[This is true, but -as Christopher pointed out recently- don't use them for evacuating aqueous solutions or slurries. If you plan to do this (for lost wax or plaster casting, for instance) you might be better off with a Kinnney, or another 'rotary piston' type pump, which are more forgiving of water contamination than vane-style pumps like the Welch.]
[If you're planning to build your own pressure system, be careful- these things can explode catastrophically. If you're not a certified welder, better hire one for this job...]
Dead simple. Find a vac-u-vin pump and stopper (sold for storing opened part bottles of wine). You can get down to about 1/4 atmosphere with these, which is enough to outgas urethanes (or rocket motors). Either use a piece of suitable copper waterpipe to adapt the stopper to your casting vessel, or cast in a small cylindrical vessel and just cut off a suitable wine bottle to make a glass funnel that fits over it. Chateauneuf du Pape works better than a bordeaux, as not all wine bottle are the same shape.
Fridge compressors are the next stage up. Then after that you're looking at S/H lab vac pumps. As chemists are brutal to vac pumps, you can sometimes collect slightly tired ones (typically mercury corrosion in an aluminium vane pump) for fairly little money.
Conley Casting Supply, Warwick, R.I. <phone>
I agree with Andrew- I was a pioneer in the use of high presure to cast fast setting materials like urethanes, and have set up systems for several companies. The safest way to set one up is to get an AUTOCLAVE. You can get these used at auction or at medical surplus salvage yards. You do not need the steam generator that often accompanies medical autoclaves (cheaper withotut them too) The nice thing about autoclaves is that they come ready made in almost any size imaginable and have the quick closing submarine style door required to seal them up in the short time that urethanes give you. They are tested and pressure rated right on the tank. The one in my studio I has a pressure regulator and a timer switch. Once I hit the presure button- it comes up to about 60 lbs/sq In and holds that pressure automatically for however long I set the timer for, then releases back to ambient. I rigged mine with a table that rolls out so that I can set up the molds on the table- mix , pour, slide in the table and slam the door in less than 30 seconds.
Because of the isocyanates, it is wise to vent the pressure release valve thru a duct leading outside the building. This alone can eliminate 70% of exposure to Isocyanates.
This pressure casting system, and adequately filling the resin results in nearly flawless urethane castings regardless of detailing and complexity. For slower setting materials like polyresin- vacuuming is still superior to pressure casting.
Also- to Gary- Synair corp makes a water clear urethan that is UV stable and will cast nearly optically clear straight out of a silicone mold,-(just a coat of gloss laquer will make it crystal clear.) It does require a post cure in a low temp oven, and it is reportedly tremndously toxic- but I have cast stuff in it and its phenomenal.
I should add that heated autoclaves are used for temperature setting resin casting. Certain epoxy and glass or graphite composites are heat /pressure cured. Another good example is Acrylic, which requires high temperatures and presures to cure optically clear without bubbles.
Acrylic is extremely hazardous to your health, I have mine cast by an outsource.
For Urethane Resins- you use pressure to eliminate bubbles, but you do not want to add any heat- urethanes set fast enough as it is. Christopher
I have had a (quite small) Welch for over a year, and use it to debubble alginate and plaster. I got it quite reasonably from Duniway Stockroom http://www.duniway.com/ as a used unit that was good enough to not need reconditioning. They were quite specific that a pump with phenolic vanes would sieze and die without a cold-trap for water vapor, but thought that a steel-vaned model would hold up. So far, no problems.
Admittedly, I change the oil frequently, and use it infrequently (perhaps 1000 minutes, 2-3 minutes at a time), and have an inexpensive roughing pump to get my chamber down to ~1/4 atm. YMMV, but at least you've got another data point.
Nice website. Well done.
I use IE5, and the images on your DIY Vacuum Chamber Page do not appear. I opened the page with Netscape Navigator, and most of the images appeared.
How do you like TufStone??
Michael Gainer wrote
No need to cuss.
Mike, have you done any testing in the weather with the GardenCast??
I like TufStone fine. The majority of my casting is done with Hydrocal White. TS has twice the compressive strength. About half the expansion. Identical castings weigh about 25% more. TS seems to really snap set at about 15-18 minutes if I remember correctly. HCW more gradually. HCW refines (carve, sand, file, etc.) really nicely, especially for the first two or three days. They both accept pigmenting real well. I like the fact that TS is available fibered or not. First time I ordered I didn't know it was available unfibered and they shipped 600 lbs unfbrd. At about $32 per 100#, I was not happy, but it did get used up eventually. What do you pay for TS in Calif?? HCW is about $18 per 100#, so the cost per cubic inch of product is significantly higher for TS. HCW (90# = 1 cu ft) / less than a penny per cu in. TS (112# = 1 cu ft) / a little over two cents per.
Use what you like and works best for you in each application.
If you don't mind I'd like to put a link on my ornamental plaster site to your site. I'm also going to build a page for homebuilt machinery and equipment and I would like to include your DIY Vac Chmbr page. Let me know if that's alright with you.