pshrynk
Member
I've been following a lot of threads, but haven't really gotten an idea of why pressure. Vacuum I can understand - to suck bubbles out. What purpose does the pressure serve?
Question: Why pressure pots?
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RileyD
Member
Vacuum takes too long for some of the faster setting resins like Alumalite.
Pressure does not actually remove the bubbles like vacuum would but either compresses them to microscopic of forces the air back into solution, I'm not sure which.
Pressure does not actually remove the bubbles like vacuum would but either compresses them to microscopic of forces the air back into solution, I'm not sure which.
I had the same question some time ago .
The reason was explained to me in the same way.
I guess you have to understand the fluid dynamics of bubbles in resin to really "get it".
It is still not completely clear ....
With vacuum, there is at most, a 35 psi pressure differential.
As I understand, most people use pressure of 50 - 60 psi above atmospheric, so that is considerably more.
I guess that squishes the bubbles AND tends to force some gas into solution with the resin.
The reason was explained to me in the same way.
I guess you have to understand the fluid dynamics of bubbles in resin to really "get it".
It is still not completely clear ....
With vacuum, there is at most, a 35 psi pressure differential.
As I understand, most people use pressure of 50 - 60 psi above atmospheric, so that is considerably more.
I guess that squishes the bubbles AND tends to force some gas into solution with the resin.
pshrynk
Member
Ah. That makes sense.
schapekop0701
Member
Euh, 50 to 60 psi !!
i am using 120 to 140 psi or 10 Bar
i am using 120 to 140 psi or 10 Bar
Euh, 50 to 60 psi !!
i am using 120 to 140 psi or 10 Bar
Good for you !! . Keep blowing and puffing !!
Most people don't get that high, or go that high !
Maximum vacuum will be atmospheric pressure of ~14.7 psi.
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Sent from my iPhone using Penturners.org mobile app
Dan,
I acknowledge my mistake above :redface: I meant to say 15, but 14.7 is closer.
Thanks for correcting.
As for pressure casting, I am worried by the statement made above by schapekop0701 ... did not know that home user pressure pots were rated to go that high ... maybe in Europe they are ... or maybe schapekop0701 is using industrial equipment. . Think of the tons (tonnes) of internal force. . I calculate 20-40 tons inside for the numbers he gave, depending on pot size.
I think it is dangerous to even mention that high level of pressure for casting.
I googled and found this article:
Myths and misunderstandings about workshop pressure pots - Australasian Paint & Panel
which includes the following paragraph:
IMPORTANT SAFETY NOTE –
Pressure pots are pressure vessels and can cause harm if not used properly. Regularly check your equipment to ensure safety blow-off value and pressure gauge are working properly. Safety valves should activate at approximately 40 PSI of pot pressure. To check this, set up empty pot in the manner we have described above. Slowly increase the pressure until the safety valve activates. If you get to 50 PSI and the safety valve has not blown, have the unit serviced. Quality pressure pot suppliers will have spare parts available and can service your equipment for you.
Read more at Myths and misunderstandings about workshop pressure pots - Australasian Paint & Panel
I acknowledge my mistake above :redface: I meant to say 15, but 14.7 is closer.
Thanks for correcting.
As for pressure casting, I am worried by the statement made above by schapekop0701 ... did not know that home user pressure pots were rated to go that high ... maybe in Europe they are ... or maybe schapekop0701 is using industrial equipment. . Think of the tons (tonnes) of internal force. . I calculate 20-40 tons inside for the numbers he gave, depending on pot size.
I think it is dangerous to even mention that high level of pressure for casting.
I googled and found this article:
Myths and misunderstandings about workshop pressure pots - Australasian Paint & Panel
which includes the following paragraph:
IMPORTANT SAFETY NOTE –
Pressure pots are pressure vessels and can cause harm if not used properly. Regularly check your equipment to ensure safety blow-off value and pressure gauge are working properly. Safety valves should activate at approximately 40 PSI of pot pressure. To check this, set up empty pot in the manner we have described above. Slowly increase the pressure until the safety valve activates. If you get to 50 PSI and the safety valve has not blown, have the unit serviced. Quality pressure pot suppliers will have spare parts available and can service your equipment for you.
Read more at Myths and misunderstandings about workshop pressure pots - Australasian Paint & Panel
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It would be interesting to pressurize resin with bubbles in a clear container to see what's really happening. My guess is that the as the resin is compressed the bubbles are forced to the surface. 60 psi is about 4 time higher than the pressure in the bubble so the bubble would shrink by a factor of only 4. Still would be noticeable when you turned it. I don't think air will go back into solution with a large chain polymer. Pressure would reduce any out gassing as the resin cures so no new bubbles form.
Then again I could be full of it too.
Danny
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Then again I could be full of it too.
Danny
Sent from my iPad using Penturners.org mobile app
Actually, the resin sustains the pressure without compressing very much at all.
Water, for example, is generally regarded as an incompressible fluid.
You say:
"60 psi is about 4 time higher than the pressure in the bubble (initially - my addition) so the bubble would shrink by a factor of only 4."
Actually, pressure is an area thing while compression is a volume issue. So, the amount of compression is less than a factor of 4.
Water, for example, is generally regarded as an incompressible fluid.
You say:
"60 psi is about 4 time higher than the pressure in the bubble (initially - my addition) so the bubble would shrink by a factor of only 4."
Actually, pressure is an area thing while compression is a volume issue. So, the amount of compression is less than a factor of 4.
Correct Sir, the volume changes by a factor of 4, the diameter of the bubble would only shrink 37% with a 4:1 pressure increase. So that isn't the mechanism for getting rid of the bubbles. It may be more like squeezing a sponge under water and the air bubbles come out. This may be why excessive pressure wouldn't be needed to cast clear.
For outgassing, the pressure would be important to a certain threshold. Above that, pressure wouldn't matter.
Danny
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I think that solubility plays a significant role in resin casting. I found a reference (http://www.iccm-central.org/Proceedings/ICCM13proceedings/SITE/PAPERS/Paper-1497.pdf) that states that nitrogen solubility in epoxy at atmospheric pressure is 1.7% by volume. And solubility of a gas in a liquid is roughly proportional to pressure. So, theoretically, you could take resin that is 6.8% by volume air bubbles at atmospheric pressure, and by raising it to 60 psi all of the bubbles would completely dissolve. Obviously this is the best possible case. Time is a factor for diffusion of the air into the resin. Also, using air to pressurize the resin introduces air that is dissolving from the exposed top of the resin (I think that this might be a contributing factor why bubbles tend to be more prominent on top of the cast). So in practice, you can only dissolve a fraction of this maximum. But once the air is dissolved it effectively disappears, and if the pressure is released after the resin is cured then the air is trapped FOREVER (well, probably not quite, but you get the point)...
Binks has pots that are ASME rated for 110 psi -- I would trust these. But I do agree that pneumatic pressure is nothing to mess around with...
Interesting paper, makes sense to me. You could have (6.8-1.7)% air bubbles that would go into solution, the 1.7% air is already in there. So if I'm casting Liquid Diamonds that is very slow to cure, do I have to keep it under pressure the entire cure time? If not the bubbles may re-form. Maybe I should also pull a vacuum first to de-gas and then pressurize giving me some more "bubble room". I can think of some fun experiments to do, but I think I'll cast some pen blanks instead.
Danny
Yeah, hold pressure until cured. Degassing appears to be pretty common in the composite material world (carbon fiber), where they also use a vacuum infusion process. The process involves using vacuum to draw the resin into the mold that already has the fabric. By degassing with vacuum and using a nucleating agent (like scotchbrite pads) they are able to get less void volume and better physical properties. That said, based on the fact that it doesn't appear to be a common practice in the epoxy casting world, I'm sure you can get excellent results without it, but if pot time allows then it's not going to hurt. For some resins, silicone and urethane, I would definitely vacuum degas. Keep in mind that this is all based on my research and not experience, would love to hear what experienced casters have to say...
schapekop0701
Member
For the people who would try 100 PSI or more !!!
I am NOT using a pressure pot of the shelf !!
The pressure pot i am using is made by photo welders and the max pressure is around 500 PSI .
The material used is a 1 cm machine welded pipe used to transport gas at a pressure of 25 Bar.
You will die, sooner or later
That's a very dangerous amount of pressure to be using. You are headed for the morgue.
I never use more than 20 to 30 psi when clear casting and that produces crystal clear blanks with absolutely no bubbles.. 70 psi when I'm casting things like pinecones, acorn caps and such. That's plenty of pressure. 120-140 psi? Geeze man, that's an accident waiting to happen.
Euh, 50 to 60 psi !!
i am using 120 to 140 psi or 10 Bar
That's a very dangerous amount of pressure to be using. You are headed for the morgue.
I never use more than 20 to 30 psi when clear casting and that produces crystal clear blanks with absolutely no bubbles.. 70 psi when I'm casting things like pinecones, acorn caps and such. That's plenty of pressure. 120-140 psi? Geeze man, that's an accident waiting to happen.
its_virgil
Member
Euh, 50 to 60 psi !!
i am using 120 to 140 psi or 10 Bar
That's a very dangerous amount of pressure to be using. You are headed for the morgue.
120-140 psi? Geeze man, that's an accident waiting to happen.
Geeze man. Read the post above. schapekop0701 explains he is using a special pressure vessel rated to 500 psi. Just saying.
Do a good turn daily!
Don
Rifleman1776
Member
I'm with you. I use only a vacuum. Theory being nature abhors a vacuum. As air is pulled out it must be replaced. In our case, it is replaced with stabilizing solution.