Vacuum chamber vs Pressure Pot for resin curing

The official proper procedure for resin casting without air bubbles is to vacuum degas the resin after catalyzing, pour into mold, and then allow the entire mold and part to cure in a pressure pot. This is what Freeman Manufacturing recommends, for example.

However, this video from Easy Composites suggests that you can use a vacuum chamber both for the degassing of the mold and casting materials and also for evacuating bubbles from the air and part during the curing process. This would seem to eliminate the need for a separate pressure pot, if true.

This latter idea appeals to me as I would rather invest in a large vacuum chamber with a high-quality pump if it can be dual-purpose. However, since I rarely see this approach recommended, I'm wondering if there is a reason (perhaps there is something about vacuum vs pressure that is inferior for the curing process). Does anyone have any thoughts on or experience with this?

Do not vac resin, all you will get is bubbles. You vac or pressure rtv but not resin

In the video they are using a clear. Most clears take a long time to set and are heavier by weight then resin

I researched the crap out of this stuff and I found several differing opinions. There are so many different applications for molding and casting, whether for hobby, industry, crafts fishing lures whatever, and they each have their own way of doing things. I just took the best ideas from each, weighed them against my needs and budget, and came up with a system that works for me.

Some of the best advice I found came from here http://resinaddict.com/forum/ They do gaming miniatures mostly, which are demanding because of the small size and intricate details. Many of the casters there use vacuum for resin, they say it's necessary to get resin into the tiny details of their molds. But you have to be quick to beat the cure time of your resin, and you need a large reservoir area in your mold to contain the resin as it rises in the vacuum.

I don't do anything so fine so I don't bother with vac, not even for silicone. I just mix my rubber, pour it and pressurize it. Some people say you can't do this because the bubbles will re-expand, others say it's fine and do it themselves. I just tried it and it works fine, I get perfect results every time. But your original model has to be pressure worthy, it won't work if it can be crushed or deformed under pressure. For resin again no vac, I just mix, pour and pressurize. This setup was great for me because I already had an air compressor, all I needed was the pressure pot and some extra fittings.

My Grizzly pressure pot is discussed here http://www.therpf.com/showthread.php?t=184777

robn1 said:

I researched the crap out of this stuff and I found several differing opinions. There are so many different applications for molding and casting, whether for hobby, industry, crafts fishing lures whatever, and they each have their own way of doing things. I just took the best ideas from each, weighed them against my needs and budget, and came up with a system that works for me.

Some of the best advice I found came from here http://resinaddict.com/forum/ They do gaming miniatures mostly, which are demanding because of the small size and intricate details. Many of the casters there use vacuum for resin, they say it's necessary to get resin into the tiny details of their molds. But you have to be quick to beat the cure time of your resin, and you need a large reservoir area in your mold to contain the resin as it rises in the vacuum.

I don't do anything so fine so I don't bother with vac, not even for silicone. I just mix my rubber, pour it and pressurize it. Some people say you can't do this because the bubbles will re-expand, others say it's fine and do it themselves. I just tried it and it works fine, I get perfect results every time. But your original model has to be pressure worthy, it won't work if it can be crushed or deformed under pressure. For resin again no vac, I just mix, pour and pressurize. This setup was great for me because I already had an air compressor, all I needed was the pressure pot and some extra fittings.

My Grizzly pressure pot is discussed here http://www.therpf.com/showthread.php?t=184777

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This is what I've found to be true and agree with.

Also, with many basic things, you don't need vacuum or pressure. Mix and pour carefully and you won't have issues. Introducing additional techniques when they aren't needed just creates more chances for error.

I vacuum long setting resins all the time...
I'm not seeing bubbles nor am I getting defects... Soooooo yah




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The most efficient way I have managed to rid air bubbles from any resin casts I pull is to dust the the mold with baby powder before you pour the resin in. The powder makes the resin seep into every crevice of the mold, and eliminates 95% of air bubbles (for me anyway).

I've tried this with both block molds (solid resin piece) and brush-on molds (hollow resin helmets). Never had any issues.

Thank you for this great thread filled with excellent information, advice and expert opinions on a subject that I have been really wanting to learn more about!

When pouring clear resins I'll first degas them in a vacuum, then move the mold into a pressure tank for the remainder of the cure. I've found that is the best way to ensure crystal clear castings. When it comes to casting parts with opaque urethane resins I'll first dust the mold with baby powder, then pressure cast to make sure all the little bubbles are eliminated.

When making silicone molds I always degas the rubber in a vacuum first before pouring it into the mold box. You can also just degas it in the mold box but generally I've found that to be unnecessary. A lot of people pour silicone molds without using a vacuum chamber and get acceptable results. I understand that. A vacuum pump and chamber capable of reaching the necessary vacuum are expensive pieces of equipment. (Pressure pots for casting are much, much cheaper, especially if you already have a compressor.)

The real reason for desiring silicone rubber in a vacuum is not to get it to flow into the small crevices in the pattern. The rubber will usually work it's way into all the nooks and crannies on it's own. However, without using a vacuum chamber, even though the surface of the mold may look fine, small bubbles (even really tiny ones) right under the surface can have and effect on the casting. Especially with larger castings, as the resin heats up as it cures (called exotherm) it can cause the air in these bubbles to expand. If the bubbles are close enough to the mold surface, the expansion can cause the rubber to deform into the mold. As the resin sets you'll get lots of little tiny dimples in the surface. This is most noticeable with larger molds (more resin, more heat) with smooth surfaces. With small molds or very detailed surfaces (like figures) you are less likely to see this happen. Softer (low shore hardness) are generally more forgiving of being poured with the use of a vacuum chamber. On the other hand molds made with softer rubbers are more prone to the dimple problem since it takes less for the air bubbles to deform the surface.

MustangGTR1 said:

Do not vac resin, all you will get is bubbles. You vac or pressure rtv but not resin

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I am no expert but I have been casting in clear resin (as testers because it is cheaper than the expensive elastomers I need to use on my finished parts) and I take the vacuum to a full -30" for a few minutes. Like the elastomer, it raises (but not as high) and falls and when you re-pressurize (you open the valve SLOWLY), any bubbles still left are crushed by the increase of pressure.

I first attempted vacuuming resin back in 2006 and all I got was the resin going off in the pot but I was never able to get the vacuum to a full -30" back then either. I think I was adding too much cat back then as well and any liquid under vacuum heats up - this would have to speed up the exothermic reaction as well. I have done 4 batches in the past week and all were perfect. Any bubbles are from the pour technique, not the resin after degassing.

Pouring is still an art form and you can trap air as you pour. So whilst I don't do the "high pour" thing, I do pour slowly and let resin run down into the mold.

I'm also in the never having used a pressure or vacuum for any molding or casting and never had a problem with bubbles. The silicone I use specifically states it doesn't need vacuuming and I've found that to be true. There are even times I've poured sloppily and not done the careful, thin, stream in a low area and not had any issues.

With my resin, I've never had any bubble issues with that unless I left it exposed to air for too long. I've done one and two parts molds, simple shapes and lots of fine details, all without bubble issues. I always do the powder in the mold as well. I find that it really helps the resin flow better than the times I tried a spray mold release.

Now, if I were to ever try a clear, I'd definitely tap into one of my friends with a pressure pot. Trying to get a clean cast with clear, that's when it's basically a requirement. But I've never found a need for it with regular resin.

You can vac a long setting resin, but not what goes off in 3 mins or less. I use a 4 cyc vac pump and can pull 30 in less then 30 secs and it is still bubbling. There is one nice thing about it tho. You can make it look like it is all rusted and pitted.

I vac all my rtv and pressure all my resin. But you can even get still get bubbles at times. I have pots from 2 gallon up to 50, but it all comes down to what type of resin and rtv, and set time.

Crank, irish uses baby powder as well. Works great.

Steel Roses

I tried vacuuming my resin before mixing it. While the theory is sound, it causes you to expose the resin to atmospheric moisture for too long, and all I would get is the "moisture foaming" effect when the resin sets. Keep in mind it is rather humid where I live.

Crank729 said:

The most efficient way I have managed to rid air bubbles from any resin casts I pull is to dust the the mold with baby powder before you pour the resin in. The powder makes the resin seep into every crevice of the mold, and eliminates 95% of air bubbles (for me anyway).

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I've considered doing this before, but was worried about what effect it might have on the outer surface of the casting. Does the powder get embedded in the resin? Do you do anything special with it (wash, etc.) prior to painting?

The powder is absorbed into the resin and just disappears, you'll never know it was there. You blow most of it out of the mold leaving just a thin film behind.

This depends if you want to achieve glossy or matte surface ... you will never got perfectly smooth/glossy surfaces with talc powder ...

Ah, nice! I'll be giving the powder a shot then. Matte surface is fine for my purposes, but good to know for future. Next project will need a glossy surface, so will avoid for that.

robn1 said:

The powder is absorbed into the resin and just disappears, you'll never know it was there. You blow most of it out of the mold leaving just a thin film behind.

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Even with ONYX black resin? Been wondering this myself.

We're talking an extremely light coating of baby powder here. Basically dust it into the mold, then turn the mold upside down and knock out and/or blow out all of the loose powder. I've never seen any indication of the powder in the castings, even with high gloss mold surfaces and black resin like Onyx. But your mileage may vary! Do a test.