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

My latest tests on paraffin itself are showing that it may not hold up well if the environmental temperatures vary too widely. On tests of small blocks, I have ended up with two cracked ones after cycling in and out of the freezer a couple of times. Granted, the HEI's won't be in those extremes and their wax potting would probably hold up ok over long term, I still don't like the test results I have been getting. So, just for kicks, I am thinking about epoxy potting, something I've never before attempted on large scale. This would be just for fun whenever I come across some spare cash again in the future (probably after the holidays of course).

The transformers in their junction box home would need a tad over 32 oz of epoxy. Again, these are the HEI type, a pair, and the HEI's already appear to be potted during manufacture. I have some questions:

1) Since the HEI's are already potted prior, would the epoxy potting need to be done under vacuum? If so, how would it be done? Single stage pump availability, but nothing available other than a small polycarbonate vacuum jar which wouldn't fit the junction box. I have read of folks placing premixed epoxy itself only under vacuum for 5 minutes, then carefully pouring the nearly bubble free solution into their circuit. Personally, I don't think a vacuum will be required as the transformers already have a robust potting so it would just be a matter of keeping arcing away from the core and ground sides (maybe I'm wrong).

2) How much heat is produced by the reaction and would the junction box be able to handle it? Since the connections into the box are already sealed, I could place it in water during the curing process.

3) Not wanting to spend a fortune even when the funds are available, here is what I am considering:

shorturl.at/fISZ6

Remember that this would be for a pair of HEI coils only as the small RC circuit I've discovered can be soldered in later. I know it's probably overkill, but always wanted to try it and will sometime after the holidays with spare funds. I will point out that, years ago when I was a teen, I tried such a feat potting insects. The result was horrible as I didn't know at the time that the insects had to be fully dried first, lol.

Comments and suggestions welcome. Thanks in advance.

Reply to
Jim Horton
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I don't know a lot about epoxy potting, but I know you don't want a lot of vacuum. If you pull too much vacuum the epoxy will boil and you'll end up with foam as your encapsulant. I know, I did this.

--

  Rick C. 

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Reply to
Rick C

I know, that was one of my concerns. That was one reason I asked about vacuuming just the epoxy premixing container ahead of time. As I said, because the transformers are already well potted for the autos, I don't even think vacuum will be necessary but thought I would throw the question out there.

Reply to
Jim Horton

Vacuum degassing is common. In a high voltage system, bubbles in the potting can cause local corona that can eventually result in HV breakdown.

Whether you need vacuum degassing depends on your voltage gradients.

Google high voltage potting bubbles

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

I'd use what Win used, Sylguard. You can dig it out if you need too. Epoxy is pretty permament, really no chance of repair. There is also a Sylguard thats like a jelly and clear, too lazy to look up the part number. We used both in a 35kv power supply. Only problem is you need to clean the Oil/wax out of your setup, and use the sylguard primer. It does not work well with oil contamination.

Cheers

Reply to
Martin Riddle

I think the point is to provide a limiter to the vacuum pulled. A valve opened for a tiny leak or something similar should do the trick.

Like others have said, epoxy is pretty much a one way street so consider that.

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  Rick C. 

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Reply to
Rick C

Why pot them at all?

When I was a kid, I used to discharge oil caps into the primary of an old-fashioned oil-filled ignition coil, through a thyratron. The spark would climb out over the insulator and back down to hit the primary terminals. I had to make an oil-filled extension tube to stop that, and got maybe 3" sparks.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

One of my HEI sets I have in mineral oil now. Running in series, they produce up to 5' sparks across the electrodes to which they are attached. After ten years of having them in the oil, leakage developed and I rehoused them recently. Turns out that it probably wouldn't have been necessary. The oil was coming out through the top of the PVC junction box cover during hot periods, like during summer. Now, even though rehoused, I also left some elbow room under the cover, and also placed an absorbent pad underneath.

The epoxy idea is just a fun experiment I was going to try after the holidays using a spare set of HEI coils in one of the unused junction boxes I have.

Reply to
Jim Horton

I plan on using a spare set of HEI's as well as a clean junction box I have as spare to avoid any such contamination issues. Sylguard's price about blew me away so it definitely won't be used, not for a "fun" experiment anyway. These are spare coils and spare box, so if they quit working after potted in epoxy, it's no loss really.

Reply to
Jim Horton

I know, that's why I'm using some old spare coils/ boxes I had lying around. I just wanted to try it to get a feel in case I ever find that I really need to epoxy pot something important down the road.

>
Reply to
Jim Horton

of vacuum. If you pull too much vacuum the epoxy will boil and you'll end up with foam as your encapsulant. I know, I did this.

I'd evacuate container the before I left in the liquid expoxy encapsulent. If it frothed at the time, that wouldn't matter, as long as you got in enou gh of the liquid to fill the container. If the process stalled before that happened (which seems unlikely - there isn't going to be much gas in those bubbles if you degassed the liquid before you poured it in - you can use a bit of atmospheric pressure to squeeze in the last of the liquid.

You've got to do this while it's still liquid and more or less free-flowing . Once it starts setting, atmospheric pressure will flatten any bubbles tha t remain.

Bubbles of atmospheric pressure air are much harder to get rid of.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

ot of vacuum. If you pull too much vacuum the epoxy will boil and you'll e nd up with foam as your encapsulant. I know, I did this.

. If it frothed at the time, that wouldn't matter, as long as you got in en ough of the liquid to fill the container. If the process stalled before tha t happened (which seems unlikely - there isn't going to be much gas in thos e bubbles if you degassed the liquid before you poured it in - you can use a bit of atmospheric pressure to squeeze in the last of the liquid.

ng. Once it starts setting, atmospheric pressure will flatten any bubbles t hat remain.

If it froths, it will expand to many times it regular volume and spill out onto the vacuum chamber. Frothing is bad.

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  Rick C. 

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Reply to
Rick C

Jim Horton wrote in news:qpfcm7$900$ snipped-for-privacy@dont-email.me:

There are several. Some even can have better thermal numbers with with little silica splinters mixed in.

I would not recommend an epoxy unless you plan on zero serviceablity. Use an RTV so you can service it if it fails. Unless you are making a device which gets replaced upon failure and you want permanent encapsulation.

GE makes an RTV called "RTV-11".

They make others as well, but prices vary as HV isolation strength goes up.

DuPont makes some to and there are a few other makers.

Dolph's makes transformer varnish which is pretty thick and can vacuum fill your voids perhaps suitably enough.

Reply to
DecadentLinuxUserNumeroUno

lot of vacuum. If you pull too much vacuum the epoxy will boil and you'll end up with foam as your encapsulant. I know, I did this.

ut

d,

't

t. If it frothed at the time, that wouldn't matter, as long as you got in e nough of the liquid to fill the container. If the process stalled before th at happened (which seems unlikely - there isn't going to be much gas in tho se bubbles if you degassed the liquid before you poured it in - you can use a bit of atmospheric pressure to squeeze in the last of the liquid.

wing. Once it starts setting, atmospheric pressure will flatten any bubbles that remain.

t onto the vacuum chamber. Frothing is bad.

That's why you out-gas the liquid encapsulant in a deep beaker, and makes s ure that the froth doesn't go over the top of the beaker ... and only after you have done that do you pour the outgassed liquid into box that contains the gear you want to encapsulate.

Doing all of that under vacuum (or a vacuum of sorts - you can't get the re sidual pressure spectacularly low) requires a proper vacuum potting setup, which isn't all that complicated, but not all that easy to improvise either - being able to tilt a beaker and get it to pour from the other side of th e the vacuum wall requires a vacuum-greased rotating joint.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

32oz of epoxy may get hot enough to burn. Set is exothermic. Why use 32oz when you can pack the spaces anyway? And why epoxy when you can use silicone?

NT

Reply to
tabbypurr

If it is a fast-setting epoxy. If it's under vacuum it won't have access to oxygen and can't burn.

Heat may be released, but the temperature rise is determined by how fast the heat is released, and the thermal resistance from the heat source to the outside world.

NT will never know. He can ask the questions, but doesn't seem to be much good at finding answers.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

a lot of vacuum. If you pull too much vacuum the epoxy will boil and you' ll end up with foam as your encapsulant. I know, I did this.

bout

aid,

on't

ent. If it frothed at the time, that wouldn't matter, as long as you got in enough of the liquid to fill the container. If the process stalled before that happened (which seems unlikely - there isn't going to be much gas in t hose bubbles if you degassed the liquid before you poured it in - you can u se a bit of atmospheric pressure to squeeze in the last of the liquid.

lowing. Once it starts setting, atmospheric pressure will flatten any bubbl es that remain.

out onto the vacuum chamber. Frothing is bad.

sure that the froth doesn't go over the top of the beaker ... and only aft er you have done that do you pour the outgassed liquid into box that contai ns the gear you want to encapsulate.

residual pressure spectacularly low) requires a proper vacuum potting setup , which isn't all that complicated, but not all that easy to improvise eith er - being able to tilt a beaker and get it to pour from the other side of the the vacuum wall requires a vacuum-greased rotating joint.

Why would you even suggest something so complicated? All you need to do is to fill the mold with the device, pour in the potting compound and then pu t it in the vacuum chamber. Apply the partial vacuum and make sure the pot ting compound doesn't foam by limiting the vacuum.

To prevent foaming it would be good to have a pressure gauge attached and t est the encapsulant to see at what point it foams. Then you can monitor th e vacuum to make sure foaming doesn't happen.

--

  Rick C. 

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Reply to
Rick C

No disrespect, but I don't think that's a fair statement. Almost always, I DO research this stuff before asking here. The big problem I have with Internet research is that there's too much information, so trying to narrow down a specific thing for a specific task can take huge amounts of time. I figured people here might have more experience in many of the things I've done, or trying to do, to help me narrow it down and save time.

Since posting here, and both with the group's help and my own, I've found out some things about both mineral oil and wax potting:

1) Mineral oil can expand right out through the container if it can. In this case, I wasn't aware how much it could expand/ contract with environmental changes. The leakage I experienced with my ten year stored spark device came from mineral oil expanding out from the junction box cover. So, once rehoused, I took measures to both allow for expansion and to compensate for any future leaks. 2) Paraffin wax probably isn't ideal for potting. My own experiments cycling a block into and out of the freezer the last few days have resulted in a block ending up with many cracks. However, now it's time to remelt the block and repeat the same experiment cycling into and out of the refrigerator since, after all, the device will never be stored outside of this range. Then, I'll know for sure. 3) I better think again before I use epoxy. Item will be non-serviceable, vacuuming could be a problem with my limited equipment available, and there is a lot of exothermic reaction with the 32 oz I would need. Such information can be hit and/or miss online, but the group has been very helpful in helping me make the decision to probably not try epoxy potting, especially for something this large. They just helped me save $30 for the cheapest electronic epoxy I would have used.

Things inconclusive or I have not found out here (or anywhere else):

1) Ideal wax and additive mix ratios to better improve wax properties. I did find information on wax-rosin ratios, but then controversial performance if used in a heated circuit. Then there was machinable wax (wax-LDPE), and wax-EVA mixes. Only information on machinable wax was found and proper EVA ratios remain unknown. 2) I was not aware of RTV being used for anything other than small jobs and/or automotive use. Now, if one wanted to criticize me, here is where they could do it by the fact that I never specified an insulator budget amount in which both the electronic RTV and most epoxy do not fall.

So, again, I apologize if I haven't already found the answers I have been looking for. I really enjoy reading this and the repair group's posts especially since so much usenet has become useless.

Reply to
Jim Horton

true of most of his anti-personnel comments. People mostly ignore it

it's fine within temperature limits, but not for all jobs. The 1930s caps I redid (paper in wax in cans) showed no cracking but the paper caps were all leaky nonetheless. So I'm not certain what the degradation mechanism is.

and of course a partial (or even a perfect) vacuum doesn't save you from that.

Silly cone has a nice assortment of upsides for your job. Wax is still practical if there's not much temp swing.

NT

Reply to
tabbypurr

Huh? RTV is 'room temperature vulcanizing' material, it sets up just as irreversibly as epoxy does. How is that service-friendly?

Reply to
whit3rd

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