From: firstname.lastname@example.org (Glen Walpert)
Subject: Re: Side effects of potting compound !
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Date: Tue, 05 Nov 2002 13:40:51 GMT
NNTP-Posting-Date: Tue, 05 Nov 2002 08:40:51 EST
Organization: Net Access (netaxs.com)
In article , email@example.com (Bob Wilson) wrote:
>In article ,
>>In article ,
>>a (Bob Wilson) wrote:
>>>Like I said in an earlier post, silicone (or other compliant) conformal
>>>coating is not compliant at all when it is fully encapsulated, and hence
>>>"nowhere to go" when subjected to stress internally.
>>And you said it again, but it is still wrong. Silicone rubbers have a bulk
>>modulus in the 100,000 to 200,000 PSI range at moderate pressures, so they
>>will compress 1% or 1/2% (linear all directions) respectively with a mere
>>PSI pressure. A few air bubbles will dramatically reduce the bulk modulus.
>>The manufacturer will provide the exact bulk modulus for your specific
>>compound (air bubble free) if you ask.
>1.2% or 1% is insignificant. Especially when it takes 1000psi to do it. It
>takes a LOT less than that to snap off a lead on an electrolytic or other
>device with a reasonable cross sectional area), and ESPECIALLY since the
>suggestion was to use a layer of silicone conformal coating (i.e. a thin
>layer, by definition). 1% of a very thin layer is 1% of nearly nothing.
>The presence of air bubbles most certainly will have a very beneficial
>effect on the bulk modulus, but they can defeat the entire purpose of
>potting since, as water diffuses into the potting compound, it can condense
>in the voids and cause the very sort of electrical problems that potting is
>intended to avoid. All polymers are permeable to varying degress to water
>vapour (with silicones generally being the worst in this regard).
I agree with you completely that the compressibility of silicone is
insignificant in this application if the epoxy shrinkage is anywhere near 1%
(in which case the OP should consider a different compound). I just could not
agree with the "not compliant at all" part, when I am in the habit of
calculating the resulting stress/strain from actual measured compliance in
The vendor may not always have a number for bulk modulus, but will almost
always have Youngs Modulus (often just called the modulus of elasticity) and
Poisson's Ratio (ratio of lateral strain to axial strain when loaded uniaxial
in a standart test machine).
Bulk Modulus = Youngs Modulus / 3(1 - 2(Poisson's Ratio)).
A material with a Poisson's Ratio of 0.5 would be completely incompressible,
but no such material exists. Silicone Rubbers have Poisson's Ratio in the .48
to .49 range. If you can only get Youngs Modulus, estimating bulk modulus
with Poisson's Ratio of .49 would get you in the ball-park.
I hope the OP was not using RTV to isolate his trimmer from stress, since the
acetic acid could be expected to cause long term corrosion. And I still think
that there is a good chance that even an extremely low shrinkage potting
compound, which would not cause any strain problems from shrinkage, could flow
into the trimmer and move the wiper by surface tension. Slitting one open
should be able to reveal this, and if it is the case, sealing is all that is