Potting in thermally insulating foam

[John Devereux]

Hi,

Looking to prevent air currents and provide thermal insulation around
some awkwardly shaped parts.

Anyone know any products that would let me pot things in something like
expanded polystyrene foam? Or maybe some small insulating beads since I
want to avoid stress on the components too.

One-off, hobbiest thing.

Thanks,

 

[mike]

Hi,

Looking to prevent air currents and provide thermal insulation around
some awkwardly shaped parts.

Anyone know any products that would let me pot things in something like
expanded polystyrene foam? Or maybe some small insulating beads since I
want to avoid stress on the components too.

One-off, hobbiest thing.

Thanks,

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.

 

[Spehro Pefhany]

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

Appears to be polyurethane, not polystyrene, based.

http://greatstuff.dow.com/products/big-gap-filler/
https://www.lakeland.edu/AboutUs/MSDS/PDFs/458/Great Stuff Foam Spray (Dow).pdf

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?

 

[George Herold]

Appears to be polyurethane, not polystyrene, based.

http://greatstuff.dow.com/products/....edu/AboutUs/MSDS/PDFs/458/Great Stuff Foam%2...

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?








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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.

 

[John Devereux]

Appears to be polyurethane, not polystyrene, based.

http://greatstuff.dow.com/products/big-gap-filler/
https://www.lakeland.edu/AboutUs/MSDS/PDFs/458/Great Stuff Foam Spray (Dow).pdf

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?

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.

 

[John Devereux]

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?

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

 

[John Devereux]

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.

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.

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.

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!

 

[John Devereux]

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.

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,

 

[Martin Brown]

Hi,

Looking to prevent air currents and provide thermal insulation around
some awkwardly shaped parts.

Anyone know any products that would let me pot things in something like
expanded polystyrene foam? Or maybe some small insulating beads since I
want to avoid stress on the components too.

One-off, hobbiest thing.

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).

 

[Spehro Pefhany]

I like melamine foam for heater insulation. (mostly because it goes
to ~170C) It cuts easily... maybe a bit too easily.

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 polyurethane stuff expands a lot.. and may not be a 'low stress'
solution.

The chemicals may not be friendly, and it may gas them off for a
while.

Maybe some glass wool?

George H.

Makes me itchy just thinking about it.

 

[John Devereux]

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.

 

[Spehro Pefhany]

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 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.

http://www.constructioncanada.net/n...americas-largest-mineral-wool-plant-in-milton

I suppose I can pull a tuft out of my ceiling just above the bench, come
to think of it!

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.

 

[George Herold]

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).

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.

 

[John Devereux]

Is the circuit static sensitive?

Not especially, but in theory I suppose. I know there would be some
risk, but it is just a hobby one-off thing. I'll try not to rub the
polystyrene sheet over my hair before fitting it!

Thanks,

 

[John Devereux]

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.

http://www.constructioncanada.net/n...americas-largest-mineral-wool-plant-in-milton


We have something that will save all that labor- raccoons.

At this point I thought you knew some osbscure detail about how raccoon
fur is the perfect insulator.

The b*ggers will rip all your insulation to shreds if they can find a
way in. Smart too, and quite dexterous.

Should be safe from the raccoon menace around here I think.

 

[George Herold]

   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.

Nice, I guess it might be a size thing that makes fibreglass itchy.

George H.

 

[John Devereux]

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.


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

Yes, of course, cotton wool. Don't know why I am trying to make it
complicated.

 

[Spehro Pefhany]

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.

You should consider milling out slots to thermally isolate the
LTZ1000. One idea I've seen uses kind of a swastika design.

They're getting harder to find- last batch of highest accuracy ones
had to come direct from LTC with a long lead time. 8-( Not cheap
either.

 

[John Devereux]

You should consider milling out slots to thermally isolate the
LTZ1000. One idea I've seen uses kind of a swastika design.

Yes, thanks, I know about that, do a image search and you see lots of
versions.

<www.google.com/search?q=ltz1000+pcb&hl=en&safe=off&source=lnms&tbm=isch>

I am trying something else, put the device on a tiny board on the end of
an FFC cable so it is embedded in polystyrene and isolated from thermal
gradients.

They're getting harder to find- last batch of highest accuracy ones
had to come direct from LTC with a long lead time. 8-( Not cheap
either.

That's where I get them from anyway, not cheap as you say. Lead time not
particularly bad though.

By highest accuracy I assume you mean the "A" versions with an insulated
die? They may be easier to control thermally but they ought to have
worse ultimate accuracy because you end up having to run the chip 10 or
20K warmer, IIRC. And the long term stability is worse with higher
temperature.

 

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Yes, of course, cotton wool. Don't know why I am trying to make it
complicated.

Two thoughts:
1) As John said, *anything* solid conducts better than air. The goal
is to break up the convection currents with the minimum possible
mass. Cotton, fiberglass, and foam are all thermal bridges. So,
since it's a one-off, how about down?

2) OTOH, if your goal is isothermal pads, some copper heatspreading
may be in order too.