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Using A Vacuum on Oil and Epoxy for the Purposes of Insulating.

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Patrick

Sun Jul 29 2018, 02:31AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Now that I have a working vacuum pump Im wondering how to use it on oil and epoxy.

ive been looking at "Raschig rings". but have no idea what special methods maybe needed, or the procedure for forcing fluid into windings. I presume the epoxy would be degassed as the part A and B separate, then mixed with minimal "bubble stirring".

Supposedly 0.1 to 5 torr is ideal with a jet/mist spray. Im not sure whats practicable or best for us hobbyists.

This is interesting :

flyrod

Sun Jul 29 2018, 10:01PM

Registered Member #61905 Joined: Sun Nov 12 2017, 03:27AM
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Posts: 23

You probably need a chamber of some sort also/next. I've done small epoxy stuff, and heating the mix does a lot to get the air out. The heat also helps cure the epoxy faster. Cold has the opposite effect, if you need more time to work. One thing to keep an eye on is differential expansion. If you have a coated and wound coil or transformer and you change its temperature, the windings could get quite loose or tight on the form. Also, if you can cycle pressure/vacuum in your chamber that seems to help get the epoxy pushed into all the crevices and collect the air bubbles. Wet winding is a mess, but in some cases it helps a lot, vs. trying to get epoxy pushed in after the fact.

I don't know much about HV oil, but I've read that you can use light duty operation to help get air out. i.e. run the device (capacitor, transformer, etc.) under light load and the magnetic/electrostatic vibration will help knock some air out.

2Spoons

Sun Jul 29 2018, 10:24PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

As I recall the best method for getting epoxy into a transformer is to pull a hard vacuum on the dry transformer, then submerge in epoxy, then let the vacuum go so that atmospheric pressure forces the epoxy into the transformer.
But you don't want too low a pressure or the epoxy will boil, and lose some of its more volatile components.
Obviously you also want a really runny, slow curing epoxy.

Patrick

Mon Jul 30 2018, 01:01AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Heres the epoxy I use :

]max_1618ab_properties.pdf[/file]
its pretty thin almost water thin.

2Spoons

Mon Jul 30 2018, 02:19AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

Might be worth doing a test run - impregnate a small coil, then cut it apart to see how far the resin has penetrated.

You may need to slow down the cure to get better penetration, which you could do by chilling everything in the fridge. Especially since blocks of resin are prone to thermal runaway - cure creates heat which accelerates cure which makes more heat. I've done it deliberately for fun - had smoke pouring out of a cup of polyester resin within 5 minutes of mixing. I've also watched my boss frantically stuffing our freezer with bubbling resin after trying to use laminating epoxy to pot some electronics (bad mistake!).
Its why there are resins specifically made for potting - low viscosity, slow cure, low exotherm.

Patrick

Tue Jul 31 2018, 12:01AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ive done the smoke thing too !

Ill definitely do a "chop test". But i need a procedure.

I was thinking of introducing already degassed and mixed epoxy to the bottom of a container. Then keeping the coil (copper and plastic only) held to the ceiling with a magnet then putting the vacuum chamber together. pulling a high vacuum, then pull the magnet off, the coil-form then falls into the epoxy totally covered, then let air pressure in. Epoxy should rush in, max1618 is almost water thin and has a 45 minute really thin usable time.

2Spoons

Tue Jul 31 2018, 08:52PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

So, if the coil is copper and plastic only, how is a magnet going to hold it? Otherwise seems a reasonable method.

Patrick

Wed Aug 01 2018, 12:03AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

So, if the coil is copper and plastic only, how is a magnet going to hold it? Otherwise seems a reasonable method.

I would add a temporary magnet.

Patrick

Mon Aug 06 2018, 02:30AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

how important is it to keep the fluid moving and in a thin layer ? does degassing only take place at the very thin fluid vacuum interface? how thin should i try to keep it ? has anyone else on the forum tried this or think its worth pursuing ?

Ash Small

Mon Aug 06 2018, 01:28PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I spent one winter working in a windings factory, during that time I vacuum potted some ferrite transformers for Raytheon, among others.

I also designed a purpose built chamber for them after they repeatedly broke the large glass beljars they'd previously been using.

They used a varnish which was oven cured, and pre-heated to thin it.

The thing that limits vacuum is the vapour pressure of the potting medium.

All we used to do was submerge the xformers in the potting medium, making sure there was plenty of excess.

Then just pumped it down until the bubbles stopped, then released the vacuum, allowing atmospheric pressure to force the potting medium between the windings, it's surprising how much the level drops, hence the need for plenty of excess.

I guess if that's good enough for Raytheon it's good enough for most purposes.

(I designed them a steel 'belljar', with a half inch thick perspex or similar 'window' in the top, and used a belljar seal around the bottom. The belljar seals are 'L' shaped rings of rubber that stretch, so you get one slightly smaller than the diameter of your 'belljar')

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