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

it can be cut off easily

NT

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

about

said,

don't

ulent. 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 befor e 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.

-flowing. Once it starts setting, atmospheric pressure will flatten any bub bles that remain.

l out onto the vacuum chamber. Frothing is bad.

es sure that the froth doesn't go over the top of the beaker ... and only a fter you have done that do you pour the outgassed liquid into box that cont ains the gear you want to encapsulate.

e residual pressure spectacularly low) requires a proper vacuum potting set up, which isn't all that complicated, but not all that easy to improvise ei ther - being able to tilt a beaker and get it to pour from the other side o f the the vacuum wall requires a vacuum-greased rotating joint.

Because that was the way it was done at places I've seen doing it.

ng compound and then put it in the vacuum chamber. Apply the partial vacuu m and make sure the potting compound doesn't foam by limiting the vacuum.

Getting the dissolved air out of the potting compound before it goes into t he mold lets you do it when there's a lot more exposed surface for the bubb les to come out of, and can speed up the process a lot.

test the encapsulant to see at what point it foams. Then you can monitor the vacuum to make sure foaming doesn't happen.

You are imagining that the encapsulant has a high vapour pressure component that boils at a specific pressure. Dissolved air doesn't work that way. An y water in the encapsulant will boil at about 20 torr if the encapsulant is at room temperature, but there shouldn't be any.

NT certainly does. He's been indulging in his little ego trips for years, and seems to imagine that complaining about being slandered is an appropriate response to being jeered at for posting bad information.

Sadly, it's fair statement about NT. He does do the research - after a fashion - but has an unhappy knack of settling on the wrong answers.

The big problem with internet research is that there's a lot of false and misleading information, primarily designed to sell particular products rather than help the researcher.

Critical thinking is required.

Some of our posters do get some of their information from deeply suspect sources. Cursitor Doom is a particularly horrible example, but he's also a particularly obvious example. None of the sane posters here take him seriously.

e:

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.

ed about

I said,

I don't

he

psulent. If it frothed at the time, that wouldn't matter, as long as you go t in enough of the liquid to fill the container. If the process stalled bef ore 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 c an use a bit of atmospheric pressure to squeeze in the last of the liquid.

ee-flowing. Once it starts setting, atmospheric pressure will flatten any b ubbles that remain.

ill out onto the vacuum chamber. Frothing is bad.

akes sure 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 co ntains the gear you want to encapsulate.

the residual pressure spectacularly low) requires a proper vacuum potting s etup, 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 the the vacuum wall requires a vacuum-greased rotating joint.

ting compound and then put it in the vacuum chamber. Apply the partial vac uum and make sure the potting compound doesn't foam by limiting the vacuum.

the mold lets you do it when there's a lot more exposed surface for the bu bbles to come out of, and can speed up the process a lot.

This is not a production run, no one cares if it takes a while. The proces s you describe is overly complex for a one-off.

nd test the encapsulant to see at what point it foams. Then you can monito r the vacuum to make sure foaming doesn't happen.

nt that boils at a specific pressure. Dissolved air doesn't work that way. Any water in the encapsulant will boil at about 20 torr if the encapsulant is at room temperature, but there shouldn't be any.

I never mentioned any numbers. I'm not talking about foam from the air. I 'm talking about the encapsulant forming enough vapor that the entire stink ing mess expands out of the container and spills into the vacuum chamber ma king a huge mess that then has to be cleaned up so you can start over. Thi s isn't imagination. This is experience.

I apologize yet again. I thought "NT" meant me somehow. Not sure who he/she is, but a big thank you for the clarification! I really do appreciate this and the other groups.

Quite right and if you have a few extra funds to experiment without putting yourself in the grave (i.e taking proper precautions), all the better!

Yes, one has to be careful. The 'net is getting better. An example of really poor information out there relates to a guy wanting to build a stun gun recently. Just out of curiosity, I decided to look up what current stun guns are for sale on the web. The majority are being falsely advertised! 9 billion or 9 million volts, yeah right, but people seem to buy them like crazy. I doubt many of them would do much other than cause a bit of pain. Videos I see of them produce maybe 1-2" sparks across their electrodes which is maybe 25-50 KV max, a far cry from their millions advertised. Not sure how they are getting away with that.

e:

ote:

t want a lot of vacuum. If you pull too much vacuum the epoxy will boil an d you'll end up with foam as your encapsulant. I know, I did this.

sked about

As I said,

s, I don't

the

capsulent. 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 b efore that happened (which seems unlikely - there isn't going to be much ga s 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 .

free-flowing. Once it starts setting, atmospheric pressure will flatten any bubbles that remain.

of.

spill out onto the vacuum chamber. Frothing is bad.

makes sure that the froth doesn't go over the top of the beaker ... and on ly after you have done that do you pour the outgassed liquid into box that contains the gear you want to encapsulate.

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

otting compound and then put it in the vacuum chamber. Apply the partial v acuum and make sure the potting compound doesn't foam by limiting the vacuu m.

to the mold lets you do it when there's a lot more exposed surface for the bubbles to come out of, and can speed up the process a lot.

ess you describe is overly complex for a one-off.

It was a prototyping set-up. It was just complex enough to avoid inconvenie nt and time-consuming problems. People want prototypes as soon as possible, and they don't like hanging around when they don't have to.

and test the encapsulant to see at what point it foams. Then you can moni tor the vacuum to make sure foaming doesn't happen.

nent that boils at a specific pressure. Dissolved air doesn't work that way . Any water in the encapsulant will boil at about 20 torr if the encapsulan t is at room temperature, but there shouldn't be any.

Surprise, surprise.

nt forming enough vapor that the entire stinking mess expands out of the co ntainer and spills into the vacuum chamber making a huge mess that then has to be cleaned up so you can start over. This isn't imagination. This is experience.

You've had experience of doing it wrong, and you want to reject the experie nce of people who had done it often enough to do it better.

I didn't invent that set-up - I got to see it in use and more or less under stood why it was set up the way it was, which you seem to be having a probl em with.

Paper capacitors were made with cheap, high acid paper. Over time, it starts to disintegrate like an old newspaper.

ote:

n'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.

asked about

As I said,

tos, I don't

ow the

encapsulent. If it frothed at the time, that wouldn't matter, as long as yo u 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 those bubbles if you degassed the liquid before you poured it in - y ou can use a bit of atmospheric pressure to squeeze in the last of the liqu id.

s free-flowing. Once it starts setting, atmospheric pressure will flatten a ny bubbles that remain.

d of.

d spill out onto the vacuum chamber. Frothing is bad.

nd makes sure 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 tha t contains the gear you want to encapsulate.

get the residual pressure spectacularly low) requires a proper vacuum potti ng setup, which isn't all that complicated, but not all that easy to improv ise either - 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.

potting compound and then put it in the vacuum chamber. Apply the partial vacuum and make sure the potting compound doesn't foam by limiting the vac uum.

into the mold lets you do it when there's a lot more exposed surface for th e bubbles to come out of, and can speed up the process a lot.

ocess you describe is overly complex for a one-off.

ient and time-consuming problems. People want prototypes as soon as possibl e, and they don't like hanging around when they don't have to.

ed and test the encapsulant to see at what point it foams. Then you can mo nitor the vacuum to make sure foaming doesn't happen.

ponent that boils at a specific pressure. Dissolved air doesn't work that w ay. Any water in the encapsulant will boil at about 20 torr if the encapsul ant is at room temperature, but there shouldn't be any.

lant forming enough vapor that the entire stinking mess expands out of the container and spills into the vacuum chamber making a huge mess that then h as to be cleaned up so you can start over. This isn't imagination. This i s experience.

ience of people who had done it often enough to do it better.

erstood why it was set up the way it was, which you seem to be having a pro blem with.

The only problem is that you insist on making it many steps when those extr a steps add nothing of benefit. Now that we have gotten to the root of the issue you stop talking about the issue and attack the person.

Bottom line is there is no need to pump the epoxy first and then again when adding it to the mold. At least you haven't talked about any reason to do that and you have mentioned the absurd complexity of adding a mechanism to the vacuum chamber to manipulate the ingredients while inside.

Just give it up. You never admit when you are wrong. You just have a comp ulsion to argue with people here, mostly the idiots who can barely put word s on a page... as if that were an accomplishment.

whit3rd wrote in news: snipped-for-privacy@googlegroups.com:

Well, son... it ain't yer tube of window caulk. ;-)

RTV for HV potting is 100% serviceable. It shears easily.

Epoxy, on the other hand cures usually quite rigid.

Remember "Stycast"?

e:

don't want a lot of vacuum. If you pull too much vacuum the epoxy will boi l and you'll end up with foam as your encapsulant. I know, I did this.

I asked about

e. As I said,

autos, I don't

hrow the

y encapsulent. 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 stall ed before that happened (which seems unlikely - there isn't going to be muc h 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 li quid.

ess free-flowing. Once it starts setting, atmospheric pressure will flatten any bubbles that remain.

rid of.

and spill out onto the vacuum chamber. Frothing is bad.

and makes sure that the froth doesn't go over the top of the beaker ... an d only after you have done that do you pour the outgassed liquid into box t hat contains the gear you want to encapsulate.

t get the residual pressure spectacularly low) requires a proper vacuum pot ting setup, which isn't all that complicated, but not all that easy to impr ovise either - being able to tilt a beaker and get it to pour from the othe r side of the the vacuum wall requires a vacuum-greased rotating joint.

he potting compound and then put it in the vacuum chamber. Apply the parti al vacuum and make sure the potting compound doesn't foam by limiting the v acuum.

s into the mold lets you do it when there's a lot more exposed surface for the bubbles to come out of, and can speed up the process a lot.

process you describe is overly complex for a one-off.

enient and time-consuming problems. People want prototypes as soon as possi ble, and they don't like hanging around when they don't have to.

ched and test the encapsulant to see at what point it foams. Then you can monitor the vacuum to make sure foaming doesn't happen.

omponent that boils at a specific pressure. Dissolved air doesn't work that way. Any water in the encapsulant will boil at about 20 torr if the encaps ulant is at room temperature, but there shouldn't be any.

sulant forming enough vapor that the entire stinking mess expands out of th e container and spills into the vacuum chamber making a huge mess that then has to be cleaned up so you can start over. This isn't imagination. This is experience.

erience of people who had done it often enough to do it better.

nderstood why it was set up the way it was, which you seem to be having a p roblem with.

tra steps add nothing of benefit.

It's one extra step. Letting the encapsulant liquid froth out before you pu t it into the mold gives you better control of the degassing process.

I don't insist on it - I'm just pointing out that the prototyping set up I saw allowed you to do it that way.

he issue and attack the person.

en adding it to the mold.

You can get away with not doing it, but out-gassing the encapsulant in the mold takes longer than doing it in an open beaker, and works better.

ioned the absurd complexity of adding a mechanism to the vacuum chamber to manipulate the ingredients while inside.

The mechanism just let you tilt the beaker through the vacuum wall. It was a conical hole in the side of the chamber, plugged with a ground glass plug which had been lubricated with vacuum grease. Demountable laboratory glass

-ware uses standard range of cones and sockets, which you can buy off the s helf.

Back when I was doing the glass-blowing on my vacuum line (as a graduate st udent) I could have whipped something up in a few minutes.

The chemistry department stores had all the parts I would have needed.

Actually I do. It doesn't happen often, but the posts where I've done it te nd to contain the word "oops" which makes them easy to find.

I'm not wrong all that often, and I'm fairly sure that I'm not wrong here.

ho can barely put words on a page... as if that were an accomplishment.

I'm just as happy to argue with intelligent people - it isn't necessary nea rly as often.

You are confusing "arguing" and "disagreeing". I do tend to point out when people get things wrong, and our crew of idiots do that a lot.

Many years ago I worked at a UK firm making HV power supplies.

We always prepared both epoxy and silicone in vacuum. It went into a bucket and into a vacuum chamber, whose evacuation rate was manually controlled (via a glass window) so the material didn't "boil" over the edge. Quite a fun job!

Eventually it would be evacuated fully and then poured into the mould (containing the electronics) and then the whole mould would be evacuated again, to remove bubbles which got in during the pouring process.

The company is long gone. They specialised in precision (ppm grade) DC power supplies for all kinds of applications.

Peter wrote in news:qpm34h$3jt$ snipped-for-privacy@dont-email.me:

I noticed that most RTVs evacuate pretty quick. The Polyurethanes were a lot more 'airy'. But the worst is transformer varnish. It 'boils' seemingly forever, but finally gets done and can be used for sealing up transformers.

Yeah, there is air trapped in the PCB layers that gets released as well. The low pressure also vaporizes any water, which PCBs also have an affinity for, especially in typically non-masked HV sections.

A big UK HVPS maker is HiTek Power in Littlehampton, West Sussex. Now part of Advanced Energy group.

Lots of precision DC supply expertise.

snipped-for-privacy@decadence.org wrote

There are several around, probably offshoots from the old Wallis Electronics. Applied Kilovolts is a good one.

It was a fun business. One of the projects done at Wallis was a 500kV unit with 1ppm-grade wirewound resistors (lots of them) for the feedback network. The whole multiplier stack was in a huge steel cylinder with some gas inside; probably sulphur hexafloride or some such. Mann Components (long gone too) made the resistors. I have a few of their 0.01% resistors still, 1972 date codes, and all are well within 0.01% now.

Brandenburg was another old one but they used air insulation. It's not as good; you get corona so a bit of noise.

Appreciate everyone's info on the epoxy. I still think if I ever went the route with the HEI's, I probably wouldn't do the vacuum because the HEI transformers themselves appear to already be potted, so it would have just been a matter of insulating HV to ground and core. However, I have decided not to try it.

The paraffin is holding up lots better cycling into and out of the refrigerator as opposed to the freezer. The freezer bottoms out at 0 F while the refrigerator at 36 F. The house is cooling fast though at 57 F now, so will probably have to continue cycling tests next spring and summer when it could reach 87 F.

Do you really think you need to test at higher temps? The stuff was poured at some 120+F. It is the low end where it gets brittle. The high end mak es the wax pliable. They use paraffin wax for dental uses and it has to be warmed a bit in your hand to make it soft enough to form. That's all it t

ured at some 120+F. It is the low end where it gets brittle. The high e nd makes the wax pliable. They use paraffin wax for dental uses and it h as to be warmed a bit in your hand to make it soft enough to form. That'

Probably not. The second HEI coil assembly had already been potted in the wax last week and has been sitting on the shelf ever since. These are just small, separate blocks I made just for testing purposes. The house will never get above 87 F during the hottest summer days, but will

soon dip to 47 in another six weeks or so. So, those being the extremes

of storage, I think the device will do well.

I may pick up a block of one of the other wax types you recommended and try cycling it throughout the next year, throwing in built up summer car

heat as an extra challenge. One block isn't going to kill the budget.

You can't achieve a very good vacuum at the bottom of a deep mould that is already filled with a liquid potting compound. Gravity acts on the liquid, causing the pressure at the bottom of the liquid to be higher than that at the top of the liquid.

Surface tension might also hinder the escape of gas from crevices in a device that is already submerged.

Depending on the purpose of the potting process you may or may not care about small bubbles remaining, but some people do need to care.

I have never needed to be extremely careful about eliminating bubbles so I have never built anything to pour encapsulant under vacuum, but I do degas the encapsulant in a separate beaker before pouring, simply because it does tend to froth and overflow unless that step is carried out in a beaker able to contain many times the volume of the liquid encapsulant.

e:

te:

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

ed about

I said,

I don't

he

apsulent. If it frothed at the time, that wouldn't matter, as long as you g ot in enough of the liquid to fill the container. If the process stalled be fore 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.

ree-flowing. Once it starts setting, atmospheric pressure will flatten any bubbles that remain.

f.

spill out onto the vacuum chamber. Frothing is bad.

d makes sure that the froth doesn't go over the top of the beaker ... and o nly after you have done that do you pour the outgassed liquid into box that contains the gear you want to encapsulate.

et the residual pressure spectacularly low) requires a proper vacuum pottin g setup, which isn't all that complicated, but not all that easy to improvi se either - being able to tilt a beaker and get it to pour from the other s ide of the the vacuum wall requires a vacuum-greased rotating joint.

potting compound and then put it in the vacuum chamber. Apply the partial vacuum and make sure the potting compound doesn't foam by limiting the vac uum.

into the mold lets you do it when there's a lot more exposed surface for t he bubbles to come out of, and can speed up the process a lot.

process you describe is overly complex for a one-off.

venient and time-consuming problems. People want prototypes as soon as poss ible, and they don't like hanging around when they don't have to.

ed and test the encapsulant to see at what point it foams. Then you can mo nitor the vacuum to make sure foaming doesn't happen.

mponent that boils at a specific pressure. Dissolved air doesn't work that way. Any water in the encapsulant will boil at about 20 torr if the encapsu lant is at room temperature, but there shouldn't be any.

sulant forming enough vapor that the entire stinking mess expands out of th e container and spills into the vacuum chamber making a huge mess that then has to be cleaned up so you can start over. This isn't imagination. This is experience.

perience of people who had done it often enough to do it better.

understood why it was set up the way it was, which you seem to be having a problem with.

extra steps add nothing of benefit. Now that we have gotten to the root of the issue you stop talking about the issue and attack the person.

when adding it to the mold.

That is true for various values of "deep". In this case you will never not ice the delta from the top to the bottom of a less than 1 foot deep mold. Materials like this are the same density as water to a first approximation. Water is 32 feet to 1 atm. So 1 foot of mold will only cause 1/32 of an atm difference in pressure.

When the volume of trapped air expands my multiple factors, it will help ge t the air out. Any remaining bubbles will be very tiny when the vacuum is released. Since these air bubbles will exist whether or not you pre-vacuat e the epoxy, it has no impact on the result.

How can you do better in getting rid of them?

It won't froth if you don't draw too much vacuum.