Metal Casting Zone

Boiling water at room temp, high school science class day one. I haven't got it to freeze yet. Too much water.


It takes a pretty stout container. I don't know what this thing is I picked up, some kind of medical equipment. It was made for vacuum, I think. My original plan was to use a cut off pressure tank, like scuba, with an acrylic cover. Smaller volume will be less likely to implode.



This is the RTV Silicone I degassed. It was a little too close on volume. It started out less than half. I think 1/3 now to be safe. 1/4 volume for plaster, it really expands.

Right on Jammer, now I have a few questions.

Does your RTV rise and fall twice?

How are you regulating the pressure? Valve, motor speed?
I have not seen the valve yet?

And how thick is the thinnest wall, lid or floor? And how large is the tank, two gallon?

The lid and the base are about 3/8" and the cylinder is 1/4". No regulation, I'm going for max vacuum, usually around 28" or 29" Mercury. The RTV rose slowly at first and then took off to the top, bubbled a little and then collapsed and kept bubbling, I think the instructions were to let it bubble for about 1 minute after it collapsed. There is a small relief valve in the lid between the gauge and hose.

I don't want to be too much of a science geek here, but just to clear up what appears to be a common misconception: The water/investment/RTV doesn't always BOIL due to the pressure drop. It takes a very strong vacuum to actually boil the water. Frequently, the vacuum is only enough to pull dissolved gasses out of the water, which is perfect for removing bubbles, and probably actually BETTER, because actually boiling the water would be creating MORE bubbles as the liquid water converted to vapor. We certainly don't want to make more bubbles for our purposes, just remove the existing ones. Our flasks aren't boiling any more than a glass of champagne is ... in both cases that's just dissolved gas releasing because the atmospheric pressure is low enough to let the gasses out - although it does look like boiling.

Think about this: Your flask stops bubbling at some point, right? And you can't get any more bubbles out. If the water was actually boiling, you would be able to continue until you converted it all to vapor (steam) and ran your little flask dry like a forgotten pot that boils dry. And there would be vapor all over the place, probably condensing on the jar and running everywhere making a mess.

You're just releasing dissolved gasses... unless you have a really powerful vacuum pump - and I don't deny that you might. But at least the first phase of bubbles is just dissolved gasses, not boiling, and that's when you should stop because after that you'll just make it worse.

I'm trying to figure at exactly what pressure (at room temp) is the threshhold. Ill let you know if I figure it out!

Of course, maybe everyone here just says "boiling" because it looks like it is boiling but they don't really believe that it is. In that case, nevermind.

Thank you for the answers Jammer. Quarter inch is pretty thin for such a large volume, but from what I have heard if there is a failure it is the lid that fails, usually around the fittings.

Personally I like redundancy in any pneumatic system. Relief valve and regulator. Surprisingly the hardware I have gotten from Harbor Freight whale being inexpensive has been durable and capable.

Dave it is plain to see you like to make your own equipment from your profile images. I look forward to seeing what you end up with.

Heres a little English geekery.

Actually boiling is the right verb via the 2nd and third listing of the Webster's 11th Collegiate Dictionary. As an intransitive verb in this case below 2,3 and especially five.

Main Entry:1boil
Pronunciation:*b*i(-*)l
Function:verb
Etymology:Middle English, from Anglo-French buillir, boillir, from Latin bullire to bubble, from bulla bubble
Date:13th century

intransitive verb
1 a : to come to the boiling point b : to generate bubbles of vapor when heated — used of a liquid c : to cook in boiling water
2 : to become agitated : SEETHE
3 : to be moved, excited, or stirred up *made his blood boil*
4 a : to rush headlong b : to burst forth : ERUPT *water boiling from a spring*
5 : to undergo the action of a boiling liquid
transitive verb
1 : to subject to the action of a boiling liquid
2 : to heat to the boiling point
3 : to form or separate (as sugar or salt) by boiling
–boil£able *b*i-l*-b*l adjective

This was after a couple minutes, lots of vapor. I stopped because I didn't want water in the vacuum pump. If I had a good enough vacuum it would freeze after losing all it's heat energy. The lower the pressure the lower the boiling point is. It's hard to boil and egg at high altitude because the water isn't really hot enough to cook it.

Probably just as well Jammer, chilling the plastic, plus an already cold garage in this frigid winter, pulling 30, with so much volume and .25" walls, well this may be less than advisable.

I am at see level and have never gotten any liquid from the air filter on my 2.5 gallon paint pot pressure tank. I imagine there are available for vacuum but wouldn't know.

Although in my experience using CO2 condensation will drip off the aluminum lid occasionally spoting the surface of the resin casting. A slight cone atatched to the interior lid would solve this. But so far I have been just intering a slihtly crumpled alluminum pie pan. Another concern when a internal drop cover is not used the rush of air can splatter a little material in the tank. Initially I used air and then nitrogen, thinking CO2 would be too wet. But I got tired of exchanging nitrogen tanks. CO2 will last me months, and have yet found a need to upgrade to medical quality CO2. It is supposed to be a bit dryer.

I made it and it works! A vacuum de-bubbler for $13 - thats right. $13!

- 1 used mini-fridge from Craigslist: $10
- 1 square of rubber gasket material from the hardware store: $2
- 1 heavy-duty glass vase from the thrift store: $1
- 1 piece of plywood that I scrounged from a "free" pile: $0
- 1 sheet of heavy copper from my junk pile: $0


Even that tiny little compressor pulled a really strong vacuum. I physically could not lift that vase from the rubber mat, no matter how hard I tried - even after 15 minutes! Since then I added a little vacuum breaker valve in the copper line, under the table. Perfect.


I nabbed the compressor out of this little fridge:



Vases and rubber gasket:



Detail of the platform on top:




The completed unit:




Not the best shot - but it sucked tons of bubbles. I was so excited I forgot to take pictures until they were mostly gone already:

Daverham wrote:
I made it and it works! A vacuum de-bubbler for $13 - thats right. $13!

- 1 used mini-fridge from Craigslist: $10
- 1 square of rubber gasket material from the hardware store: $2
- 1 heavy-duty glass vase from the thrift store: $1
- 1 piece of plywood that I scrounged from a "free" pile: $0
- 1 sheet of heavy copper from my junk pile: $0

Pulling nasty air bubbles out of my investment...Priceless!!
Sorry, I couldn't resist.
I've looked at hundreds of vases and glass jars and never found any that I would trust. Your's look perfect, short, stout and pretty thick. What is the size? About a quart?
Just be careful, if it did break it would send glass flying in all directions.

Yeah, about a quart and a half maybe. I'm being wary of flying glass, to be sure. I have clamped up a big sheet of lexan scrap that I had - and I stand behind that WITH safety glasses on when the pump is in action.

Now I'm trying to adapt this to vacuum casting. My centrifugal caster is causing me new problems. I guess I just need a silicon rubber pad and a tank to build up vacuum in so I can release it/suck all at once. Maybe try steam casting... this is endless!