Potting in thermally insulating foam

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.

I'm assuming that "great stuff" is the aerosol expanding foam stuff;

You could lay a sheet of glad wrap / saran wrap / cling wrap over the assembly so the expanding foam doesn't stick to the parts.

In my experience that stuff sticks VERY well to most things.

Would a scrap of rock wool ceiling insulation work?

Appears to be polyurethane, not polystyrene, based.

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?

Still air is a prety good insulator. Try putting a small plastic box around the parts.

We make a couple of OCXOxs and temperature-controlled analog circuits, and we put them under small deep-drawn aluminum covers. All the foam type things that we tried made the thermal insulation worse.

Maybe some fiberglass insulation inside a box would help. The only function of any foam or insulation is to break up convection. The material itself will be a better thermal conductor than air.

Potting is always messy.

ke

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.

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.

Hi George, yes, I guess that is what Sphero suggested too.

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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! :)

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,

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).

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.

Yes me too actually... Martins suggestion of polystyrene and putting all the little bits inside is starting to sound better.

Take some dishwashing liquid and rub it on your hands & foreams before you start. Let it dry. It plugs the pores to keep the glass out, and simply washes off when you're done.

Now you tell me. A week ago I went into the attic to add a backing plate to an anchor that had pulled out of the ceiling on a fluorescent in the laundry. Had to fish around in 8" batts to find it :-( After much hand lotion and a prescription from the Doc, the itching has finally subsided. ...Jim Thompson

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.

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.

Is the circuit static sensitive?

I've mentioned it before, and was laughed at. I've used it since the '70s. and never had a problem.

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I've seen the low frequency phase noise of a cheap XO go down by 5:1 when a small cover was added to block air currents. We used a plastic potting shell. Opamp circuits can behave similarly.

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Try some cotton balls; my wife keeps them around for some mysterious female cosmetic use.

My experience, measuring the power drawn by a small OCXO under a cover, is that, in a small volume, insulation makes things no better, and usually worse. There must be some size scales where convection matters.

Here's a temperature-controlled analog thing:

and an OCXO with its cover, lower right

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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.