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What's the best off-the-shelf solution for a vacuum encapsulation chamber?

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Steve Conner

Thu Oct 29 2009, 07:06PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Paraffin wax is cool. You can vacuum impregnate it in while it's molten, then let it cool and solidify. I think Tesla himself used this trick.

jpsmith123

Thu Oct 29 2009, 07:11PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well I went to a local company that supplies restaurants, and all of their aluminum stock pots had riveted-on handles, and not the welded-on handles I was looking for, so I didn't buy anything.

I suppose I'll take a chance and get the food storage canister.

@Teravolt: Paraffin with its low melting point and high thermal expansion is probably ok for quick kitchen table experiments, etc., but for what I'm doing, especially with the time and money invested so far, I need something better suited to the purpose.

Edit:

Today I happened to find a hermetically sealed canister made by Oggi in a department store (although it was a smaller model than the one I linked to above), and unfortunately, it just doesn't seem robust enough to handle full vacuum. Mainly, the lid seems too thin to accomodate the over 700 lbs of force that will have to be supported as the air gets pumped out.

So it looks like I'm back to the stock pot and plexiglass sheet idea.

jpsmith123

Mon Nov 02 2009, 01:52PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

This is an update for anyone that may be interested.

The only aluminum stock pots I can find that have spot-welded handles (rather than riveted-on handles), are those made by Vollrath; like the following for example:

If this will suffice for a vacuum chamber (and it seems to me it should), then the two remaining issues are the gasket and the lid.

As it turns out, the 8, 10 and 12 quart Vollrath stock pots all have a 10" ID (and I think the wall thickness is close to 0.125"), so a standard off-the-shelf L-gasket for a 10" bell jar may work.

The last issue is the clear plastic cover. It seems that polycarbonate sheet is available in 12" x 12" x 1/2" sizes for $20 to $25 (for greater thicknesses the price gets ridiculous); and acrylic sheets are available in 3/4" to 1" thicknesses for about the same price.

Having never worked with such thicknesses of these plastics before, I have no feel for what would be required here. Would 1/2" thick polycarbonate be stiff enough and strong enough to reliably seal a 10" OD vacuum vessel, or would it be better to use 3/4" or 1" thick acrylic?

klugesmith

Tue Nov 03 2009, 04:16PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

jpsmith123 wrote ...
The only aluminum stock pots I can find that have spot-welded handles (rather than riveted-on handles), ...

Would 1/2" thick polycarbonate be stiff enough and strong enough to reliably seal a 10" OD vacuum vessel, or would it be better to use 3/4" or 1" thick acrylic?

1. Are handle rivets a big problem? I would think it very easy to seal them against leakage, using (for example) epoxy or silicone on the outside. Just makes it ugly when you use the pot to cook for guests.

2. A) wait for someone that knows the answer.

B) Work it out as a standard strength-of-materials problem -- see handbooks or online calculator such as

C) Test an inexpensive 1/4 inch plate at proportionally lower vacuum. I think you get same stress at 1/4 of the vacuum, and same deflection at 1/8 of the vacuum.

D) You might be surprised how much stiffer are steel and Al compared to strong plastics. If 1/2 inch polycarbonate deflects too much, it could be reinforced with a metal bar across the diameter.
Or make a lid from metal plate, with plastic windows of less then 1/2 the pot's diameter.

MinorityCarrier

Tue Nov 03 2009, 05:02PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

If you can get 3/4" or 1" acrylic for little extra cost, I'd go with that. LAM AutoEtch plasma etchers used 3/4' Acrylic plates to cover their loadlocks, these run at millitorr vacuum. You could see a little surface deflection during pumpdown, but not much.

The advantage of thick plastic is the ability to machine threaded holes for ports, without losing too much structural strength.

Clear acrylic covers also allows one to see what's going on inside their vacuum chamber, something I personally prefer. Watching RTV12A silicone degas for example; when it's done frothing, and the suds collapse down to the liquid I know its ready to be removed from the vacuum chamber.

jpsmith123

Tue Nov 03 2009, 11:35PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Hello Klugesmith:

In the sense that I hate to introduce unnecessary variables into a vacuum system, yes, I see the rivets as a problem...so I decided to spend the extra $10 or so for the Vollrath pot rather than bother with with sealants.

As far as the plastics go, theory and calculations are always a useful guide, but nothing beats real-world experience/examples, IMO.

@MinorityCarrier:

It's nice to know that acrylic plates were used in a commercial vacuum system. Do you know roughly how big the 3/4" plates were in the example you mentioned?

MinorityCarrier

Wed Nov 04 2009, 05:38AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

It was a ~18cm by ~28cm rectangle. I'll take a photo of the loadlock cover of a LAM Rainbow plasma etcher tomorrow and post it, it's a bigger piece of plastic.

MinorityCarrier

Wed Nov 04 2009, 08:35PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Here's a photo of the plastic loadlock cover for a LAM Rainbow etcher. The plastic cover is 32mm or 7/8" thick. The ruler gives a sense of area dimension.

The loadlock chamber is pumped down to 200mT, the pressure crossover point where the main process chamber door opens up.

Scott Fusare

Wed Nov 04 2009, 09:42PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

Now that brings back memories. Spent 12 years working as a field rep for Lam. Rainbow 4600s were my specialty.

Can't say I miss the semi industry much.

Scott

MinorityCarrier

Wed Nov 04 2009, 10:57PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

We don't have anyone here who knows how to rebuild the LL harmonic drives. You probaly understand the ordeal I am experiencing trying to develop processes on this tool, but at least I am still employed.

BTW, the plastic cover looks to me to be Lexan, one of our Maint folks thinks it is acrylic, but it has the Lexan bluish cast. Since you were in LAM Field Service, what is the plastic this cover is made of?

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