Alternatives to Potting compound

Discussion in 'The Projects Forum' started by RiG615, Dec 13, 2008.

  1. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    my question sort of morphed into a new thread
    What are alternatives to using Potting compound?

    I'm building a cheap project (under $10),- the electrical part will be stored in a piece of pvc pipe which will have to absorb impact from being thrown when the project is used, say consistently being thrown across your lawn.
    The electrical part of the project consists of a 556 timer and the resistors, capacitors that go along with it.

    I need some way to fasten the electrical components down and protect them from breaking and against wear and tear on joints...
    Potting compound seems expensive to use...

    I want to know the best way to achieve this at the minimal cost or just using household items, i.e. glue, tape, foam

    Suggestions?
     
  2. leftyretro

    Active Member

    Nov 25, 2008
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    Well I haven't tried it for potting purposes but clear silicon caulking sealant (GE brand is good) that is sold in tubes is pretty inexpensive and I would think it would make a good candidate for a inexpensive potting solution with good electrical properties.

    Lefty
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Pack it in foam, wrap with rubber bands, and put it in the cylinder.

    John
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Foam (like polystyrene aka "peanut" packing material or coffee cups) wouldn't be good, as static electricity would be a big problem.

    Pink bubble wrap has been treated for antistatic properties.

    RTV silicone would be fairly good. You may not have to completely fill in the tube; just more or less put a layer on the electronic components so that they'll have pretty rigid support, and then use a couple lines of the RTV to "glue" the board to the inside of the tube. I don't know what you're using for batteries, but "button" cells would likely take abuse a lot better than AA or larger batteries.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Spray the assembled and tested components with something like clear Krylon, and then use some expanding insulation foam. It you cap the ends of the tube, the foam will set up with smaller (and more resistant to collapse) bubbles. You will need a pinhole or two to allow the evolved gas to escape, or you may have a low-order pipe bomb.
     
  6. SgtWookie

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    Jul 17, 2007
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    Following along on Beenthere's recommendation,
    Ace Hardware and other stores carry "Great Stuff" insulating foam sealant. If you want to try using this stuff, I suggest the "Window and door" variety:
    http://greatstuff.dow.com/greatstuff/diy/products/wd.htm
    Don't use the "Big Gap" variety unless you've practiced with it on something you don't care about. The "Big Gap" type expands like crazy, and is high pressure. I'm afraid it may rip the components right off your board as it expands. The "Window and door" type is relatively low pressure and low expansion.
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Be aware that the standard RTV contains acetic acid (strong smell) that will corrode electronic parts and solder connections. There are versions that are safe for electronic circuits.

    Ken
     
  8. SgtWookie

    Expert

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    Yep, I mentioned that in our OP's other thread. There was a particular GE red silicone RTV that had low acetic acid emissions, but I forget the part number for it now. It's been awhile.
     
  9. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    Just curious, would the insulation foam be a problem with static electricity or electrical noise? if I didn't use the coating?

    thanks good idea
     
  10. SgtWookie

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    Yes, the insulation foam would be a problem if you didn't use the clearcoat first.
     
  11. jpanhalt

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    The problem with Great Stuff and similar hard setting foams is that your electronics will be denser than the foam. Thus, as the package is repeatedly subjected to shock loads, the part may well break loose and rattle inside a cavity it forms in the foam.

    The small-cell, pliable, pink, polypropylene/PVC-like foam (like DigiKey and many electronic places use) use will remain flexible and shock absorbing. Moreover, it will not stick to your electronic parts. If you are stuck with the non-static dissipating foam, then a coating on the board may help, but frankly, I think the risk from static after the whole thing is assembled and in inside the PVC pipe is overplayed.

    John
     
  12. SgtWookie

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    John,
    You'd be surprised at how strong that foam actually is once it's set up. It's much like a light-to-medium density wood in consistency.

    At a former employer, we used a product called Eccofoam to encapsulate LC and xtal filters, which has a somewhat similar consistency and appearance when cured - except it's specifically designed for electronics encapsulation, along with being quite expensive.

    As a test, we took an LC filter, plotted it's response using a network analyser, then threw it as hard as we could repeatedly against a concrete wall, and then re-plotted it's response. There was no detectable difference in the filter's response.

    I think that the low-expansion foam will actually perform quite well.
     
  13. DickCappels

    Moderator

    Aug 21, 2008
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    Alternative:
    Fill the pipe with sand and then top off the sand with paraffin or wax. Or, just fill the whole pipe with wax.

    Can't get much cheaper than sand :)
     
  14. SgtWookie

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    Sand would likely work pretty well, if the added weight wouldn't be a problem. It sure would make it easier to get it apart later if needed. You can get a big bag of white sand from Big Blue or Big Orange hardware stores for a couple bucks.

    One thing about sand is that it tends to harbor moisture. This isn't much of a problem in low humidity environments, but here in Florida it is. You might want to make sure it's pretty dry by baking it at low heat in a shallow pan in the oven for a few hours before using it.

    The sand will still shift around unless it's packed in tightly. I suppose you could install one end cap on the pipe, fill it most of the way with sand, and then use a small shot of low-expansion foam before slapping the other end on.

    Paraffin wax has a pretty low melting point, somewhere in the mid 120°F. If you left a tube filled with paraffin wax sitting in the sun here in Florida, it would become liquid.
     
  15. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    Sand is an interesting idea, have to think about that
    do you think it would be- better, same or worse 0than the foam idea??

    could the sand short something or scratch something or hurt anything? I would never of thought of sand because of its coarseness- great thinking out of the box

    wax would probably not be all to great here in SoCal
     
  16. SgtWookie

    Expert

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    Certainly worth a try. It would sure be easier to dig the electronics out of sand than it would be to try digging it out of foam. :confused:

    Well, sand is silicon. Basically, it's pretty neutral, if it's more or less dry. Sure, it can be abrasive, if it's not held in check.

    You haven't said what your application is, but sand might be a good application, if it's constrained.

    I can tell you from experience that it would not be LOL!
     
  17. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    I just thought of something....
    Is clear Krylon, or other types of coating flammable?
    i know it says it on the can, but once it drys, could it catch fire or explode, if something got too hot or a cap exploded?


    expanding insulation foam
    is this stuff flammable when cured?
    if something got hot or a cap exploded what would it do?

    I dont want to be making a potential bomb...
     
  18. SgtWookie

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    Both Krylon and "Great Stuff" are extremely flammable until they are fully cured.

    But during curing, the volatile chemicals evaporate. Once fully cured, you could still get them to burn, but they're no more flammable than paper or wood. I wouldn't suggest trying to burn them indoors, as polyurethane foam when burning will give off a very black, sooty ash that will get all over the house.

    If enclosed in a sealed tube, there would be a very limited amount of oxygen for combustion to take place. If something caused heating due to a catastrophic failure, you'd probably have some melting of the foam, but not likely a fire.
     
  19. jpanhalt

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    I am sure you mean outdoors. One other aspect of burning polyurethanes is that they tend to produce more toxic gases than acrylics produce. But, that is a minor issue and negligible concern. I am just anxious to see the OP make this device and tell us how it works.

    John
     
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