to vacuum or not to vacuum... HEI epoxy potting?

A 2-stage rotary pump like the one I use can very easily achieve 0.01 Torr (though perhaps not with the resin present if it is at all volatile), which is 1/76000 Atm, so 1/32 is comparatively a poor vacuum. If there is a certain volume of bubble that is large enough to be buoyant enough to rise to the surface and pop in a reasonable time (i.e. before the goo cures), then one would want to make as many bubbles as possible expand sufficiently under vacuum that they exceed this volume and rise and pop. Roughly, halving the pressure doubles the volume of all of the existing bubbles, (though I suspect that there may be some limitation due to surface tension). If at one atmosphere, a bubble is 32 times smaller than the volume at which it would rise and pop in a reasonable time, then lowering the pressure to 1/32 Atm ought to do the trick. If the bubble was smaller to begin with, then it is my expectation that this level of vacuum would not be good enough to remove that bubble.

The size of the remaining bubbles will depend on what the "multiple factors" are, i.e. how good the vacuum is, at the location of the bubble. If the spaces between for example turns of a coil, are already at 1e-3 Torr when you pour encapsulant over them, any bubbles that were not already suspended in the encapsulant should be very small indeed after the chamber is vented to atmospheric pressure. I suspect that the result might not be so good if the pouring is done at atmospheric pressure and you are then trying to get existing bubbles out from between the turns of a coil.

Commercial equipment seems to do the dispensing under vacuum:

Putting the two components of the encapsulant under vacuum, stirring under vacuum then mixing under vacuum, then pouring the mixed encapsulant under vacuum onto the dry assembly needing potting.

If the vacuum is poor, it will not get the smaller bubbles out. If the vacuum is good, but applied very gradually then that approach might work, but I find a deep, wide beaker (but filled to only about 1/2 inch depth) gets the job done quickly without requiring so much time and attention to avoiding spills.

Agreed. IIRC (from 50 years ago) we used RTV 11. White, rubbery.

n when adding it to the mold.

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. So 1 foot of mold will only cause 1/32 of an atm

There is your first mistake, thinking there is any real advantage to the be tter vacuum. The issue is not how low the vacuum can go, it is the differe nce in pressure to 1 atm. So what you call a poor vacuum will work pretty much as well as your good vacuum in this case.

Your second mistake is in thinking your pump will ever achieve 0.01 Torr wi th the epoxy resin. The resin itself will foam at that pressure and I'm no t sure you can find a large enough vessel to hold it. Consider how large a soap solution will become if it is turned into bubbles 100%.

Yes, well thought out. So what is the bubble size of this threshold?

r not you pre-vacuate the epoxy, it has no impact on the result.

The context is the OP's setup where he is encapsulating an encapsulated dev ice.

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If the vacuum is "good" it won't get the smallest bubbles out. You have ra ised many theoretical questions and answered none. This is a practical app lication where the theory is too complex to be of any use other than a gene ral guide. Then you have missed the lesson I learned by trying this myself many years ago.

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  Rick C. 

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Perhaps. The commercial units seem to mention 5mbar, and perhaps that is because there is not much point in going below that, at least for devices operating at ordinary mains voltages.

I specifically mentioned that it might not. BTW the resin that I have used is a 2-component silicone rather than an epoxy.

I agree there is not much point in trying to achieve a pressure so low that a major component of the encapsulant boils at a significant rate. Such an encapsulant might not be well suited for vacuum encapsulation anyway.

For the silicone that I have used, I would guess about 3mm-6mm, or say

1/8 to 1/4 inch diameter. That would depend on the viscosity and how long you can wait (limited by the cure rate of the encapsulant, unless you get rid of the bubbles before mixing and then mix under vacuum, which seems to be the best approach for really critical applications).

I'd like to find a really good book or article on the subject.

Hi. I've reviewed your circuit and I would suggest not even bothering with epoxy. With something taking that many ounces, it's unlikely you'd have the proper equipment to vacuum it sufficiently. What you did with both mineral oil and wax was probably sufficient. You found out that mineral oil has to "breathe" so you compensated for this after your oil changeout. I believe your paraffin version will be ok too, but don't let it liquify in the car during the heat of summer (of course I'm sure you already knew this). If you want to try other waxes, just pick one with a higher melting point like has been recommended here, but I don't think you have to go anything else.

With the two HEI's already being potted, you are just concerned standard arcing issues and nothing between the windings. Oil/ was will do nicely. Now if you *didn't* have already potted windings, then you might want to think more about epoxy but even mineral oil often suffices for that too as long as you let the bubbles work out of it. You can even heat the oil slightly to help hasten bubble and air pocket release.