Well, Bill Shuey was right. The lid blew off. Fortunately it blew off during a pressure test after I had convinced Dad that it might not be safe, so no one was leaning over it and no one was hurt.
I went over for lunch today, and talked him into following the advice of Greg Reynolds and filling it with water for a pressure test. I carried it outside and set it on a chair outside the basement door, filled it with water, and ran the hose from the compressor out to it. My idea was that we would fire up the compressor from the safety of the basement, with the concrete wall as our bunker to protect against any fragments if it shattered. Instead, Dad fired it up while I was still outside, having just tightened the lid and checked the connections. I had not quite made it to the basement door when the lid blew off, but lucky for me it went straight up and didn't come anywhere near me. I think it was at about 80 psi when it blew.
I think now he is convinced to buy something a little more substantial and to check the pressure rating before hooking it up.
FYI Harbour Freight (look them up on the internet) has 2.5 gallon paint pressure pots for under $75. IIRC they are are rated to 80psi. I have one and its fine for most small castings. Its dimensions are
I take it the "fingers" chunked off? Dad's in a hurry isn't he? Whether you're blowing up models with fire crackers or destruction testing equipment always start the camcorder FIRST! You'd be entertaining people for weeks! Cheers,
Well, Dad had his fun and no one died! Sounds like a success.
Buy the paint pressure pot.
No, you can't use a vacuum chamber for a pressure pot.
1) The seals are built to go the other way.
2) A vacuum chamber is only rated for 1 atmosphere, i.e. 14.7 psi.
Some of the responding posts were a little confused about when to suck and when to blow in the casting business.
1) You want to "degas" the RTV silicone BEFORE you pour it to make the mold. This is to remove all the air entrained from mixing it. You need a laboratory grade vacuum pump (and bell jar) to this properly. The uncured silicone will expand to nearly 10X its original volume and then "collapse". After degassing, carefully "paint" the first layer of the silicone onto your pattern and brush out all the bubbles from nooks and crannies. Only pour in the bulk of the material after all the pattern surfaces have been "wetted".
Resist the temptation to pull a vacuum on an uncured mold after filling the mold box. This just makes a BIG mess and you'll have more bubbles than you ever thought possible. It seems logical, but it just doesn't work. All it seems to do is condense a lot of microscopic air pockets on the surface of the pattern into larger objectionable ones. The other thing that happens is that it drives silicone under and into tiny cracks where you'd rather it didn't go. Particularly distressing when making a two-part mold.
I guess you could put the uncured mold into the pressure pot. I've just never found it necessary. I'd be more concerned about driving the silicone into places I don't want it to go.
2) You want to pour your resin into the mold and pressurize it while it cures. Again, you want to fill the mold carefully using a brush or toothpick to work out the big bubbles as you go. You should have only tiny bubbles in your filled mold. When you put it in the pressure pot, they vanish. How much pressure? As much as you can SAFELY handle. I know some folks who pressurize their EXTREMELY STRONG pots with welding gas tanks at 1000's of psi.
You CAN experiment with removing some of the gross entrained air BEFORE pouring by using a low vacuum. A high vacuum can, as some posters noted, make the stuff boil. The boil can even cause some resins to cure instantly. It's better to figure out how to mix and dispense the resin to minimize the entrainment of air. You can use bits of nylon stockings to filter out large bubbles. You can also "pour" the resin from the bottom of the container with syringes or funnels. Commercial mixing machines are available which can mix resins in the correct ratio without exposing it to the air at all. (One inexpensive example is the "static mixing nozzles" found on Devcon products at the hardware store.)
Again, resist the temptation to pull a vacuum on a mold filled with uncured resin. Sounds logical. Doesn't work. The same effects takes place.
If you're getting more than a 3X expansion you whipped it instead of mixing it. I average a 2X expansion.
A properly made mold and properly degassed RTV can be vacuumed after pouring into the mold without causing those problems. The only problem I have is finning on the outside of the mold if I make the mold box out of Lego.
65psi is sufficient for all the hobby resins and most industrials as well.
Well, the 10X figure WAS just shooting from the hip! I can't lay my hands on the Corning literature at this moment, but it was sure more than 2 or 3X. The only figures I could find quickly on the web quote
6X. If you are only getting 2 to 3X, I you may not be using an RTV silicone or your vacuum pump is not really degassing it throughly - although it is obviously working fine for you. If your pump has a rating like "29 inches of mercury" (a gage scale - relative to atmospheric pressure) it is a roughing pump. If it has a rating like "10 torr" (an absolute scale - relative to a perfect vacuum), it's a true high vacuum pump. Again, what you should see is the RTV foam up quickly to "many times" it's original volume, the bubbles break abruptly and the mass fall back to nearly its starting volume. The whole process takes only about a minute. If you see the RTV expand only 2X and it bubbles away for as long as you keep the vacuum on, you aren't giving the goop the full treatment. But if it works, it works!
For a solid master and a simple pocket or open face mold, you are right. But again, why? If your RTV is properly degassed and the mold filled carefully, why deal with the messy expansion again? (And it will expand again. You'll be pulling gas off the surface of your patterns and mold box.) I tried it and always found it made things worse. But, I was making a lot of two part molds with the pattern half buried in clay or on a mold-board to create a complex parting lines. Throwing it back in the chamber would just pull air out of nooks and crannies I just as soon RTV not go - like behind my pattern! This was particularly a problem on the second pour as any RTV that "leaked" around passed the parting line would bond to and ruin the first half of the mold.
Not to mention SAFER! I think the guys going over 1000psi were just doing it because they could. But, even placing safety first, more pressure IS better. If we call 65psi about 4 atmospheres, then the bubbles will be 1/4 their original volume when placed inside the chamber. However, that isn't quite as good as it sounds. 1/4 the VOLUME is actually only about 5/8 the DIAMETER - less than half. Pressure pot or not, you still need to avoid bubbles at every step.
Try using an old autoclave available at most surplus medical supplies. My company has two that have been cycled "pressurizes and depressurized" thousands of time. You can find a small one for about 100.00 to 250.00 make sure the pop off valve reads a least
50 psi. You will need about 25 to 35 psi for most resins sometimes as much as 80 psi but that is rare mostly for very thick resins. My company is masterpiecemodels we go through at least two
55 gall>Well, Bill Shuey was right. The lid blew off. Fortunately it blew off
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