Where does the air go when pressure casting?

red4

Sr Member
I'm not sure where to post this question, and couldn't find any pertinent information elsewhere.

I've read and seen videos that say when you cure resin in a pressure pot the "bubbles shrink". What exactly does that mean? Is the air inside a bubble being compressed into a microscopic area, or is the air being squeezed out of the resin, with only minor residual air remaining inside?

If you apply, for example, 40 psi to the liquid resin, and that pressure shrinks a bubble - is the hardened resin permanently applying 40 psi of pressure to that bubble even after you take the resin out of the pot? Is the microscopic bubble a tiny bomb of 40 psi waiting to expand? Because if it's not, then that means most of the air from the original bubble escaped the resin. The pressure inside the bubble can't be neutral once cured if most of the air did not escape.

Does anybody know what's actually happening?
 

cavx

Master Member
RPF PREMIUM MEMBER
As far as I know, they get crushed, forced to be much smaller by the increased pressure in the chamber. Once the resin cures, the bubble remains at that much smaller size which hopefully are invisible to the eye. Will they ever pop? Depends on the wall thickness of the part containing the bubble. Even if they did, the bubbles can not expand back to their original size because the resin is now much harder than air. If they were just under the surface of the part, then they might pop at some point. I doubt that you will be under any risk from "shrapnel" though as the parts that broke away would be as small if not smaller than dust particles.

A similar thing happens to bubbles when degassing under vacuum. In this case, the bubbles have expanded to up to 15 times their original size and have a wall thickness much thinner than they would at normal atmospheric pressure. Whilst most bubbles pop, some remain and as you re-pressurize the chamber, the bubbles which formed under vacuum are now crushed by the new increase in pressure. They don't come back either. It is pretty neat to watch them shrink to nothing.

I don't have pics of pressure casting as I don't own a pressure pot, but here is some from my vacuum chamber.




The time between these two images is about 1 second where you can clearly see large bubbles, then as you open the valve, they crush to nothing.

So whilst at opposite ends of the pressure gauge, the end result is pretty much the same - no visible bubbles.
 

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robn1

Master Member
My observations are that the air bubbles are pushed out of the resin. I've made resin parts that I cut into and even broke and there is no sign of bubbles in the interior. I also cure silicone molds under pressure and have observed the same effect, if I cut into the silicone there is no sign of bubbles, just smooth continuous material. Molds I made without pressure were full of holes like a sponge.
 

msleeper

Sr Member
My observations are that the air bubbles are pushed out of the resin. I've made resin parts that I cut into and even broke and there is no sign of bubbles in the interior. I also cure silicone molds under pressure and have observed the same effect, if I cut into the silicone there is no sign of bubbles, just smooth continuous material. Molds I made without pressure were full of holes like a sponge.
You pressure cure your molds? Interesting. I've always found that simply degassing the silicone is sufficient to get bubble-free silicone. I would be afraid of damaging a master under pressure and not knowing until the mold comes apart.
 

GhostMinion

Sr Member
RPF PREMIUM MEMBER
You pressure cure your molds? Interesting. I've always found that simply degassing the silicone is sufficient to get bubble-free silicone. I would be afraid of damaging a master under pressure and not knowing until the mold comes apart.

Yeah, I thought the same thing. Well, at least that's what I use, a degassing chamber. No bubbles in any of the silicone......
 

robn1

Master Member
Yes the master has to be pressure worthy, I just make everything with that in mind. I don't have vac equipment, but if I ever find myself doing something that can't be pressurized I'll have to consider it.
 

cavx

Master Member
RPF PREMIUM MEMBER
My observations are that the air bubbles are pushed out of the resin. I've made resin parts that I cut into and even broke and there is no sign of bubbles in the interior.
Pushed out or crushed smaller than what is visible to the eye?

What would make the bubble move under pressure?

Under vacuum, bubbles rise as they get bigger because they are now lighter than the surrounding material. Once on top, the bubbles continue to expand until they reach a point where the surface tension breaks releasing the trapped air.
 

robn1

Master Member
Pushed out or crushed smaller than what is visible to the eye?

What would make the bubble move under pressure?...
That's what some people say, that the bubbles are just reduced in size. But if that were the case the bubbles in silicone would re-expand when removed from pressure. But in my casts there is no sign of bubbles at all. I cast a small part with old resin that had turned brittle and it broke. The broken surface was smooth and shiny like glass, with no pitting. The left over resin in the mixing cup blew up like a sponge and the part would have too without pressure. Where'd the air go?

As near as I can tell the bubbles are squeezed out of the resin/silicone.
 

Knightjar

Sr Member
RPF PREMIUM MEMBER
The bubbles are compressed. You can prove this to yourself by making a pressure cast using a silicone mould that not made under pressure. The bubbles in the silicone compress, resulting in little dents in the surface of the silicone and thus little bumps on the surface of your casting. This is why you always need to make moulds under pressure if you plan on casting that way.
 

cavx

Master Member
RPF PREMIUM MEMBER
But if that were the case the bubbles in silicone would re-expand when removed from pressure.
Why would they? If the silicone has cured and even if it is a soft shore product like A20, the rubber is still much denser than air.

But in my casts there is no sign of bubbles at all. I cast a small part with old resin that had turned brittle and it broke. The broken surface was smooth and shiny like glass, with no pitting.
They won't be visible to the naked eye. If you look at one of those BK7 trophies, it appears optically clear with no visible pits. Yet they are are miles away from being even close to a commercial optical surface of SD (scratch and dig) 60/40 (microns).

The left over resin in the mixing cup blew up like a sponge and the part would have too without pressure. Where'd the air go?

As near as I can tell the bubbles are squeezed out of the resin/silicone.
I just don't see from my experience (with a vacuum chamber) how the air will move through a dense medium like resin or silicone without a force pulling it out. A pressure pot applies pressure to the surface. If anything, it is pushing down to the extent that it will simply crush what ever is less dense (like air) under the pressure.

When I degass the elastomer, it has a single raise and fall before it boils. With the silicone I use (shore is A40), it will raise and fall 3 times before all the air is sucked out. So what happens is that the stuff is so thick, all the air can't escape during the first rise. It may even trap some air as it falls in on itself. The 2nd and 3rd rises are no where as high as the first as each has less trapped air.
 

cavx

Master Member
RPF PREMIUM MEMBER
So when in doubt, ask an expert and he said that the bubbles are crushed smaller than the eye can see. Given 1 atmosphere is about 14.7PSI,. when you take resin to 40+PSI, it is quite a lot of pressure.

If you blow up a balloon to be about the size of your head, then take that balloon under water it will be reduced in size. It gets smaller the deeper you go. As you bring the balloon back to the surface, it expands back to its original size. The amount of air inside the balloon does not go anywhere and is not reduced, it get compressed by the pressure of the water surrounding it. If the pressure was high enough (to go up 1 atmosphere under water, you have to dive to 30 feet), the balloon would crush really small. The same thing happens to the bubbles in the resin. The difference is that once the resin cures, the bubbles can't expand back to their original size once the pressure is reduced.

He also said that some poly resins cure faster under pressure than they do at normal atmospheric pressure and so there is a time gain benefit as well as the "no bubbles" benefit.

Now I want a pressure pot.
 

red4

Sr Member
So when in doubt, ask an expert and he said that the bubbles are crushed smaller than the eye can see. Given 1 atmosphere is about 14.7PSI,. when you take resin to 40+PSI, it is quite a lot of pressure.

If you blow up a balloon to be about the size of your head, then take that balloon under water it will be reduced in size. It gets smaller the deeper you go. As you bring the balloon back to the surface, it expands back to its original size. The amount of air inside the balloon does not go anywhere and is not reduced, it get compressed by the pressure of the water surrounding it. If the pressure was high enough (to go up 1 atmosphere under water, you have to dive to 30 feet), the balloon would crush really small. The same thing happens to the bubbles in the resin. The difference is that once the resin cures, the bubbles can't expand back to their original size once the pressure is reduced.

He also said that some poly resins cure faster under pressure than they do at normal atmospheric pressure and so there is a time gain benefit as well as the "no bubbles" benefit.

Now I want a pressure pot.
This is interesting, and now I have to know: Are the micro-bubbles permanently under 40psi of pressure inside the cured resin? Is the hardened resin forever applying 40psi of pressure to those bubbles? And if so, doesn't that compromise the strength of the resin?
 

robn1

Master Member
So when in doubt, ask an expert and he said that the bubbles are crushed smaller than the eye can see. Given 1 atmosphere is about 14.7PSI,. when you take resin to 40+PSI, it is quite a lot of pressure.

If you blow up a balloon to be about the size of your head, then take that balloon under water it will be reduced in size. It gets smaller the deeper you go. As you bring the balloon back to the surface, it expands back to its original size. The amount of air inside the balloon does not go anywhere and is not reduced, it get compressed by the pressure of the water surrounding it. If the pressure was high enough (to go up 1 atmosphere under water, you have to dive to 30 feet), the balloon would crush really small. The same thing happens to the bubbles in the resin. The difference is that once the resin cures, the bubbles can't expand back to their original size once the pressure is reduced.

He also said that some poly resins cure faster under pressure than they do at normal atmospheric pressure and so there is a time gain benefit as well as the "no bubbles" benefit.

Now I want a pressure pot.
Not to dispute your expert but his analogy is flawed. Air in a balloon held under water is not the same as air in resin/silicone under pressure. The air in the balloon can't escape because it's sealed inside the balloon! Open the balloon and the air will escape into the water and rise to the surface. The water is more dense than air but it will still rise.

I run my pot at 60psi. That 60 pounds per square inch. I don't know offhand the surface area of my pot but let's assume it's 300 square inches. 60 x 300 = 18,000 pounds of total pressure, that's enough force to squeeze air out of the heaviest bodied liquid.
 

cavx

Master Member
RPF PREMIUM MEMBER
Not to dispute your expert but his analogy is flawed. Air in a balloon held under water is not the same as air in resin/silicone under pressure. The air in the balloon can't escape because it's sealed inside the balloon! Open the balloon and the air will escape into the water and rise to the surface. The water is more dense than air but it will still rise.

I run my pot at 60psi. That 60 pounds per square inch. I don't know offhand the surface area of my pot but let's assume it's 300 square inches. 60 x 300 = 18,000 pounds of total pressure, that's enough force to squeeze air out of the heaviest bodied liquid.
He has pressure pots and why I asked him. And what he says is that air does not escape from bubbles in a pressure cast system. The air is compressed. Compressed hard enough, air becomes a liquid. Only a vacuum can pull bubbles because they expand and raise to the top where they can pop and release the air. That said, if the mix runs out of potlife under vacuum, you end up with a part that looks like Swiss Cheese - big air bubbles. The idea is that you let your parts cure under pressure, so the bubbles are reduced and can't re-expand. 60PSI is massive pressure and why you can't see them. Vacuum systems are also called "degassing" systems because they pull the air out of the mix.

Back to the balloon. Divers that use compressed air to breath underwater have to come back to the surface slower than they went under. As they breath in the air which containt nitrogen, tiny bubbles of nitrogen are present in their bloodstream. If they come back from being under pressure too fast, these bubbles expand and cause massive pain known as "the bends". So the expansion from a heavy pressure system to a normal pressure system is kind of like taking a resin mix of resin from normal atmosphere to a negative one. The bubbles which were small (but annoying because they are visible at normal atmosphere) expand to the point where most will pop.

Look at my photos I posted of the vacuum system. First photo has bubbles in the mix. 3rd last photo is basically boiling as the air escapes. 2nd last photo is virtually bubble free. At this stage, I have returned the chamber back to normal pressure. So what happens to the bubbles that were present and did not pop under vacuum? Because they were formed at 1 negative atmosphere, they are crushed when (normal) atmospheric pressure is introduced. That pressure is just 14.7PSI and your taking it up to 60PSI. If less than 15PSI can crush bubbles, then 60PSI will annihilate them. And once the resin cures, they ain't coming back.
 

cavx

Master Member
RPF PREMIUM MEMBER
This is interesting, and now I have to know: Are the micro-bubbles permanently under 40psi of pressure inside the cured resin? Is the hardened resin forever applying 40psi of pressure to those bubbles? And if so, doesn't that compromise the strength of the resin?
Yes they are there forever. They are so small you can not see them. Therefore their impact to the strength is negligible.
 

red4

Sr Member
Yes they are there forever. They are so small you can not see them. Therefore their impact to the strength is negligible.
How is their visibility related to their impact on the resin's strength?

Imagine a concrete wall filled with CO2 cartridges. It doesn't sound negligible.
 
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cavx

Master Member
RPF PREMIUM MEMBER
Can you explain what this means more clearly? Is the strength of the resin not compromised due to the compressed bubbles having 60 psi perpetually applied to them inside the cured resin? If those compressed bubbles are under perpetual pressure, then the resin does not have neutral pressure. It has a bunch of tiny bubbles inside it trying like hell to push outward. You used the word "annihilate" as if to suggest the bubbles either don't exist anymore, or are otherwise neutralized. How is that possible if the air pressure never escaped the bubbles? It seems like they're considerably more volatile than bubbles that form from mixing the resin. The compressed bubbles would be constantly flexing the resin molecules outward with significantly more force than bubbles that form from simply mixing the resin. It sounds a lot like the unrelieved pressure makes the resin brittle.
Have you ever had a part explode or even crack from expanding air or gas? I will take the answer as a no, because the ratio of surface area of the bubble(s) (remember I keep stating smaller than the eye can see) Vs that of the part itself is massive. Even at 60PSI, the pressure force inside the "invisible to the eye" cavity is insignificant next to the resin's own molecular forces that hold it together. They won't break from that. If a part does break, air temp changes causing rapid expansion/contraction of the resin would have greater effect than the pressure inside the bubbles.

What is the ratio of "bubble" to solid resin anyway? Unless you were whipping the resin like scrambled eggs, you will probably be well under 10% per volume. If you pour correctly, you should not trap bubbles or if you do, you may be able to tap them out, viscosity of the product pending. I use a really thick product (a polyurethane) that does not pour in fine streams so high pours generally don't work. In my case, I have to degas under vacuum to remove as much air as I can before I pour. And I get pretty good results from this. I can get 100% bubble free in an open back mold. I do find that I trap air in my two part molds due to this stuff being being as thick as honey and the air not being able to freely float to the surface of the vent. A pressure pot would probably solve those issues.

Something else worth considering - resin is thicker than water and it get thicker as it cures. If you stir a glass of water, there is next to no resistance and the bubbles formed raise really quickly to the surface and generally pop. The resin presents a lot of resistance and as it cures, that resistance goes up to the point when it becomes a solid. If the pressure really could squeeze out the air to raise up, would you not have a cavity or at least a slight shrinkage of your part in your two part mold upon de-molding? Even if you overfilled it, would the fill level not have receded?
 

cavx

Master Member
RPF PREMIUM MEMBER
Please watch THIS at about 4min.
 
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