Vacuum Chamber Build

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cecrops props

Active Member
Now that I am getting closer to the molding step of my Borderlands 2 Jakobs pistol (http://www.therpf.com/f9/borderlands-2-jakobs-pistol-build-230069/), I find myself in need of a larger vacuum degassing chamber. The one that I currently have is way too small for larger quantities of silicone mold making resin. In order to assure that my new chamber will be large enough for all future projects, I have decided that it must fit a 5 gallon bucket, giving me plenty of room for the expansion of the silicone that takes place during degassing. For reference, a standard Home Depot 5 gallon bucket is 14.5 inches tall by 11.8 inches in diameter.


After looking into the construction details of several different chambers online, I see that there are four basic approaches to construction.


One of the most common approaches is to use a large heavy walled aluminum stock pot for the chamber itself. A 40 quart aluminum stock pot will barely fit a 5 gallon bucket. There are three or four large enough aluminum stock pots listed on Amazon for around $50, with free shipping, so this is an economical approach to building a vacuum chamber. This is also a nice and easy approach since you start with ready made chamber. The only disadvantage is that the 40 quart pot is a pretty tight fit for a 5 gallon bucket, and larger sizes of pots jump way up in price.


A second approach is to use large diameter plastic pipe, like 12 or 14 inch PVC drain pipe, or a large PVC pipe coupling fitting. This approach works well if you have a source for scrap pipe or fittings, but the pipe is usually sold new in ten foot length for a painful amount. The fittings are also expensive. The other downside is that the plastic plate for the bottom, since it is flat, needs to be quite thick to withstand the full vacuum, probably 1 inch thick for a 12 inch diameter chamber, and that adds quite a bit to the cost of this method, unless you have access to scrap plastic of the appropriate size.


A third approach is to use an appropriately sized section of steel pipe, and weld on a flat plate bottom. This approach may be cost effective if you have access to large diameter scrap pipe, but purchasing the pipe new is very expensive. Also, purchasing, cutting and welding on the flat plate bottom greatly adds to the expense unless you have access to scrap and someone to cut and weld it for you. Given that the bottom plate is flat it will have to be fairly thick (1/4”?) to withstand flexing during full vacuum. The whole chamber will end up fairly heavy.


The fourth approach is a modification of the steel pipe approach, a steel walled tank with a formed dome bottom. This makes for a much lighter tank. Of course, forming and constructing such a tank is beyond the capabilities of most people, as the forming of the domed bottom requires a large die press. Fortunately, you can purchase and already constructed tank in the form of a compressed air tank, and cut off one end to form the opened top of your vacuum chamber. There a several options for acquiring a tank. Lowes carries a 7 gallon compressed air tank for $60, Walmart has a 10 gallon compressed air tank also for $60. A tank from a used air compressor is also a good option. Using a compressor tank, you can move up in to a larger sized tank, as portable compressed air tanks seem to top out at the 10 gallon size. A used compressed air tank can often be found on craigslist in the form of a complete or damaged air compressor. I have seen older compressors with 20 or 25 gallon tanks sell there in the $50 to $80 range. The disadvantage of using a compressor tank is that you may have to remove various welded on brackets and plug one or two air outlets to get a clean tank, but that’s not too much work. In this regard, a vertical compressor tank is easier to use than a horizontal tank, as most of the extra brackets are removed when the top is cut off.
 

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cecrops props

Active Member
I have chosen to construct my vacuum chamber using a formed steel tank. Although I have chosen this one option, much of the construction details will apply to the construction of a vacuum chamber using any of the other methods.


For the constructed steel tank, I was able to find a large propane tank, actually two of them, for $30. I found these by listing a “wanted” ad on Craigslist. These are 100 lb. tanks, which refers to the propane capacity, not the weight of the empty tank. They are about 14.5 inches in diameter, and 40 inches tall. If I chose, I could make two vacuum tanks from each propane tank.


Before I proceed, I wish to discuss the use and dangers of a propane tank for this project. I do not recommend that you chose to use a propane tank. Propane, mixed with air, is highly explosive. However, as I proceed with my project, I will show how I have safely used one.

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Veritech

New Member
Interesting tutorial... Thank you for sharing. :)

What type of vacuum pump would one need in order to effectively degas let's say a 5 gallon metal container?

I vaguely recall a DIY tutorial stating that the vacuum pump would need to be explosion proof, due to the vapors from the catalyst.

Does anyone know whether or not this is true?
 

cavx

Master Member
My chamber is about 20" x 10" (dia) and I would probably opt for a larger pump if doibg this again. My pump is 3cmf and takes out about 120 secs to reach a full vacuum. Still works a treat though.
 

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cecrops props

Active Member
Yes, I guess a pressure pot would do double duty as a vacuum chamber. One would have to trade out the lid for a clear one and figure out a seal for it to see the de-gassing stage. Also, a pressure pot large enough for a 5 gallon bucket is pretty pricey, but if you've already got one...

I'm not yet sure at this stage what capacity pump is required for the size chamber I am building, but I already have a big old Welcsh Duo-Seal laboratory vacuum pump, so I think I'll be OK. When I find more information on pump capacities, I'll post it. I have never heard of anyone exploding a vacuum pump de-gassing silicone, I think that it's pretty unlikely. Anyone else have some input on this? Anyone blown up their vacuum pump?
 

cecrops props

Active Member
After a bit of violence with a large pipe wrench and a small sledge hammer, I was able to remove the valve from the propane tank. I then filled it with water after adding some dishwashing detergent to purge any remaining propane. Since propane is heavier than air, even removing the valve will not assure that the tank is empty of propane. By filling it with water, any remaining gas is forced out the top. The soap is added to the tank to help to cut the oily residue that is added to propane to give it it’s distinctive smell. This oily additive is somewhat flammable itself, so it’s a good idea to remove it as well. Notice all of those warnings on the tank. Of course, you could skip all of this by buying a new empty 100 lb. tank for about $135.


In the next couple of days, I will cut this guy open, and see how rusty it is inside. I’ll also let you know if I was successful at removing all of the propane…

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asalaw

Sr Member
As far as rust removal from large industrial thingies and stuff, electrolysis is probably the most effective method for removing years of heavy rust (if that's what you're faced with). There are a number of great tutorials on YouTube on cheap DIY electrolysis for anything from small nails to car engine parts to big-ass machinery. File this one under big-ass:


(Sorry if I'm being too technical. :p )
 
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cecrops props

Active Member
OK, I've cut the propane tank in half with no unfortunate consequences.

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cecrops props

Active Member
Thanks for the link for electrolysis. Fortunately, there is surprisingly no rust inside the tank, and very little on the outside. I wouldn’t have trusted it with pressure as an air or propane tank, but it will be fine for vacuum.


After marking out the cut line, I drilled a little row of small holes, enough to sneak in a saber saw blade. I used that to make a slot long enough for my reciprocating saw blade. The reciprocating saw cut quite quickly around the whole tank. Unfortunately, the foot on the tank was badly rusted, so I cut it off and ground the welds down smooth. I will have to make some sort of new base for the vacuum chamber.


A note once again about using a propane tank: I know that this tank had been sitting with the valve opened for some time. I also purged it with water. It still made me nervous, because the propane odorant oil still smelled. If I were to cut the other, newer tank that I received, I would have purged it with water several more times, and tested it with a gas sniffer to make sure it was completely empty. Also, if you should ever wish to cut an empty propane tank, use a saw. DO NOT USE A CUTTING TORCH. Ok?. If there is even a tiny chance of any gas reside, a torch will light it. Even if the tank is empty, quite often unburned acetylene will build up in the tank and ignite.


I now have an order in to McMaster/Carr for a bunch of parts, so this project will be on hold for a couple of days until they arrive.

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cecrops props

Active Member
Plumbing!


Now that the three day weekend is over, I have received my parts for the vacuum chamber. Here’s what arrived:


starting at the top:


1 1/2” dial vacuum gauge, 1/8” pipe rear inlet


brass 90 degree elbow, 1/8” pipe


brass 1/8” pipe nipple


forged black steel thread pipe fitting, 1/8” pipe


brass ball valve, 3 Way, Female NPT 3/8” pipe


brass 3/8” pipe nipple


forged black steel thread pipe fitting, 3/8” pipe


left of valve: quick-connect fitting, 3/8” pipe


right of valve: exhaust muffler/filter, 3/8” pipe




I will be mounting all of my vacuum connections to the side of the tank itself instead of through the lid. Since the flat lid is the weakest part of a vacuum chamber, I do not wish to weaken it more by putting a hole through it. Also, mounting through the side of the tank it is tidier, there is no vacuum line to mess with when opening and closing the chamber lid.


The 3 way ball valve shifts the vacuum chamber between the vacuum input and an external air input, to bleed the vacuum off. While some chambers control the vacuum input and exhaust functions with two separate 2 way ball valves, the 3 way ball valve is easier to control, and actually turned out to be less expensive than two valves.The vacuum input side is served by the quick connect fitting, and yes, air hose quick connects also seal under vacuum as well as compressed air. The outside air side of the 3 way valve is fitted with an air exhaust filter/muffler, so that no dirt gets sucked into the ball valve and chamber when the vacuum is released.


The vacuum gauge is a small diameter gauge that I will be mounting facing upward on the 90 degree fitting from the rear mount of the valve so that the valve can be easily seen from the top of the vacuum chamber while I am looking down into the chamber at the degassing process. I am mounting the gage on it’s own inlet fitting. It is also possible to mount the vacuum gauge on a T fitting that also mounts the ball valve. A vacuum gauge is not absolutely necessary, as it is possible to judge the degassing progress visually, but it’s handy to see the vacuum drawing down on the gauge.


The forged black steel pipe fittings are special fittings meant to be welded or brazed to a steel tank. Later I will discuss alternatives for fitting vacuum connections to a plastic pipe or aluminum stock pot chamber.


Now I can get to work, and put all these parts together!

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cecrops props

Active Member
I’ve just worked on some miscellaneous parts for the vacuum chamber. The first photograph is a wire grate floor for inside the tank that I welded up. The second photograph is the beginnings of a new foot for the chamber. Since the original foot on the tank was too rusted, I cut it off. Here, I am making a new foot from a section of the other half of the propane tank. I have braze/welded in a section of flat steel rod all around the inside to beef up the edge. You can see the line scribed a few inches in from the edge on the outside where I will cut it off to create a ring for the base. Not too exciting yet.


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MatterMaker

Active Member
Hey! I made myself a vacuum chamber a few years back and I have a great dual stage pump I picked up from ebay. I can get it to a vacuum of 28" which is just under a full vacuum which is 29". If I bought a rebuild kit with new gaskets filters and springs, I'd be able to get a full vacuum. I found a two foot long section of 8" diameter pvc from a scrap bin at a construction site and it worked perfectly. I used a flat square piece of clear 1/2" acrylic as the top cover. Reinforced it with two strips crossed on top of it. For my hose lines, I used 3/16" ID (inner diameter) hose, thin so it doesn't collapse. I just drilled a hole sized for the hose into my PVC lid and sealed it in place with tube silicone. The gauge I did the same with. Directly into the lid and sealed with silicone. I did find I had a small leak in the seal around the gauge when I first tested it so all I did was spray around the edge with 3M #77 spray glue to seal it. The leak draws it right into itself while vacuuming, worked great.

I'd like to see how you decide to seal the top of your chamber. My solution was to make a silicone gasket for the open edge where the lid sits. I used 2 part RTV silicone. I first built a trough around the inner and outer edge of the chamber out of flattened strips of clay. Build right in place, on the opening edge of the chamber. Then I poured the mixed RTV right into the trough, up past the top edge of the chamber to the edge of the clay walls. I then took the flat lid piece and gently placed it on top being sure to not trap any air in the silicone. So the silicone, poured right to the top of the clay trough sticks right to the lid. Then I let it set for 24 hours. Peel the lid off of the silicone (as it doesn't stick to anything but itself) and pull off all the clay, and I was left with a perfect fitting custom silicone ring gasket for the top edge. Seals perfectly every time against the lid when you turn the pump on.

The most useful thing I did was to make a clear lid. This allows me to look into the chamber while running to make sure nothing is overflowing. Vacuuming silicone will expand 3 or 4 times it's volume so your container has to be big enough to hold it all. The other thing I did was to put one valve near the lid on the pump line and then a T junction and one more valve leading to the open air. So I close the release valve (leading to the open air), open the pump line valve, run the pump to full vacuum, close the pump line valve, open the air intake valve which is on the pump side of the line to release the pressure from the vacuum pump. I can then turn the vacuum pump off and the chamber is still under vacuum. This prevents me from overworking the vacuum pump. It's also better for the pump to shut it off without a vacuum being pulled on it. Hence opening the release valve before I shut the pump off. So it's very useful to have two valves so you can close the chamber but open the vacuum line at the same time.

Good luck with your build! Can't wait to see the final result.

mattermaker.
 
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asalaw

Sr Member
Do you absolutely need a full vacuum? Doesn't a large pressure differential alone force the bubbles out? What role does the viscosity of your medium play? Do more viscous compounds require more pressure difference? Trying to remember my high school thermodynamics here, but I'm completely crapping out, cuz 50.
 

cecrops props

Active Member
Do you absolutely need a full vacuum? Doesn't a large pressure differential alone force the bubbles out? What role does the viscosity of your medium play? Do more viscous compounds require more pressure difference? Trying to remember my high school thermodynamics here, but I'm completely crapping out, cuz 50.

Yes, viscosity does play a part. There are some low viscosity silicone mold compounds that do not require vacuum de-gassing (Mold Star 15, 16, 30 for example), but most do. A more viscous compound will require 28" of vacuum to degas.

- - - Updated - - -

Today i spent some time grinding the top edge of the chamber as flat as I could. I will have to devise a method to get it very even all the way around before I am finished, so that the lid will seal.


I also brazed on the two threaded ports on the front of the chamber, one for the valve and one for the vacuum gauge. I will not be doing very much on the vacuum chamber for the next couple of days while I’m waiting for a new jig saw from Amazon. I’ve got some more metal cutting to do and my old one is just not up to the task…


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cavx

Master Member
Do you absolutely need a full vacuum? Doesn't a large pressure differential alone force the bubbles out? What role does the viscosity of your medium play? Do more viscous compounds require more pressure difference? Trying to remember my high school thermodynamics here, but I'm completely crapping out, cuz 50.

I have not vacuumed silicons yet, but with elastomers, I found that all the magic (the raise and fall) happens at 29.5"HG of vacuum.

I double vacuumed a small amount the other day and saw the for the first time, the liquid 100% air free. It was at 30"HG and producing no bubbles at all.

Showed the kids that water boils at room temp in a vacuum and then it freezes (or in my case icy cold) when you re-pressurize.
 

cecrops props

Active Member
The new saw is very nice, it’s so much better at cutting steel than my old saw. I cut out some parts for the vacuum chamber…


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