Hey, an engineering/materials question!

[angry.bob]

So.... I've decided I need a vacuum and/or pressure chamber. I want to build it out of 12" x 12" clear acrylic sheet in the form of a cube. So what I need to know is, will 1/2" thick be structurally sound at -29.92 inHg vacuum? How about 70psi of pressure? Or will I need 3/4"? 1"? Thicker? I MUST KNOW THESE THINGS! Mostly because sharp shards of acrylic being propelled into my torso and neck is not my desired outcome.

Thanks in advance for the help!

[Sky]

Is this some kind of fancy bong? Rock on! 

[angry.bob]

Sadly no, it's for degassing RTV and epoxy resin for moldmaking and casting, the pressurized aspect for pressure casting - though that's a secondary concern to the vacuum.

[bhodi]

How are you going to cement them together? That's what you need to be concerned about. If it's got to be a cube, the seams are going to be a bitch @ 70psi.

Also, what type of acrylic? Is it going to be laminated?

[angry.bob]

I'm planning on using WeldOn #40 to cement it. the acrylic is just plain old acrylic as far as I know. The place wants $9.00 for a 12" x 12" sheet and says it's 10X as strong as glass. They also have polycarbonate for twice the price and they say it's 250X as strong as glass. the equivalent strength of 125" of glass seemed a little like overkill, but so did 5" for that matter. 

If the pressurizing aspect is too big a pain, I can just drop it. It would be nice, but not worth spending a ton more money nice...

[Murgos]

I'm not a materials engineer but here are some things I would consider if you wanted to 'engineer' a solution.

Pressure is force per unit area.  So P = F/A.  If you know the range of P and the A of the design you can determine the range of forces (don't forget to account for atmospheric pressure, 1 Pa = 1 N/m^2), pay very close attention to units, if you use metric for all your calculations you should end up with F in Newtons.  If you don't really know P and are just guessing you will also need to consider the properties of the gas inside the vessel, P=nRT/V (ideal gas law) is probably a good start, you can look up R and n for most things and you know the range of V and the range of T.

You will probably need to request material data from your supplier on the physical properties of the specific acrylic you're using (i.e. something like http://www.polymerweb.com/_datash/pmma.html), whoever you are buying it from should have this data available.  You are probably more interested in the Flexural Modulous.  Do a little analysis with fluid dynamics and that should tell you how much the acrylic at the thickness you are using can bend before failing which should lead you to how much Force (above) you need to bend it that much which will tell you what pressures are safe.

Also, how do you plan to form the container?  One piece molded or some kind of adhesive to hold it together or maybe screws and a caulk?  Each method is going to have different properties and different failure causes.

The end result is that you will know exactly how much you can stress the vessel and the exact thickness of acrylic to use and a good idea of how best to assemble it.  Good luck!

---

Personally, I'd just build it, stick it behind a wall and test it well beyond (5x?, 10x?) what you ever anticipate it actually seeing in use.  If it fails, make it stronger.  If you're really worried about it bursting and it doesn't need to be mobile build a steel safety box around it.

[CharlieMopps]

right angles are bad when dealing with pressure. Does it have to be a cube? 70PSI is signifigant. I'm fairly certain you'll blow your seams.

[Chimpy]

right angles are bad when dealing with pressure. Does it have to be a cube? 70PSI is signifigant. I'm fairly certain you'll blow your seams.

That.

It would be difficult enough to make it airtight with (what I assume you have) standard tools/fixtures.

[Furiously]

Steel and welds.

I'd look at modifying an empty propane tank I think they are rated for 100-250 lbs.

[CharlieMopps]

the easiest and cheapest way to do what you want to do is... Steel.
Cheap stick welder: $75
Angle Grinder with cutting wheel: $15-$20
Steel: cheep as hell

You could use 1/8" steel and it would hold a couple of hundred PSI. It's pretty easy to weld an air tight seam. AND, you could make it any shape you wanted.

[Lantyssa]

Any way you can make it a bell?  We used a metal plate with a solid glass bell in my research lab.  Cylinders and domes are much more structurally sound when it comes to pressure.

Edit:

Maybe too small for your needs, but probably better to buy.  Do a google for vacuum bell jars.

[Salamok]

Steel and welds.

I'd look at modifying an empty propane tank I think they are rated for 100-250 lbs.

definitely stress the word empty and not just gee my bbq is out of propane empty either.

[Miasma]

I misread the first post as twelve by twelve feet and assumed you were building some crazy room to protect yourself from a chemical attack.

[Murgos]

the easiest and cheapest way to do what you want to do is... Steel.
Cheap stick welder: $75
Angle Grinder with cutting wheel: $15-$20
Steel: cheep as hell

You could use 1/8" steel and it would hold a couple of hundred PSI. It's pretty easy to weld an air tight seam. AND, you could make it any shape you wanted.

An even cheaper and easier way is probably to just go get an old hot water heater from a junk yard.  Pretty sure they rate them to over 120 psi.

[CharlieMopps]

He wants it 12"x12"
Hot water heater is a LOT bigger than that.

[Salamok]

the easiest and cheapest way to do what you want to do is... Steel.
Cheap stick welder: $75
Angle Grinder with cutting wheel: $15-$20
Steel: cheep as hell

You could use 1/8" steel and it would hold a couple of hundred PSI. It's pretty easy to weld an air tight seam. AND, you could make it any shape you wanted.

An even cheaper and easier way is probably to just go get an old hot water heater from a junk yard.  Pretty sure they rate them to over 120 psi.

What am I missing isn't this just an air compressor tank (that can be had for under $100)? or does he need the ability to open it up and place things inside?  If the latter is the case i'd stay with the heavy duty acrylic box or the bell jar.

[Draegan]

You don't make pressure vessels in the shape of cubes at those pressures.

Spheres or Cylinders.

Tell me exactly what you want to do, since I deal with compressed air in my job, and perhaps I can dust off my Mech. Engineering degree and help you out.

[rattran]

This is one of the things I do for a living. We bubble/degas rtv with a piece of 12" pvc coupler as a vacuum chamber, using 2 rubber gaskets and 3/4" lexan for the bottom and top. Fitting screwed into the side of the pvc for the pump.
Basically this http://www.alumilite.com/ProdDetail.cfm?Category=Equipment&Name=Vacuum%20Chamber but 14" tall.

<edit> of course, just for vacuum.

[Draegan]

You have to do something like that but with a flange coupling on either end of the cylinder to be able to contain 70psig.

[angry.bob]

Once again, thank you all for the help. Like I said, the pressure aspect would have been nice, but I can get rid of it if need be - the vacuum is what's important. Adequately degassing the RTV and epoxy should eliminate the need for the pressure casting, I just thought it would be handy to be able to degass the epoxy in the mold and then compress it in with 50-60 psi.

For the vacuum aspect of it I Have a Schedule 80 PVC 10" coupling I got for a measly $20 at a yard sale to make something similar to the Alumite thing Rattran posted if worst comes to worst. A bell Jar would work too, but some of the molds I need to make are 11" x 6" and bell jars that big are really expensive. I thought it would be nice to have something like these for doing basic science stuff with my kid, but without spending several thousand dollars since the materials appear to cost about $100 (on the surface at least).

If I do decide I need to pressure cast too, It sounds like it's easier and safer to buy one of the hobby pressure casting pots for a couple of hundred bucks.

[Chimpy]

I thought it would be nice to have something like these for doing basic science stuff with my kid, but without spending several thousand dollars since the materials appear to cost about $100 (on the surface at least).

Those boxes answer your original question about if 1/2" acrylic would be sufficient:

All models are fabricated of transparent 1" thick acrylic

Just doing a quick ole McMaster-Carr search, 12x12 sheets of 1" will run you around $40 each. Polycarbonate would cost you about $200 per 12x12 sheet @ 1" thickness. McMaster is usually a little more expensive than you would pay to get a piece of acrylic from a local plastics supplier, but they are usually a better gauge of how much things cost than "random google search z".

[angry.bob]

All models are fabricated of transparent 1" thick acrylic

Doh, i must lern2read

[CharlieMopps]

Pressure and Vacuum are going to have the same effect on your chamber. Either one will probobly blow the seems on what you plan on fabricating. I've worked with Polycarb and Lexan before and it just plain sucks to try and make right-angles joints with. You have to drill a whole through the edge of 1 sheet (which cracks as soon as any load is out on it) and then tap the joining piece to run screws through to it... which will eventually strip out. Even with epoxy on the joints the entire thing will fail fairly quickly when any angular stress is put on it. Someone correct me if I'm wrong here... but 70PSI x 12 x 12 = 10,080 Pounds of force pushing out on each side of that box.