phase change madness!!!

[anchange]

Hey everyone, I have a rather odd question. I'm trying to make a tricky kind of interior mold for metal casting and I’m wondering if there's an accessible element or compound that has these particular properties -

 

1. It will perform sublimation at a fairly easily attainable pressure and a temp less then 300 degrees C. Pressure will probably have to be below 1 atm but it has to be something that I can create without super fancy gear.

2. I can easily transition it from a solid state to a liquid state by adding or subtracting heat with in the range of -20 degrees C to 200 degrees C.

3. It's not toxic etc. I can work with it ok.

 

Any ideas?

 

I'm guessing sense I'm dealing with all three forms - solid,liquid,vaper the key is that the TP point be as close to atmospheric pressure as possible (while still under it) and also be as close to room temp as possible. But I'm not a chemist so you tell me

 

Thanks in advance for the help!!!

 

Bradley

[insane_alien]

umm, water actually fits all your criteria

 

1/ will become vapour at 100*C (less than 300*C)

2/ will melt at 0*C (between -20 and 200)

3/ is non toxic

[anchange]

Ha!!

 

Thanks, that might actually work, not completely idea but obtainable.

 

What about these parameters?

 

1. It will perform sublimation at a fairly easily attainable pressure and room temp (or close to it)

2. I can easily transition it from a solid state to a liquid state by adding or subtracting heat with in the range of -20 degrees C to 90 degrees C.

3. It's not toxic etc. I can work with it ok.

Edited August 16, 2008 by anchange

[John Cuthber]

Naphthalene and menthol fit the bill but are not really non toxic. (nothing is).

[anchange]

Nice!

 

Thanks John.

 

Where can I find phase change graphs for these guys / what the sublimation pressure at room temp?

 

Any idea how expensive menthol is in larger quanities say 20lbs or so?

Edited August 17, 2008 by anchange

[frosch45]

Well, usually methanol is a liquid at room temp, so I'm not sure it would be measured in lbs....

 

As for your diagrams, I searched google for a little bit and couldn't find it, thats not to say that someone else will.... It might just be easier to look up the values yourself and make your own graph, or even expirimentally test it

 

methanol is antifreeze (ALTHOUGH SOMETIMES THERE ARE OTHER ANTIFREEZES, like Naphthalene so be sure you check what you're getting by looking at the back of the bottle or check an msds for the particular product) so you could buy it from a gas station I guess, but if you want to buy it elsware...

 

http://store.hvchemical.com/search.htm?step=2&viewfrom=1&numresults=10&searchterm=methanol

[John Cuthber]

Frosch45,

What the F*** has methanol got to do with it?

I also wouldn't expect to see an insoluble solid like naphthalene in antifreeze.

 

I don't know what menthol costs as a bulk chemical but a 1999 fine chemical catalogue lists it at about £50 per pound.

Naphthalene is probably 10 times cheaper.

 

The vapour pressure of menthol is about 1.3mmHg at 56C based on an MSDS I found, but this

http://jeb.biologists.org/cgi/reprint/20/2/159.pdf

gives it as 1mmHg at room temp.

 

Naphthalene seems to have a vapour pressure of about 0.1mmHg or less at room temp.

 

It seems odd that even for so well known a chemical as naphthalene there isn't any good data on the vapour pressure on the net.

[anchange]

0.1mmHg = .00013158 atm correct?

 

Is that achievable for a 3ft by 3ft cube with some kind of simple box/vacuum that I put together?

 

Seems like it isn't but I don't really know the scale of pressure.

[frosch45]

Frosch45,

What the F*** has methanol got to do with it?

 

 

woooohhhh, why the hostility? seriously, I wasn't offensive was I? I mean.... I thought methanol has lots of things to do with it.... thats what our friend wanted, right....?

 

seriously, if you want to talk, just pm me, I know I'm not the very smartest of all the people on here, but I really like science and I'm learning (look at some of my beginning posts any you can see that....) but feel free to tell me if I'm wrong, you don't have to start cursing... and I would welcome the correction

 

edit:

 

OH!

 

Jeez, sorry, I just reread a little bit more closely.... menthol is quite different from methanol....

 

now I feel a lot stupider.... oh well, my mistake

Edited August 17, 2008 by frosch45
reread some posts

[iNow]

It happens, frosch. No worries. Don't spend too much time worrying about kicking yourself while you're down. There are plenty of other people around to do that for you.

[John Cuthber]

0.1mmHg = .00013158 atm correct?

 

Is that achievable for a 3ft by 3ft cube with some kind of simple box/vacuum that I put together?

 

Seems like it isn't but I don't really know the scale of pressure.

 

Sorry, but that's going to be quite difficult, even with a good pump etc.

Unfortunately, solids generally are not very volatile.

What are you making? could you use something that can be dissolved away, rather than sublimed?

[YT2095]

Frosch45,

What the F*** has methanol got to do with it?

 

cool it will you! he made an honest mistake, there`s no need AT ALL for that sort of response!

[CaptainPanic]

You're going to do metal casting, and you want a mold that melts at less than 200 deg C? What kind of metal are you using? How does the casting process look? You got me puzzled

 

I'll explain what puzzles me: I don't see the whole point of having a mold that melts, when the casting process is based on the principle of the mold having a higher melting point than the metal that you put into it. Sublimation temperatures seem even less relevant.

 

In industry, sand is used for casting. You just pack it together, and put the metal in all places where you have no sand. You will need special sand for it because some sands can be shaped, while others (like beach sand) really are hopeless for this (look up the perfect mold-sand on google or wikipedia). Even Ferrari uses sand for casting their engine blocks!! (Like any other car manufacturer).

 

Finally, if your shape is too complicated, you can create the shape you need out of polystyrene foam (styro foam) and pack the sand around that. Polystyrene melts at 90 deg C, but because it's so light its liquid volume is nothing compared to the solid foam. So, the hot metal will melt the foam, and immediately replace it, so the sand has no chance of collapsing.

 

John Cuthber's idea of making a soluble mold might also work... although I think it's going to be tricky. The advantage is that you can wash it out... The sand will have to be knocked/shaken out.

Edited August 19, 2008 by CaptainPanic
little more info/explanation added

[insane_alien]

i think he is talking about making the mold rather than casting at this stage.

[CaptainPanic]

Me too... but I was just talking about sand molds that are used for metal casting... But I just realized that you can cast something that becomes a solid without a temperature change... like a resin or gelatin. So, then you can actually use ice or something that would be totally useless for metal casting.

[anchange]

It's not the negative mold that will actually pour metal into that I'm discussing. I'm talking about replacing the wax in wax lost casting with something else that evaporates.

[John Cuthber]

Is this just for interest or is there some reason you can't just use wax?

[anchange]

If I can find something that evaporates I might be able to only use packed sand around it instead of a ceramic coating that has to be fired/tempered. Could be quicker and more efficient.