Vac chamber for degassing

Getting ready to cast new bumper finials for the Roadster. They are the rubber trim pieces the rear bumper ends fit into on the body. Have
my plugs and molding boxes ready. I need a small vac chamber to degas the mixed molding compound. I would rather make one (satisfation) than buy one (expense); was thinking a capacity of 1 pint or so container. How about HDPE shedual 80 pipe and end caps? I have a good enough vac pump, and don't really need to see the degassing; just pull it for 1/2 hr or so. I was thinking glue the bottom cap, and just have a sliding fit of the upper, with an O ring to seat against. A fitting with valve for the vac pump, open it for re-pressure. Ideas? JR Dweller in the cellar -------------------------------------------------------------- Home Page: http://www.seanet.com/~jasonrnorth If you're not the lead dog, the view never changes Doubt yourself, and the real world will eat you alive The world doesn't revolve around you, it revolves around me No skeletons in the closet; just decomposing corpses -------------------------------------------------------------- Dependence is Vulnerability: -------------------------------------------------------------- "Open the Pod Bay Doors please, Hal" "I'm sorry, Dave, I'm afraid I can't do that.."
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On 6/21/2010 8:38 PM, JR North wrote:

http://www.zaccardis.com/bean-vac-vacuum-coffee-canister.html http://www.beanvac.com/getting_started.htm http://cgi.ebay.com/Bean-Vac-Coffee-Canister-ED150-NEW-/300422919295?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item45f299f47f
Might work in a pinch.
--Winston
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Just might...I have my doubts the little battery powered pump can pull a really decent vacuum. Mebbe I can tweak it to use my pump. Inner liner is removable; it's ABS, might be hard to clean the compound out of it without damage. Mebbe wax the liner, and peel the compound out after setting. JR Dweller in the cellar
wrote:

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On 6/21/2010 9:53 PM, JR North wrote:

I see that the bottom 2/3 of a plastic 1/2 gallon jug is a very loose fit inside the liner. Would make a great 'liner liner'.
I would be tempted to take out the liner and use it as a form to make liners out of heavy aluminum foil.
A 4" dia. PVC end cap is a very loose fit inside the liner.
--Winston
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Take the PVC pipe approach, a piece of clear acrylic / polycarbonate sat on the top with an o-ring or some silicone grease would hold the seal and let you see whats going on inside.
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kelly wrote:

I think you had better use the "window" to see what is going on. I have seen degassing being done and if you don't monitor and control the vacuum it's going to "boil over" and you'll have a terrible mess in the chamber. ...lew...
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Even then...
I bought a vacuum oven cheap at an auction that does have a large window and was caked nearly an inch thick on the bottom with many thin layers of spilled plastic. It all came out fairly easily with a sharp chisel.
jsw
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Many moons ago I used to work in a physics dept research lab at a university. When we needed to degas potting compound and then pot electronics assemblies we used an aluminum tube about 10" in dia and a foot long with 1/4" walls. The bottom end had a 1/4" thick aluminum plate welded on and the top had a o-ring RTV'd around the outside. A piece of clear 1/4" plexy set on the top and we were good to go. While the vacuum was being pulled the plexy bowed in but never broke. A tube fitting was installed in the side of the tube for the plumbing access. As you guessed the operation created somewhat of a mess on the inside but it cleaned up quite easily. Be careful how fast you let air reenter as it will create quite a lot of turbulence inside. Art
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    That sounds like something which would work. I would first suggest that you test it empty and pump on it a half hour or so to see how it does.
    Hmm ... does the molding compound have anything which could soften the HDPE? If so, then you want something else. Perhaps make a second test with a small amount of the compound smeared on the ID of the Sched 80 pipe. A similar test -- but perhaps a full hour to give it time to soften things if it is going to.
    Good Luck,         DoN.
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On Mon, 21 Jun 2010 20:38:32 -0700, JR North wrote:

Thrift stores sometimes have old pressure cookers; maybe one would work as a vacuum chamber...
--
jiw

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The rubber sealing ring wouldn't work. It's a sideways V pointed out. Maybe one of those really old ones with the screw clamps would work but I don't know what the seal looks like on them. karl
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I tried one, with a new gasket, and found that it did hold a vacuum. I had to remove the fittings and retap the holes to pipe threads.
When the cooking time is done you cool them under the faucet, which condenses the steam into a vacuum and would suck in water if the gasket leaked. The lid compresses the gasket pretty well, they aren't too easy to open.
jsw
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They are made for both pressure and vac. The pressure is greatest. The heavy weight on top that spins, drops down with the lack of pressure and prevents the air from re-entering the chamber as it cools. It would depend on the specific model if it could be rated to a very low torr value.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net "Our Republic and the Press will Rise or Fall Together": Joseph Pulitzer TSRA: Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
On 6/22/2010 6:04 AM, snipped-for-privacy@gmail.com wrote:

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On Mon, 21 Jun 2010 20:38:32 -0700, JR North

Hello JR:
Some amateur rocket folk make solid propellant** which may be degassed under vacuum. A good 5 quart stainless steel mixing bowl (e.g., Kitchen Aid mixer bowl) with a thick polycarbonate lid and a rubber ring as a gasket will generally take full vacuum.
Also: Search for "Blue Sky Rocket Science". He shows a heavy plywood box that encloses a KA mixer for vacuuming while mixing. Several coats of good paint to seal inside and outside, if memory serves. -- Best -- Terry **To those who are about to write "these guys will kill themselves", ammonium perchlorate composite propellant (APCP) can be darned hard to light and does NOT 'function by explosion', it just burns nicely. Last year the two big hobby rocketry organizations (TRA/NAR) finally won a 10 yr suit removing APCP from the BATFE explosives list...
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wrote:

Does anyone still use the sulfur/zinc powder mix for propellant? When I was interested in it, as a kid, that was the standard. _Scientific American_ gave the proportions and mixing instructions. IIRC, it was just 50:50.
--
Ed Huntress



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I made lots of engines with Zn/S propellant in my yout. Dangerous/unpredictable burning. One turned aluminum engine 3/4" ID, 1/8" wall, 4" long with machined-in nozzle, cast Zn/S, literally detonated at ignition when fired. Good thing it was in a concrete pit I made for just such occasion. Blew it to pieces. JR Dweller in the cellar
Ed Huntress wrote:

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Hmm. So they cast it? The _SA_ instructions were to tamp it as powder.
--
Ed Huntress


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You can do it either way-cast Zn/s is made by melting the sulpher, and stirring in the zinc. Has to be pretty hot, and the motor case preheated if possible. It's like molten wax-shrinks considerably on solidifying. The tamped propellant has it's own set of problems-mostly disintigrating under G forces, with resultant increase in burning area-BLAMMO. JR Dweller in the cellar
Ed Huntress wrote:

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Something tells me I was better off sticking to tying flies and flying control-line models. d8-)
--
Ed Huntress


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You're thinking of Candy fuel. Zn/S heated to the temperature that will fuse the sulfur will start to react immediately -- injuring you.
There are solvents that will dissolve sulfur, but the traditional way to treat Zn/S so it will not break up during thrust is to VERY slightly dampen it with a tiny bit of alcohol (just for processing, to keep down the dust), then pressing it under hydraulics to about 6800 psi on the composition (which makes the sulfur go amorphous, acting as if it had dissolved, but at a much lower temperature). When it dries completely, which takes weeks, it becomes a solid grain than can be lightly machined, and will not break up easily.
But --- WHY? Zn/S has one of the lowest impulse figures of any fuel available. APCP is a strong fuel, inexpensive, easy to make, and fun to shoot.
Despite what a prior poster said, it's NOT "difficult to light". It burns like Hell's Fury with the mere touch of a match. But it was accurate to say it doesn't function by exploding, and has a shallower P/R curve, so it doesn't run away in a casing as easily as some other sorts of propellants.
LLoyd
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