Molding polyurethane for potting electronics?

Discussion in 'General Electronics Chat' started by gimpo, Apr 6, 2016.

  1. gimpo

    Thread Starter Member

    Jan 27, 2016
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    After long reading about potting materials (epoxy, polyurethane and silicone) I've decided for polyurethane as a trade-off between costs and shrinkage problems. Nevertheless, the price for professional stuff are quite high on Digikey, Mouser or Farnell.

    Anybody here has some experience about using one of that simple compounds used for casting/molding? :)
    This products costs around 40-50% less than professional ones and I can easily found them here in Europe.

    I need to pot 30 or more 5x5x3cm boxes for outdoor use. The box contain one arduino, one TFT display, one led and a couple of resistor (max total current absorbed is around 110 mA in the worst case).

    P1030897_1.JPG


    p.s: for example I could buy Politek products in England at good price and low delivery costs.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Probably work just fine.. probably the same stuff..
    The markup on "industrial" products can be quite high..

    The "industrial" might be modified for low outgassing, thermal conductivity, etc... too

    You might want to look into its curing methods to ensure its not like most caulks that release "acid" when curing that will cause corrosion on electronics
     
  3. #12

    Expert

    Nov 30, 2010
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    First, name your primary purpose. Do you want to protect the parts from external pollution? Make the assembly un-repairable? Conceal the contents? Avoid vibration induced lead breakage?
     
  4. gimpo

    Thread Starter Member

    Jan 27, 2016
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    Good point. I have to investigate on it, even if technical sheets for such kind of products usually does not go into fine details. Any suggestion is welcome.
     
  5. gimpo

    Thread Starter Member

    Jan 27, 2016
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    Firstly protect it from water and humidity; secondly provide vibration (not hard shocks) reliability.
     
    #12 likes this.
  6. #12

    Expert

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    My first impression is to spray it with Krylon Clear or the equivalent in Rustoleum Brand (suggested by tcmtech) then flood it with the non-acid version of silicone caulk.
     
  7. gimpo

    Thread Starter Member

    Jan 27, 2016
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    I have alcohol-based RTV silicone and works great but it has too high viscosity to be used in my case (I cannot reach the bottom of the box when the board is placed inside... :( ). I need a low-viscosity product.
    Furthermore, RTV silicone can cure good only when the layer is no more than 7-9 mm deep. Yes, I could use a professional silicone for electronics but, again, costs rise up and the wife gets angry.
     
  8. #12

    Expert

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    You don't have to fill the box to reduce movement due to g forces. You only have to coat the parts.;)
     
  9. gimpo

    Thread Starter Member

    Jan 27, 2016
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    Polytek EasyFlo60:

    Selection_041.jpg

    Thanks again mcgyvr !!!!

    p.s: I should avoid contact with water too? Polyurethane is so sensible to water or it is just the specific formulation of this company?
     
  10. gimpo

    Thread Starter Member

    Jan 27, 2016
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    and what about water and humidity?
     
  11. #12

    Expert

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    Which water do you think can get through a coating which covers all the parts?
     
  12. gimpo

    Thread Starter Member

    Jan 27, 2016
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    I cannot coat the TFT display. If moisture gets in then I will have vision problems across the plexiglass window. :(

    (The TFT is screwed - and gently pressed -on a plexiglass plate, the plate is then screwed to the bottom of the box. Vision is allowed by a cut in the bottom of the box)
     
    Last edited: Apr 6, 2016
  13. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    You might also consider a two-part polysulfide rubber polymer. Much like silicone and others. Very water proof. One use is for acrylic windows in aircraft, but also sold at Home Depot and cheaper. They can be quite pourable and self leveling. They are quite safe with acrylics and do not cause crazing -- hence use in aircraft. Here is just one of many from Google: http://www.wrmeadows.com/deckoseal-polysulfide-joint-sealant/

    The versions I have used were both pourable or slightly thickened and black. The pourable version made a great casting mold for fiberglass parts. One advantage is they do not outgas during curing.

    Finally, if cost is a factor, you can always add an aggregate, phenolic spheres, glass beads (small), polystyrene beads, and sand are some options. Including a silica based filler will frustrate those who want to remove it.

    John
     
  14. gimpo

    Thread Starter Member

    Jan 27, 2016
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    Hi John, thanks for the idea. Polysulfide rubber polymer can be aggressive for copper metal? There is no clear indication about that.
    More than this, I see that max recommended width x depth of the joint is 2 x 0.5 inch. That is too small for my box (around 2 x 2 x 1.2 inch).
     
  15. tracecom

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    Apr 16, 2010
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    I have recently been using Liquid Tape, which is not a potting compound, but is sold for electrical insulating purposes. It is painted on and dries to a rubbery consistency. It looks to form a waterproof coating. It is probably not suitable for potting, but might suffice for waterproofing, and the remainder of the void could be filled with plaster of paris.

    If you don't mind, please post additional photos and describe what the devices are for.
     
  16. #12

    Expert

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    This sounds like you need to seal the box. "Seal" doesn't mean, "fill".
    I kept humidity out of some lasers (which were inside boxes) without any potting at all. The boxes were sealed by o-rings. I used "dry nitrogen" as a filler because it did not contain humidity that would condense on the lenses at low temperatures.
     
  17. jpanhalt

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    Jan 18, 2008
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    I was talking about casting, not caulking. What you quote is very common for caulking. This is similar to the material I used: http://www.smooth-on.com/Polysulfid.../index.html?osCsid=qgg22shetrca6v1eqkv2gb30l0 The military and FAA (USA) accept it for use in contact with aluminum.

    Anyway, it was just a suggestion for an alternative. John

    Edit: If it is just for protection, have you considered a simple wax?
     
  18. gimpo

    Thread Starter Member

    Jan 27, 2016
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    As said above, I cannot reach every zone of the box when the PCB + TFTdisplay + plexiglass-window are screwed to the bottom of the box. So using a brush is not possible for me.
    I need some "fluid" that can freely flow inside the box and fill all the gaps.
     
  19. gimpo

    Thread Starter Member

    Jan 27, 2016
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    I'm really really interested about Polysulfide Rubber. Expecially for what is reported in the link you posted:

    Unlike polyurethanes, FMC® 200 is not sensitive to moisture and can be poured directly over wet plaster or water/sulfur based clays.

    So protections against water and moisture seems outstanding when compared to polyurethane. On the other hand, I see on wikipedia that

    Polysulfides, as sulfides, can induce stress corrosion cracking in carbon steel and stainless steel.

    But I cannot get no clear informations about copper... mmmmh....
    Anyway it seems a stuff with high mixed-viscosity, 6000 cps versus 60 cps of the polyurethane (!). So I think it will no flows easily in my little box... :(:(:(
     
  20. gimpo

    Thread Starter Member

    Jan 27, 2016
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    The device should be mounted on a motorcycle. Parking the vehicle under the sun (maybe in August) after a long run (hot engine)... will make the wax screaming for the pain.
     
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