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Daedronus

Fri Nov 09 2012, 10:23AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

I need to make a view port on a vacuum chamber and I'm having a bit of trouble sourcing (at a reasonable price) a thick 6" glass disk and I was thinking of bonding a couple of 6mm thick disks with some soft of epoxy.

Would the laminate have comparable pressure resistance with a single piece of glass?

macona

Fri Nov 09 2012, 09:16PM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

How thick are you looking at? You would be surprised at how little you need. A full vacuum is only about 15psi.

Daedronus

Fri Nov 09 2012, 09:52PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
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Only 15psi means about half a ton for my view port area

I'm looking at about 14cm diameter window, 20mm of glass should be enough.

The only local company that I found want's about 80euro for a piece.

But I can get 6mm thick glass for a couple of bucks, cut to size, etc....

Ideally I would want a 300mm x 25/30mm disk, to have all of it clear.

Lower flange is ISO250, small one ISO63
Now I need a top.
I can have the aluminium stock machined for a smaller view port...or I could use a all glass view port.

1352498254 2329 FT1630 Img 4480p

klugesmith

Fri Nov 09 2012, 11:22PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Daedronus wrote ...
Would the laminate have comparable pressure resistance with a single piece of glass

Depends on the shear strength of your bond between glass surfaces.

Like sitting on a long bench made of two 3/4 inch thick boards, vs one 1.5 inch thick board.
Two layers of single thickness, if not connected together, have 2x the stiffness and 2x the strength of a single layer.
A single piece of double thickness (or two layers adequately connected for shear) has 8x the stiffness and 4x the strength.

If the 6mm stuff is so cheap, how 'bout doing a proof test with a single layer and full vacuum (or maybe even positive pressure -- could be hydrostatic). You could measure the deflection to validate your application of strength-of-materials formulas, e.g.

Whatever one layer withstands, it would be conservative to expect multiple layers to withstand greater loads in direct proportion to overall thickness. The glue only needs to prevent air at ambient pressure from getting between the layers.

I can attest that acrylic plastic 12mm thick is plenty strong for a 150 mm diameter round window into vacuum. It deflects about 1/2 mm in the center.

As for adhesives, you might search the internet for lens cement and UV-curing cement.

p.s. the Internet page I cited mentions "Circular Plate Defection Moments and Stress".

klugesmith

Sat Nov 10 2012, 03:47AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Bumping my own post w/ new information about Circular Window Thickness.
Hope you don't mind somebody investigating and learning on your behalf. Like you did in response to Gren's transformer design calculator.

This online calculator gives the minimum thickness of circular windows made of fused quartz.

Is says that for unsupported diameter of 140 mm, thickness should be at least 9.5 mm.
Doesn't give the formula, but does say it uses a design stress of 1000 psi.
The result agrees with this online calculator:

.

Another reference says "Rule-of-thumb tensile design strengths are typically at 1000 - 1500 psi for nominal glass materials." Yet another says "For most types of glass a nominal strength of 70 MPa (10000 psi) and a design strength of 7 MPa (1000 psi) are typical."

Here are two references that give the formula:

.
The latter includes an Excel calculator pre-loaded with material strengths.

Looks like two layers of 6 mm glass would do it, without depending much on bonding strength (see previous post).
Not because calculator says 1 atmosphere requires 9.5 mm -- that would have to be one piece, or well bonded laminate.
But because calculator says 1/2 atmosphere requires 6.7 mm, with a good safety factor.

Hey, since you presumably have a vacuum pump, you might use a vacuum-bagging process to "clamp" your glass disks together while the laminating adhesive cures. How come you're using glass and not, say, acrylic plastic?

p.s. That's a very pretty stainless steel fitting you've got. Did you weld it yourself, or have it made specially?

johnf

Sat Nov 10 2012, 07:53AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Nothing wrong with 20mm thick perspex

also some polycarbonates ok but some sublime making getting a good vacuum difficult

Ash Small

Sat Nov 10 2012, 10:10AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Depends on vacuum required. All plastic (perspex, etc) outgasses to some extent.

Daedronus

Sat Nov 10 2012, 11:16AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

Thanks klugesmith, don't mind at all.

Ebay made it :)

I try to stay away from plastics for two resons:
They can outgass/sublimate/etc...
and, they react badly to plasma

I try to keep this as clean as possible, I'm aiming for single digit microns with my mechanical pump, or as low as possible if I also use my diffusion pump....

Patrick

Sat Nov 10 2012, 06:45PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

you could use 6mm of glass facing the vacuum, then Acrylic 9mm or more, for safety and strength. then epoxy them together. if you dont abraid teh glass you may get future parting as the epoxy peels away though. if you do abriad the glass it may may things less transparent... a single thick pane of glass would be best.

2Spoons

Sun Nov 11 2012, 09:05PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Abrading the glass would be a mistake - it would result in thousands of crack initiation points. You simply need to choose an epoxy with good glass bonding properties (as used in fiber-glassing), use a thin layer, and make sure the glass is absolutely clean before gluing.

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