Vacuum chamber vs Pressure Pot for resin curing

[clayslinger]

I use a vacuum chamber for most everything. Rubber and resin. You need to use a resin with a longer setting time and also have a large enough vacuum pump to work quickly enough for your application.

Your mold has to have the room for the resin to expand to while in the chamber and the chambers need to be large enough for the job but not a 5 gallon size to evacuate a golf ball sized cast. I use half a dozen different sized chambers.

I use pressure only occasionally for larger solid pieces that I can't fit into a vacuum chamber.

For some large hollow casts I talc the mold and roto cast by hand or use a fiber filler and brush it in. I do not vacuum the resin in either of those occasions.

The key is to use a big fast vacuum pump. Getting the rubber or resin to rise and collapse in seconds makes all the difference.

That's my two cents!
Chuck...

 

[robn1]

Even with ONYX black resin? Been wondering this myself.

I use Onyx but so far only for parts that have a dull finish anyway. But like DaveG said you blow most of the powder out of the mold, I use my compressor with a dusting nozzle, leaving only a thin film behind. If you need a glossy finish just polish the part.

 

[Crank729]

And, on a side note, double check to make sure the talc powder hasn't collected anywhere in the mold before you cast. If it pools, there will be large imperfections in your casting since the resin can't breach the powder.

 

[irishamericanlad]

The powder will take the onyx from a shiny black to a more flat black, but just a small layer of powder or it will coagulate as not set .. Dump some in, brush it into the low ends then blow it out, you may not see powder but there's a small later ...

 

[norbauer]

This is all super excellent info, everyone. Please keep your perspectives coming. It seems mileage may vary when it comes to this issue. I think I'm likely to get both a vacuum chamber and pressure pot to experiment with both.

 

[irishamericanlad]

Norbauer I have both a huge pressure pot (up to 80psi, and a huge vacuum, both serve their rolls perspectively, I even sometimes put a complex mold in the vacuum for a few minutes then in the pressure pot till it cures


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[cavx]

I hope this helps - I am still learning but thought I would share given I have had some success with this.

Mixing stirs air into the resin or this case elastomer.

As the pressure decreases, the bubbles begin to expand.

Once full vacuum is achieved the mixture rises right up as the air is evacuated.

Then collapses back into itself.

The mixture appears at this stage to be boiling as the last of the air is pulled out.

I found by slowly increasing (in the video the guy opens the valve very quickly) the increasing pressure back into the chamber crushes any bubbles still left in the mix.

Bubble free pouring. Technique here still applies because it is easy to trap air back into the mix during the pour. Air temp also plays a big part to the success. When I took these images, the day time temp maxed out at about 24 degrees C so I had to warm the product in a hot bath first. Yesterday, the local temp was 35+ and the pot life was shortened significantly to the point where I had to throw away some of the product. A cold bath prior to mixing and degassing would have extended the pot life.

 

[sycor]

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.

Just curious, what are you using?

 

[wolfchild59]

Just curious, what are you using?

I typically use Alumilite HS II for my silicone though have recently used some Smooth-On Mold max 10 as well. I find both to be pretty comparable, with the Mold Max being a little softer after curing. I only actually ordered the Mold Max 10 because there is a supplier in SoCal that can get Smooth-On products to me two days after I order, with regular shipping, and I needed it faster than the Alumilite would get to me.

For my resin, I use Smooth-On Smooth Cast 300 for almost all of my molding. Though I recently also purchased some Smooth Cast 310 so I had the longer working time to pour a larger mold all in one go. In a pinch, for a little while I was using some resin from a local plastics shop (Tap Plastics Quick Cast). I never had any bubble issues with it, but I feel that it's an inferior product when doing finishing and tooling work on it.

I actually also heat my molds before pouring. It was something I picked up from the Alumilite site and it doesn't really do anything for the resin (though, it does seem to cure a little faster the warmer the mold), but it does help extend the life of the mold. When I was casting big runs of some figures a few years back, I could get 65-75 clean pulls from a single mold before it started going south. When I did a small run last year for a project for the Halloween festivities at work, I didn't heat the mold at any point, and I got about 30 pulls before the mold starting failing.

And, though probably not part of the question, I use regular Johnson & Johnson baby powder as my release. I sprinkle some in, use a soft brush to coat the entire mold and get it into all the small crevices/areas, shake out any loose powder, and then hit the entire mold with some sprays from a can of compressed air. The pieces come out cleanly and easily and can go straight into cleaning up seams and to the paint. They take paint perfectly without any washing up first.

I do have a can of spray release, but I mostly use it for spraying plastic containers that I need to mix resin in if the piece I have is too big for disposable containers and occasionally for two part mold separation if I'm out of my brush-on release. I've used it in the molds before, just to see how it goes, and there aren't any issues with it, per se, I just prefer the powder overall.

 

[Duncanator]

I like to think about what each process is doing to the air bubbles. I've done a lot of molding and casting over the years both with and without vacuum/pressure.
These days I vacuum my silicones and pressurize my casting. I find it more consistent, and here's why:

When mixing either rubbers or resins; no matter how hard we try not to, we whip air bubbles into the mixture. So what do we do about it?

-Vacuuming allows the bubbles to expand and hopefully rise to the surface to escape before the mixture has a chance to harden and trap them. This works great on silicones and resins, but I have had castings from open faced molds that kept having tiny surface bubbles that were hard to get rid of.

-Pressurizing squeezes the bubbles smaller - they are still there, just smaller and now pressurized. The thought is that the squeezed bubbles will stay small after the resin has hardened and the part is released to live its life in normal atmospheric pressure. I figure the pressurized air will eventually leak out in time with no ill effects to the casting.

I've also had a couple occasions where I had to de-air the mixed resin (to get most of the bubbles out), and then pressurize it to squash down any remaining bubbles. That was for a REALLY picky client and required a slower setting resin to do.

Experiment!
Everyone has there favorites.

 

[Sinned]

Definitely taking this thread to heart now. I've been getting by without a vac, or pressure tank, but I did some casting last night which really highlighted the need. I'm making some scissors for a costume (SmoothCast 65D, so they're slightly bendy), and they were really tough to get right without a tank. I finally got some decent ones by cutting in more air channels, but sure would have been nice to have the tank. Yeah, I could have made my life easier by adding more channels when molding too.

Am I assuming correctly that a pressure tank would have fixed this problem?

Bummer


Better

 

[DaveG]

Sinned - no doubt a pressure tank would have helped but what you really need to do is give the trapped air someplace to escape as the resin filled the mold from the top down. What I would do is cut little tiny bleed channels, not even 1/32" wide from the very bottom most parts of the mold, the tips of the open scissors and the bottom of the handle loops of the closed, out to the edge of the mold. Yes, a little bit of resin will leak out before it sets, but your fill reservoirs should hold plenty of resin to keep the mold from "bleeding out". I would also consider widening the fill inlet on the pointy end of the closed scissors to let the resign in faster, while it still has the least viscosity. Yes, you'll have a littler bit more to trim and file off the finished casting but that won't be too bad.

Good luck!

 

[Duncanator]

DaveG is right. I think setting up your mold a little differently would make a difference in how it pours up. You need to give the escaping air a way to get out, which is really hard if the air is trying to get out the same hole the resin is coming in.

I'm a fan of running a "fill" sprue down to the bottom of the part, so that it fills from the bottom. The "vent" sprues can then easily allow the air to escape out the top.

 

[cavx]

I have had few fails with my pours and learned something the other day that was neat. I was able to the check the edges of a part without fully demolding and then ablebto top up the air gap. The new material bonded to the older stuff and you can not see the join. I also was able to inject material into the mold which forced the air out. I kind of think I should have been doing this from the beginning.

 

[Kubla Kraus]

Hi everybody,
I am working on my setup and am wondering if the base of the paintpot itself can be used as the vessel for the vacuum chamber... has anyone ever tried this? I am doing the usual 1/2" clear lexan with a 1/16" stick-on silicone pad to make the seal.
thanks.

 

[robn1]

As long as the gasket on the Lexan fits the top edge of the paint pot it'll be fine.

 

[Circassian]

Hello Everyone

I read your articles while I was searching an answer to my problem
If anybody helps me about it I would be very happy
I took of the mold of a model first I degas the silicone and then I put it in pressure chamber during the cure time and everything is perfect up to I REALIZE THE CASTS WERE ALL SMALLER 1/2 CM THAN THE ORIGINAL MODEL !!!!!

Does anyone tell me is this problem because of I put my silicone rubber during cure time in pressure chamber or the shore of silicone or something I did wrong at any other points

Thank you

 

[cavx]

Hang on. You pressure cast the silicone? Nothing to say you can't do this, but it won't shrink if the master part is still inside whilst curing, unless the master part shrunk? You generally degas silicone to pull out air. Once it degassed, and provided you pour from a low point in the mold box, you should not trap air. Therefore you shouldn't need to pressure cast to cure.

Shrinkage is generally resin based. Resin like epoxy tend to shrink up to 10%, but they don't pull from the sides of the molds, the upper level just drops. This can be fixed with a small top up.

Photos would certainly help here.

 

[Circassian]

Hi

Yes the original model was inside while pressured. So the problem is resin as you say
I use polyester resin and marble dust mixture and as you say not from the sides but from length it shrunk )) and as volume indeed

For your solution what you mean by small top up I could not really understand how to do it ? I am very new to casting and molding )

Thank you so much for replying

 

[cavx]

Is the mold two part or open back?

Regardless, when you fill your mold, you pour an open back to be level with the top. You have a pour spout on a 2 part mold and you need to fill that to the very top. When you get shrinkage, you will notice this pour spout is empty. You DO NOT disturb the mold, rather mix a very amount of resin and fill the in the cavity.

When you demold, you should have a perfect part.

Again, post some pictures and to help everyone help you.