I've always wanted a good vacuum pot!

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Lloyd
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I've always wanted a good vacuum pot!

Post by Lloyd »

When I worked in the transformer manufacturing industry in 1969, we had a vacuum 'chamber' for de-gassing epoxies in which transformers were potted.

I built a TINY little one a few years back, just to test some principles, but never had a 'real' pot.

Yesterday I built one. I first tried a really-stout 2.5 gallon(US) stainless steel bucket. I couldn't deform it by hand, so it looked to be the perfect receptacle. I made a nice Lexan cover for it, and fired it up. It held until I reached about 20" Hg below atmospheric, then collapsed like a cheap tin can!

So I fetched an old R-22 refrigerant bottle, cut the top off, and spent a few minutes first grinding then filing the top edge to make it smooth. The metal is so thin that it cuts my gasket material, so I have to work on that. BUT... I pulled it down to 25"Hg, and it held.

Today, I degassed about 3lb of 2-part silicone I'm using for a customer's mold. It took longer than it should have, but I used up less than half the pot-life of the mixture getting it fully de-aerated. The mass swelled to almost 4 times its volume! Wow. But then it subsided, and all the bubbles were gone after about 50 minutes (2-hour pot life).

Next... weld a smooth 'top T-bar' all-around the top edge of that tank, in order to stiffen the top and prevent the gaskets from being cut.

Lloyd
"Pyro for Fun and Profit for More Than Sixty Years"
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richardh08
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Re: I've always wanted a good vacuum pot!

Post by richardh08 »

Welcome to the world of air pressure. A pressure of 14lb/sq, in. doesn't sound like a lot, but it means that each of us has a force of about 20 tons pressing on our skin.
Even when I'm wrong, I'm convincing.
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Lloyd
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Re: I've always wanted a good vacuum pot!

Post by Lloyd »

So, Richard! I created a dilemma! <This is copied from another site I frequent, but I figure YOU would benefit from my 'accident'.)

Because I failed to do my research, I may have thrown away about $108USD worth of silicone resin! Dadgumit!

I re-used some wax chips resulting from my last machining of a mother mold to make a new mother mold. The chips were slightly contaminated with coolant and with way oil from the mill. OK... so? All it did was slightly discolor the wax. It still melted and solidified and retained it's normal lubricity.

However, this morning, I returned to the molding I'd poured yesterday, using 3lb of the $36USD/lb platinum-cure resin. The 'main body' was cured. But anywhere it was in contact with the mother mold, the resin is still tacky... not quite liquid, but not 'cured' either.

WTF? So, I started researching. It turns out, even in small quantities, sulfur and sulfur compounds can inhibit or prevent cure of Pt-cured silicones. Both my extreme-pressure cutting fluid AND the mill's way oil (Mobil Vactra #2) contain sulfur compounds... and that darned wax was contaminated with both!

Eh... I have more 'virgin' wax on the mill right now. Just allowing those fluids to contact the surface of virgin paraffin doesn't contaminate the mother mold enough to inhibit cure, so long as the mold is thoroughly washed and dried before use. But apparently re-using contaminated milling chips does!

Well... since SOME curing occurred, it may just be cure-rate inhibition, and not an outright failure. I'll give it a few more days, then warm the thing up, and see if it cures.

Durn it!

Lloyd

PS... after about 8 hours of warming to about 105F (can't go much higher, or the wax mold will melt), the uncured resin has lost a LITTLE of its tackiness, but still has not cured. I'm willing to give it a couple of days to see, but it doesn't look good! DADGUMIT!
L

PPS... 14.7lb/sq.in. <grin>
L
"Pyro for Fun and Profit for More Than Sixty Years"
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richardh08
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Re: I've always wanted a good vacuum pot!

Post by richardh08 »

More like 14.696lb/sq.in. if we're being picky :D

I've had an almost equally frustrating day, but it only cost me wasted time.

I'm looking to extend my computer model to cover a more general crackle composition, with two oxidisers and two fuels.

My question was, if only part of the material has reacted, what portion of each substance gets consumed? Obviously, in real life, it will depend on many factors, such as the temperature, the relative rates of the four possible reactions and their activation energies. But I wanted a simple answer that ignores all those complications.

It seemed that the answer should be obvious, but I couldn't see how to work it out. In the end, I went at it methodically, using algebra - with lots of variables. After a lot of head scratching and twiddling of the equations, the variables I wanted to evaluate just cancelled each other out and disappeared!

It turns out that there isn't an answer. I now realise that this makes sense as, without the chemical factors, the order in which the various reactions take place doesn't matter - the end result will be the same in all cases. Sigh.
Even when I'm wrong, I'm convincing.
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