    want to avoid stress on the components too.

    One-off, hobbiest thing.

    Thanks,
     
  2. mike

    mike Guest

    For thermal, use "Great Stuff".
    Get it at home depot.
    Comes in several varieties. You want the soft stuff.
    Have no idea the electrical characteristics of the stuff.
     
  3. Appears to be polyurethane, not polystyrene, based.

    http://greatstuff.dow.com/products/big-gap-filler/
    https://www.lakeland.edu/AboutUs/MSDS/PDFs/458/Great Stuff Foam Spray (Dow).pdf

    EPS is not trivial to mold- it's done by expanding PS beads with steam
    in a mold. You can do it in a lab, but I don't think it's suitable for
    encapsulating circuitry.

    Can't you just cut chunks of sheet insulation up (with a hot wire, or
    a long kitchen knife works really well) and glue them together with
    Pliobond or something like that?
     
  4. I like melamine foam for heater insulation. (mostly because it goes
    to ~170C) It cuts easily... maybe a bit too easily.

    The polyurethane stuff expands a lot.. and may not be a 'low stress'
    solution.

    Maybe some glass wool?

    George H.
     
  5. Yes perhaps, that is what I have been doing. Was hoping for something
    less fiddly. It is not possible to get a really good fit like that,
    though not sure how important it is.

    It *could* be important, it has a self-heated component (LTZ1000) and
    want less than 20mK fluctuations of solder pad temperature due to air
    moving about.

    Yes might be better.
     
  6. Hi George, yes, I guess that is what Sphero suggested too.
     
  7. It's more thermal fluctuations due to air currents I am worried about,
    than absolute performance in terms of insulation. Although more is
    better there too.
    Yes glass fibre / glass wool / rock wool sounds good. I assume these are
    all the same thing (I have only come across glass fibre over here I
    think).

    I suppose I can pull a tuft out of my ceiling just above the bench, come
    to think of it! :)
     
  8. Hi Mike, thanks for idea. Thermal resistance seems to be similar to the
    expanded polystyrene I was looking for.

    Think I am going to try the glass wool first but might get some of that
    if does not work.

    Thanks,
     
  9. Martin Brown

    Martin Brown Guest

    For a one off how about just roughly carving something out of bulk
    polystyrene packaging and loose fill the interior with the bits that
    break off or come with your next consignment of fragile parts?

    Rigid polyurethane foam will tend to stress the components. I have seen
    damaged car panels by putting too much PU foam into a confined space! It
    is still the method of choice for fridge freezers.

    The semi rigid packaging grade isn't too bad for stressing things if you
    can get a hold of it but wrap the part in cling film or similar first. I
    have a feeling it isn't a particularly good closed cell foam.

    Urea formaldehyde foam for cavity wall insulation would be another
    option if you can find someone who will make you a bit.

    Flexible PU foams use more noxious reagents (serious lung sensitisers).
     
  10. That's good.. PS is not only low temperature stuff, but it's not
    exactly petroleum solvent resistant. I clearly remember the one time I
    poured a bit of gasoline into a styrofoam cup. When the revolution
    comes, it will be useful for DIY napalm.

    There's also Polyimide insulation if you're feeling rich, or even
    aerogel (which outperforms all the others by a large factor).
    The chemicals may not be friendly, and it may gas them off for a
    while.
    Makes me itchy just thinking about it.
     
  11. Yes me too actually... Martins suggestion of polystyrene and putting all
    the little bits inside is starting to sound better.
     
  12. I think rock wool has coarser fibers or something.. anyway it's not as
    itchy. I did some controls for the Roxul plant which makes the stuff
    (just down the road), years ago. It costs a bit more than fiberglass.

    http://www.constructioncanada.net/n...americas-largest-mineral-wool-plant-in-milton
    We have something that will save all that labor- raccoons. The b*ggers
    will rip all your insulation to shreds if they can find a way in.
    Smart too, and quite dexterous.
     
  13. Well I don't think they are all the same. I'm not sure what rock wool
    is, then there's the fibre glass that's used for insulation. (As
    someone said that makes my skin itch.) But we also used a glass wool
    in the lab. It was white... and never made me itch. I went looking
    for it in McMaster-C, but didn't find it. Listed in Cole-Parmer...
    kinda spendy. (~$100/lb.)

    George H.
     
  14. Not especially, but in theory I suppose. I know there would be some
    risk, but it is just a hobby one-off thing. I'll try not to rub the
    polystyrene sheet over my hair before fitting it!

    Thanks,
     
  15. At this point I thought you knew some osbscure detail about how raccoon
    fur is the perfect insulator.
    Should be safe from the raccoon menace around here I think.
     
  16. Nice, I guess it might be a size thing that makes fibreglass itchy.

    George H.
     
  17. Yes, of course, cotton wool. Don't know why I am trying to make it
    complicated.
     
  18. You should consider milling out slots to thermally isolate the
    LTZ1000. One idea I've seen uses kind of a swastika design.

    They're getting harder to find- last batch of highest accuracy ones
    had to come direct from LTC with a long lead time. 8-( Not cheap
    either.
     
  19. Yes, thanks, I know about that, do a image search and you see lots of
    versions.

    <www.google.com/search?q=ltz1000+pcb&hl=en&safe=off&source=lnms&tbm=isch>

    I am trying something else, put the device on a tiny board on the end of
    an FFC cable so it is embedded in polystyrene and isolated from thermal
    gradients.
    That's where I get them from anyway, not cheap as you say. Lead time not
    particularly bad though.

    By highest accuracy I assume you mean the "A" versions with an insulated
    die? They may be easier to control thermally but they ought to have
    worse ultimate accuracy because you end up having to run the chip 10 or
    20K warmer, IIRC. And the long term stability is worse with higher
    temperature.
     
  20. Guest

    Two thoughts:
    1) As John said, *anything* solid conducts better than air. The goal
    is to break up the convection currents with the minimum possible
    mass. Cotton, fiberglass, and foam are all thermal bridges. So,
    since it's a one-off, how about down?

    2) OTOH, if your goal is isothermal pads, some copper heatspreading
    may be in order too.
     
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