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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Hey everbody!
First time poster, long time reader. Ive looked on this site and googled many others but no tecnical details on how vacuum potting is done. I am 27yo, have much HV experience, but none with vacuum potting, any help or comments apreciated.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
What are you using as potting compound?
It is usually a good idea to pull a vacuum on the potting compound seperately for some period of time to de-gas it. If you are using oil, it may also be a good idea to dehydrate the oil first. The transformer, sitting in it's potting form, needs to be in a vacuum chamber, under vacuum. The potting form itself may be the vacuum chamber if it is well designed. The de-gassed potting compound is then fed into the vacuum chamber from a seperate leak-tight resevoir through some plumbing, and flowed into the potting form. It's nice to have a window in your vacuum chamber so that you can watch the form filling. Generally speaking, that is how I have vacuum-potted things. Others may have better ideas.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
minorityCarrier,
I will try water thin epoxy (Heated 15 secs in microwave to reduce viscosity) , but i have never done this before, also what about the vacuum pressure? how much or little does it take? (psi)
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Patrick -
Before you get too far into the specifics of vacuum impregnation, what type of a device or assembly are you trying to pot? Why do you want to pot it? What are the design goals? Have you considered the thermal issues (heat dissipation as well as thermal expansion), both as the epoxy cures, and during device operation?
I would be careful about heating the epoxy prior to impregnation/potting, unless you have a means to carefully monitor and control the temperature, and you know exactly what the pot life is after mixing and heating. It would be disastrous to have the epoxy start to gell right when you are trying to vacuum degass the partially-potted device.
Depending on how you mixed the epoxy, and how much air is entrained, and the viscosity, you will probably find that the epoxy will foam and froth vigorously when vacuum is first applied, and you'll have to carefully control the vacuum level to prevent the material from "overflowing". It works well to have an appropriately-sized "bleed valve" T-connected into your line to the vacuum pump, and start the pump with the valve fully opened. Gradually close the bleed valve to start applying vacuum to your epoxy mix, but be ready to open the valve quickly to increase pressure if it starts to foam excessively. You don't want or need a "high-vacuum" pump; a cheap dry-vane pump will work fine.
I'd use the lowest viscosity epoxy with the longest pot life (slowest cure time).
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Yay! Herr Zapp,
yeah, these are the things i need to know, ( t-valve and low pressure vac) i was going crazing looking for high vac devices. For background i am making 10 transformers (really normal HFHV with new ferrites) for a 500w to 1 kw SMPS using the FAN4800 PFC and then the UC3526 SMPS controller from ti/unitrode.
and Zapp, most of the device is solid PVC and Acrylic, (I have a CNC machined Pie wound bobbin I made myself) so only a tiny amount of epoxy is needed, and exothermic heat accumulation wont be a problem.
I have been investigating ion propulsion (Alexander P. DeSeversky device) since freshman year high school (in 1996.) Now i am working for my pHd in EE. And i hope i dont get killed:-)
Been reading every Ti/unitrode pdf for more then a year, learned so much, now i am ready to attempt this task.
Ps. to the "Balloon boys" Dad, electrogravitics and antigravity are bogus, just so you know.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
I have not used those epoxies.
Patrick, they will be a bit expensive, but there are elevated-temperature-cured epoxies available. Some of these have fairly low viscosity, and the pot life (time to gel) can be controlled by the temperature the epoxy is held at. For vacuum potting you really should use one of these epoxy systems.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Patrick -
I forgot to mention, it's also a good idea to bake your completed assemblies for several hours at elevated temperature to drive off moisture before potting.
Allow the assembly to cool before potting, so you don't inadvertantly accelerate the jelling of the epoxy before the impregnation is complete.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Design your transformer to allow easy access for the potting compound. Do a test on your potting compound under vacuum. Some epoxies etc. contain solvents and/or form stable foams under vacuum and just aren't suitable. Ideally find a compound recommended for vacuum potting.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Although they don't mention it on the data sheet, when I contacted them about it, MG Chemicals stated that their potting epoxies are vacuum compatible.
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Need help! How do i vacuum pot transformers?
