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Weekend_Scientist
02-06-2015, 04:05 PM
Hey Guys,


I'm in need of some engineering advice. I'm designing a lid for a small vacuum chamber.

Constraints:

Material must be transparent

Must span a circular opening of 12"

No NASA budget, can spend up to ~$200 to make this happen but would be happy to spend a lot less

Material must be suitable for vacuum of 0.001 Torr (most plastics should be ok, wood is not acceptable)

No holes will be drilled inside the 12" circle and no fittings will be screwed into the lid.

I intend to mill an O-ring groove into the lid. This will be ~1/4" deep ~1/4" wide at ~12.5" diameter.


Atmospheric force bearing down on a 12" circle is:

(6^2)*pi*(14.7lb/in^2) = 1663 lbs of force


My gut instinct says go with 1" thick cast acrylic. Is that sufficient? My other choice is maybe half inch thick polycarbonate. I would expect that to bow in a bit but be stronger and more shatter resistant.

Any comments?

Thanks!

jep24601
02-06-2015, 04:28 PM
A 10" diameter is only about $125 with shipping:
http://i.ebayimg.com/00/s/NzA4WDcwMA==/z/X-MAAOxy4YdTT-7Q/$_12.JPG

I would have thought you could find a 12" for around $200:

http://i.ebayimg.com/00/s/Nzg4WDEwMDA=/z/O~QAAOSw0e9Ut~Ks/$_57.JPG

Here's a 12" for $170 including shipping

macona
02-06-2015, 04:55 PM
I have used 3/4" on larger chambers and it was fine.

Normanv
02-06-2015, 04:58 PM
Is this of any use?
https://www.youtube.com/watch?v=zmWYGb0EB-8&list=PLUK8GR4xS78uz-JDvyEazNJZFeV3Y6l4o&index=16

Weekend_Scientist
02-06-2015, 04:59 PM
Thanks Macona.

JEP24601, I appreciate the input but I'm just trying to nail down the plan for the lid. The rest of the vacuum chamber is already taken care of. I don't want to use a pot because I have a lot of electrical pass throughs and stuff.

Thanks!

Fasttrack
02-06-2015, 05:14 PM
Ditto what Macona said. I had a 12" window made from 3/4" acrylic with a number of feedthroughs and no issues. It was routinely pumped down to 10^-6 torr.

sandiapaul
02-06-2015, 05:36 PM
DO NOT use acrylic, polycarbonate only. Why the emphasis? I had an acrylic one impload on me while degassing some silicone. Still remember it clearly as I was intently watching the bubbles when it happened. Mine was 1/2" or 5/8" on about 10" diam.

Also I just used a piece of rubber sheet, about 1/8" thick or so and it sealed quite well, no need for the o ring, not sure it will work for the level you are pumping to though.

CalM
02-06-2015, 09:54 PM
Acrylic is so unreliable. a wiff of acetone, and it just goes crazy! ;-)

Thicker is always better! No need for an o-ring grove, a step will do, and so much easier to polish up, to better assure a good seal. O-rings super glued into rings from stock work fine. Just wet sand the joint if not made "perfectly".

As for strength as related to vaccum level? There is no real difference between 99% of 1 Bar and 99.9999% of 1 bar.

darryl
02-07-2015, 01:41 AM
Agreed- high vacuum is no different than 99% or whatever, as far as needed strength goes.

My vacuum pot is about 8 or 9 inches across. I used 3/8 polycarb and domed it by heating in the oven, then pressurizing to raise the dome shape. I haven't used it a lot, but no problems.

tc429
02-08-2015, 03:38 PM
might sound silly, but tv picture tubes are very high vacuum, faces are very thick... a 36" tube can weigh hundreds of pounds...get a old monitor, bust the vacuum tit off the rear, sandblast the 'bell' of the tube off, sand it flat, seal it in a steel lid...just dont drop it :)

HAP
02-08-2015, 06:32 PM
might sound silly, but tv picture tubes are very high vacuum, faces are very thick... a 36" tube can weigh hundreds of pounds...get a old monitor, bust the vacuum tit off the rear, sandblast the 'bell' of the tube off, sand it flat, seal it in a steel lid...just dont drop it :)

Can you provide a little more details on this? Perhaps a sketch?

Thanks,
HAP

darryl
02-08-2015, 07:13 PM
I think you'd have quite a time extricating a usable piece of picture tube from the whole. There's no doubt the face can withstand full vacuum, but you'll need a pretty good cutting technique to get through the entire operation without a hitch. Besides that, the face may not be a good representation of a fraction of a sphere- being rectangular you might find that even if you cut out a round of the size you need, it may not leave a flat suitable for sealing.

FWIW, my technique for doming the polycarbonate was to first make a round from mdf, then another round with a nice round hole in it. The bottom round would have a hole in it through which you could apply some air pressure. I used a valve from an old inner tube, and epoxied that into the mdf, after a through hole was made. I laid a piece of polycarb in the oven, set on low- about 150f or so, then left it for several hours. That's low enough that it doesn't soften, but hot enough to eventually drive out all moisture. That's important.

Then it's all laid up as a sandwich - bottom round, polycarb, top ring. Many holes were predrilled in the top ring. Once it's sandwiched, I drilled through the plastic in two opposite spots, then fastened it with wood screws. The rest of the holes were then predrilled through the plastic and then filled with wood screws.

I set the oven to 275 or so then let the assembly heat for about an hour. A couple times I pulled it out early so I could test using a bicycle pump to see whether the plastic was soft enough to form. The plastic was not soft enough at the first test and I had to let it heat more. The end result I was looking for was a fairly low distortion at the edges of the hole as the polycarb was raised by air pressure. The thing won't be ready until you are getting a good whiff of the gasses from the mdf.

By the way, you will probably be tempted to not use enough screws to hold the sandwich together. One screw every inch is not too many- there will be enormous pressure pulling on each hole as the dome is raised, and you don't want multiple local distortions to foul up the visual once you begin using it as your vacuum chamber cover.

deschmid00
02-13-2015, 05:48 PM
At 12" the surface area is 113" x 14.7psi (dang near full vacuum) you got some serious pressure! We have lots o vacuum chambers at work, lexan doors but thick! Like 1.5" for a 12 x 12 opening

kf2qd
02-13-2015, 10:32 PM
At 12" the surface area is 113" x 14.7psi (dang near full vacuum) you got some serious pressure! We have lots o vacuum chambers at work, lexan doors but thick! Like 1.5" for a 12 x 12 opening

Sounds like a rectangular window. Forces are worse on a rectangular window than on a round window.A round window is evenly stressed, a rectangular window is not.

the 1700 lb force is less a problem for the window than for the vessel that you are evacuating. the plastic will only see -15PSI at any point on its surface. that 1700 lb force is distributed across the window and not in just one point.

macona
02-14-2015, 12:04 AM
1.5" is massive overkill for a door.

Virtually every vacuum pot I have seen has used 3/4".

You can find the calculations to figure the stress on page 491 of this book:
https://vinodhchennu.files.wordpress.com/2013/04/arthurp-boresi-advanced-mechanics-of-solids.pdf

Poisson's ratio for acrylic is .35 and PC is .37