Vacuum Potting in the "At Home" Shop

[Kazooman]

Anyone who is going to try this needs to be very careful. The container and lid may look very robust, but they are probably not designed for vacuum use. The flat lid looks like the weak link in the chain. If it cracks air will rush in very rapidly and hot wax will come flying out along with pieces of the lid and probably your nice, new pickups. I would certainly do several test runs without heating the wax and I would wear safety glasses. I have seen the results of several failures of glassware under vacuum in the lab and they are very sobering.

Don't be lulled into a false sense of security by thinking that it is only a little pump and it is only on for a short time. What matters is the pressure differential between the outside and inside. You achieve most of the potential energy with even a modest vacuum.

[RedHouse]

Is there any way to set the MAX vacume setting on the pump? (like an air comressor's regulator) just curious because I see a knob and valve in the picture.

[Kazooman]

The oil in the vacuum pump is intended to a medium for trapping gas molecules being extracted. Not sure how you would determine what exactly was being drawn off during the process.

Usually a cold trap (cooled with dry ice or liquid nitrogen) is used between the apparatus and the pump to condense as much vapor as possible to protect the pump. This would not be very practical for your situation. You should change the pump oil regularly (just like in your car) to get rid of any contaminants that might be corrosive to the pump. Also, the maximum vacuum you can achieve is limited by the vapor pressure of the most volatile component in the oil.

[kayakerca]

Anyone who is going to try this needs to be very careful. The container and lid may look very robust, but they are probably not designed for vacuum use. The flat lid looks like the weak link in the chain. If it cracks air will rush in very rapidly and hot wax will come flying out along with pieces of the lid and probably your nice, new pickups. I would certainly do several test runs without heating the wax and I would wear safety glasses. I have seen the results of several failures of glassware under vacuum in the lab and they are very sobering.

Don't be lulled into a false sense of security by thinking that it is only a little pump and it is only on for a short time. What matters is the pressure differential between the outside and inside. You achieve most of the potential energy with even a modest vacuum.

That's an excellent point. I will reinforce the acrylic lid with aluminum flat bar bracing.

[kayakerca]

Is there any way to set the MAX vacume setting on the pump? (like an air comressor's regulator) just curious because I see a knob and valve in the picture.

You could force a reduction in the vacuum by opening the value on the high side of the gauge set a bit and let the pump draw air from the atmosphere as well as the crock pot. The low side gauge would indicate how much vacuum is being drawn on the pot.

[Kazooman]

I do not know about this particular pump. The valve may just be there to allow you to break the vacuum by opening the system up to the atmosphere (you DO need a way to break the vacuum or else you will need to wait for leakage to do it for you).

More sophisticated vacuum pumps usually include a "ballast valve" that allows some air to enter to help purge volatiles from the oil. These are not intended to control pressure.

There are many devices (manostats) for controlling vacuum. They would be overkill here. The simplest is simply a bleed valve that lets some air into the system (between the pot and the pump, NOT upstream of the pot). The problem here is that this can be hard on the pump and the oil. Also, remember that the energy potential is still there with even a modest vacuum.

The cheapest way to generate enough vacuum for this purpose would be a vacuum aspirator.

Aspirator (pump) - Wikipedia, the free encyclopedia

This is a simple device that attaches to a faucet. The force of the running water creates a vacuum. One word of caution here. You MUST put some sort of trap between the aspirator and the pot. If you have a good vacuum established and the water pressure drops (your kid just flushed the toilet) then water can be sucked back from the aspirator. Not good to pour cold water in your hot wax.

[Kazooman]

I would avoid drilling a bunch of holes in the lid. They would provide points for a failure crack to begin.

Thickness would be your friend here. Also some duct tape applied on the surface where you do not need to look through would help hold pieces together in case of a failure.

I recall MANY years ago the guy in the lab next to mine was using a vacuum desiccator to dry a chemical sample. These are usually a very heavy glass bowl of sorts with a domed lid (handles the pressure better than a flat on). However, these things are notorious for having the lid get stuck on and they were very heavy. A company called Nalgene came out with a Plexiglas version. Problems solved! This guy had used his Nalgene dessicator for several years without incident. Until one day..... WHAM! Luckily no one was in the room at the time. There were sharp shards of plastic all over the place. Just like repeatedly bending a piece of metal until it fatigues and breaks the same can happen for plastic.

http://en.wikipedia.org/wiki/Vacuum_desiccator

[RedHouse]

You could force a reduction in the vacuum by opening the value on the high side of the gauge set a bit and let the pump draw air from the atmosphere as well as the crock pot. The low side gauge would indicate how much vacuum is being drawn on the pot.

Sounds good.

[J M Fahey]

Don't think molten wax has any air trapped in it.
At most, it might be contaminated with some low vapor pressure solvent, which evaporated under vacuum but don't find it much probable either.
I think (may be wrong, of course) that all those bubbles you see are just air.
And then, how can you pull, say, 5 cubic inches of air from a pickup with less total volume than that?
Easy: air is a gas; under low pressure it expands and occupies a much larger volume.
1 cubic inch of air at 1 atm. will occupy twice as much under 1/2 atm. and so on.
So you will see "large" bubbles, but they do not contain that much air (measured in weight).

[RedHouse]

...it might be contaminated with some low vapor pressure solvent, which evaporated under vacuum but don't find it much probable either...

That was my notion also.

[big_teee]

I think it is just the air in the pickup.
You have the same effect with regular potting, just more gradual.
I shake my pickups a few times in a 15-20 minute potting cycle.
When I quit getting bubbles when I shake them I take them out.

[David King]

Kazooman,
Thanks for the aspirator tip. That looks like the ticket for me since I do my "wax potting" in the basement bathroom.
Kayakcera, I made my vacuum lid from 3/4" polycarbonate (Lexan). I found the chunk in the $1/lb bins at my local TAP plastics store. The outer shell I used was a 6" ABS coupling. It seems plenty rugged for the job but I might wrap it in gaffers tape after reading what Kazooman wrote.

I would definitely consider adding a trap to your system and possibly a filter. You might want to use it later for degassing a mold should you ever get into making your own covers or bobbins. Nothing like urethane resin to slow down a pump.

[Kazooman]

WOW! 3/4" lexan. That isn't going anywhere!!! Well, at least I don't think so. If it ever did let go I wouldn't want to be anywhere near it.

The aspirator will pull plenty of vacuum for what you want to do. Just be certain to include a trap and a valve between your waxing pot and the trap so you can close things off if the water pressure goes down. Water aspirators have been used for decades (centuries??) in labs as a cheap, reliable source of a moderate vacuum. Turn the water on full blast.

One thought I had after reading the original post and watching the video was about the vigorous bubbling of the wax. It would be nice to have a way to have the pickup suspended above the wax, pull the vacuum, lower the pickup into the wax, and release the vacuum. I can almost hear the wheels in your brains turning as you try to figure out a slick way of doing just that.

[kayakerca]

Kazooman/David King:

Well, I made an 1 1'4" X 1/2" cross brace out of T6061 aluminum to mount flush along the top surface of my acrylic lid. I am certain that there will be no stressing anywhere along that axis and any stressing in the other direction would now be substantially reduced as the aluminum brace has reduced the unsupported distance down by 50%. It certainly feels a lot meatier.

I'll try out the new lid tomorrow.

[kayakerca]

Don't think molten wax has any air trapped in it.
At most, it might be contaminated with some low vapor pressure solvent, which evaporated under vacuum but don't find it much probable either.
I think (may be wrong, of course) that all those bubbles you see are just air.
And then, how can you pull, say, 5 cubic inches of air from a pickup with less total volume than that?
Easy: air is a gas; under low pressure it expands and occupies a much larger volume.
1 cubic inch of air at 1 atm. will occupy twice as much under 1/2 atm. and so on.
So you will see "large" bubbles, but they do not contain that much air (measured in weight).

Also, Strat style single coils have large pockets of air between the AlNicCo magnets that may well be what produces the larger bubbles when that air it is liberated from those cavities. Possibly?