Snake Casting Question

[JimGo]

For those doing pressure casting, are you reducing the amount of catalyst compared to if you were casting without pressure? I tried casting snake skins this weekend and learned a few things:

1) Cocobolo does not make good stand-off material. It will bleed into the PR like crazy! Though, interestingly enough, the PR won't bond to the wood itself.

2) Harkening back to chem class in high school, the old PV=NRT equation (can't remember whose law that is) says that as you increase the pressure, the temperature will go up. Well, apparently the temperature increased SIGNIFICANTLY; my casting actually CRACKED. Granted, it was 90+ degrees outside, but there was a big crack in the PR, which was disappointing.

3) Heat sufficient to cause PR to crack while curing will also cause snakeskin to either shrink or simply dissolve. I had a LOT of air bubbles around the skins themselves, but from what I could see though the PR, the skin had actually either turned transparent, or dissolved, or something, out near the ends.

My guess is that I used too much catalyst for casting under pressure, and the catalyst made everything hotter (there's that chem law again). But I'd be curious to get feedback from you and others. I have some leaks in my system, and was thus only able to pressurize the pot to about 20-25 PSI, but since that's the same pressure Don Ward said he uses, that should have been OK.

Thoughts/suggestions?

 

[its_virgil]

[<br />Jim,
I'll give my answers to your questions...but someone else may have better answers.

1) Cocobolo does not make good stand-off material. It will bleed into the PR like crazy! Though, interestingly enough, the PR won't bond to the wood itself.I visit a cabinet shop the evening before the dumpster is emptied and get the 1" square cutoffs from the cabinet trimmings. I find maple, ash, and redoak which have produced no problems. The thicker the little standoff thingies are, the more the tubes want to float in the resin. I cut them as I safely can on my chop saw. Make a zero clearance fence from a piece of lattace and the little standoffs don't fly off behind the saw, but stay right next to the bland in a nice little pile.

2) Harkening back to chem class in high school, the old PV=NRT equation (can't remember whose law that is) says that as you increase the pressure, the temperature will go up. Well, apparently the temperature increased SIGNIFICANTLY; my casting actually CRACKED. Granted, it was 90+ degrees outside, but there was a big crack in the PR, which was disappointing.I have dropped back to 3 drops per ounce. It takes longer to cure, but less heat is produced. I do not cast when the temp is over 90* unless I turn on the AC[][] in the shop. Heat is a real menace to casting snake skins. I have had the heat actually dissolve the skins from the tubes and they were gone. The only place for the heat to dissipate is through the top of the casting and if it can get out vast enough, stays in the middle of the cast where the skin is. I think most of my failures were due to heat problems and pressure that fluxuated. I was using 3 drops even before I started using prressure.

3) Heat sufficient to cause PR to crack while curing will also cause snakeskin to either shrink or simply dissolve. I had a LOT of air bubbles around the skins themselves, but from what I could see though the PR, the skin had actually either turned transparent, or dissolved, or something, out near the ends.Yep, they just vanish sometimes...into thin air. Where did they go? Another discussion for the Phd's...don't we keep 'em busy[]! The heat cracks tend to be fan shapped with the straight side parallel to the tube and the curved fan shaped side radiating up to the surface and back to the tube.


My guess is that I used too much catalyst for casting under pressure, and the catalyst made everything hotter (there's that chem law again). But I'd be curious to get feedback from you and others. I have some leaks in my system, and was thus only able to pressurize the pot to about 20-25 PSI, but since that's the same pressure Don Ward said he uses, that should have been OK.
Steady pressure is another key factor. When the pressure fluxuated I had more failures. I think the air pressure in the tubes was reduced, then it was increased as the pressure on the tank decreased. It eacaped from the tube and was trapped between the skin and the resin around the skin and after this happened a few times it made the skin look cruddy. I now hook the hose to the pot straight from the compressor and use the pressure regulator on the compressor to regulate the pressure. I leave the air hose on the tank for the whole curing time.

Thoughts/suggestions?I do hope my thoughts and suggestions from above helps and I'll be interested in what others that do this have to say.

Do a good turn daily!
Don

 

[JimGo]

Thanks for the feedback Don!