Pressure Casting In Resin

[masterpuppet]

hi
the pressure pot is the best way to make reproductiones i build one pressure potand one vacum chamber, the first one with poor closer sistem ( i was think that are secure, i am wrong) , and kabum, blow up, and one piece of the was fixed on my left hand, the hand was most faster than the piece, becouse my head are so closer, and i dont tell this history.
now i got a most stronger pressure pot, but the problem is that in argentina we dont got poliurethane resin, may be is good for my body.. jeje
my ask is how the great companys make your own molds and theys castings sistems, they use pressure castings, or vacuum chambers only, make singles molds or more molds for one piece.
take cares with the pressure pot
jorge

 

[garyo37388]

Safety first!

Jorge,

Glad to hear you're ok. Scary experience. All the more reason one should make sure their equipment is rated ( certified ) at a higher rate than they will actually use.

And one must have safe guards to ensure they don't accidentally apply too much pressure.

When I blew out my seal it was because I had used the compressor on another machine and run the pressure up too high for my 80psi pot.

Now I have 2 regulators. One stays on pot ( please note this is a PAINT POT, not a kitchen pot ) and stays fixed to 60 psi.... well below its rated pressure of 80 psi. Cost an extra $10 , but safety can not be valued enough!

Hope this helps,
Gary O

 

[T50]

Hello gents
I read the thread with interest as casting is what I do every day.
A few years ago, I had to find and learn a way to cast all Alpine
figures in a hurry as the casting company I had hired quitted on me.
Guys like Miguel from MIG recommended vacuum system, and I am
glad that I decided to go with it. I use my vacuum system for
both resin and silicone. I just don't see the point of having both
system when either one can get the job done.

Given that both vaccum and pressure system achieve good results
in casting "resin figures", I can only think of one advantage that pressure
system has over vacuum. It's the initial setup cost.
Other than that, vacuum system is faster, quieter, and less messy.
Faster because you don't have to lock and unlock the pressure chamber.
Quieter because vacuum chamber almost hums when a pressure chamber
can cause an avalanche. Loud noise stresses me out also.
Less messy because you don't have to worry about the harden excess
resin on top of the mold (unless you know exactly how many grams of
resin it requires for every single molds). In vacuum system, you get to
wipe out the excess resin off the mold top before the resin hardens.
Faster and cleaner process means better productivity.

Of course in order for your vacuum system to work, the combination of
the power of the pump, size of the chamber, and gel time of resin have
to all work together.

"Many people think that you can vacuumize Resin, but this is not true. *Resin cures in a minute or two (depending on the temp of the material) and can be removed from the mold in 4-5 minutes. *To be able to pour the resin, put the lid on the chamber, and deaerate, it could take a few minutes. *So, if you want to do solid resin, then pressure is the way to go. *for that, you can get a pressure pot from a local idustrial supplier store."

You can't say that vacuum system is not ideal or suitable for casting resin
becuase of the short gel time. Get a resin with a longer gel time then.

My question is: why silicone molds expand in volume after some castings?
All my silicone molds tend to expand in volume slightly enough that I can
notice the parts are slightly "slimmer"...
Is there any way to prevent this?

TS

 

[Anders Heintz]

Good thread! As someone who has just started casting with both vacuum for the molds and pressure for the casting process its very interesting.

Im not sure how you would cast the resin with Vacuum though, when I have tried it all the resin just bubbles out of the mold over everything!

So do you vacuum the resin before you pour it? Or do you vacuum the mold with resin in it? How does it work?

 

[T50]

Anders, you don't have any type "wall" aroung the mold when
you pour resin into the mold??? Some people build "wall" by
putting a strip of wide tape or thick sheet of paper around the mold
secured by a rubber band. This has pros and cons. What ever
works for you! I think people develope their own way of casting
with the given equipments.

TS

 

[garyo37388]

Hello gents

My question is: why silicone molds expand in volume after some castings?
All my silicone molds tend to expand in volume slightly enough that I can
notice the parts are slightly "slimmer"...
Is there any way to prevent this?

TS

Just a guess... If you have microscopic bubbles trapped within the set silicone, then when you pull a vacuum on the mold ,those microscopic bubbles get bigger. Silicone will stretch, powerless against the now expanding bubbles. This distorts the mold.

You can demo this by putting a half filled ballon into your vacuum chamber and watch it swell and bust as the vacuum allows the trapped air to expand.
Pure physics. Those trapped bubbles ( however minute ) will get bigger as vacuum allows them to expand.

I use a Welch 1402 125lb pump that will pull down to 20 micron and bubbles still can be trapped in thick RTV.

Now for the magic of pressure. When a mold or casting is done with pressure, you hold the material in that state until it hardens. And then it is very stable... much more so that the vacuumed molds.

It's a long debated arguement vacuum vs. pressure. But I say... If it works for you, don't fix what isn't broken.

But I think it's the vacuum process that is causing your problem. Borrow an electronic gauge ( that will measure 29.99xx) and see how much vacuum you're really getting. 10-20 micron is fantastic. 50 micron is good. Less and you may may issues.

If so the the fix is :
1) seal the system tighter ( assumming pump is not issue )
2) Buy better pump ( assumming chamber/system is not an issue )
3) Switch to pressure ( sorry... had to say it )

Best of luck,
Gary O

 

[garyo37388]

Anders, you don't have any type "wall" aroung the mold when
you pour resin into the mold??? Some people build "wall" by
putting a strip of wide tape or thick sheet of paper around the mold
secured by a rubber band. This has pros and cons. What ever
works for you! I think people develope their own way of casting
with the given equipments.

TS

I cast extra large sprue that can hold a PVC insert. Excess goes up tube and can collapse back down into mold. PVC pipe is very cheap.

 

[Calvin]

I can only think of one advantage that pressure
system has over vacuum. It's the initial setup cost.
Other than that, vacuum system is faster, quieter, and less messy.

Vacuum is not better than pressure in the meaning that these are just techniques, each one with its own pros and cons.
Acrylics are not better than oils as Sculpey is not better than Magicsculp and viceversa. Each different method or medium requires a different technique, defined as skill once learnt. But exactly as for painting and sculpting, casting too requires an 'artistic' attitude, defined as talent once learnt. Mediums and techniques per se are nothing and will bring you nowhere.

Personally I use pressure to make molds and for resin casting and I am more than happy with it. It is impressive to see how many people still think that vacuum is the right and the only way to make molds. As Gary already pointed out the best way to get a perfect mold (with no physical imperfections) is to pressurize it but generally people does not believe it until they try it.

The problem pointed out by Gary about the deformation of the little air bubbles still into the silicone due to an inappropriate vacuum level cannot be solved using more vacuum, air will get trapped anyway while pouring. The only way to get rid of air bubbles is pressure.

Regarding the shrinkage issue consider that molds (once set) does not expand giving a slimmer piece. A mold could get temporarily deformed during the casting process due to the used technique (pressure/vacuum) or get permanentely deformed because the stress of the heat generated by the resin while curing during each pull. If your pieces look slimmer once casted is because you are applying a wrong amount of vacuum which deforms the mold and keep it deformed while the resin cures.

 

[T50]

I'd like to try the pressure system once they have a quite pump.
I just don't like my workshop to sound like a construction site.
Maybe there's a way to mute the noise? Now that I have a larger
workshop, I have some room for a pump and a chamber

Regarding the shrinkage issue consider
that molds (once set) does not expand giving a slimmer piece...
get permanentely deformed because the stress of the heat generated
by the resin while curing during each pull.

Thanks for the help, Luca and Gary!
I do suspect that the reason for deforming silicone is due to
the stress of the heat generated by the resin while curing.
Once this deforming of the silicone is starting to happen, the
cured resin parts are starting to have "oily" or shinny surface.
I tried to age the silicone molds at least a week (usually more)
before the first casting, and it seems to help a bit. But this problem
does come soon or later. The aging of molds only delays the
problem just a little.
Is this resin heat related issue unavoidable?

TS

 

[Ray]

technique

For pressure casting I find it most expedient to have a large carrier attached to the cast piece’s bottom, (mold side up) to increase the volume of material being forced into the mold cavity. In theory, and seemingly at least, in practice this extra material increases the forces being applied inside the mold, and helps crush the gaseous air in the resin to a size too small to be visible in the finished casting. As one of the previous respondents noted, however, the mold/casting must be left under pressure until the resin has set, usually not to the point of being ready to de-mold though.

As regards pressure cookers as pressure pots; I have used these things to effect with satisfactory results. The biggest problem encountered with using a pressure cooker was that pressure is limited to about 30psi, never more, which reduces the effectiveness of the system in crushing air in the casting. As to safety, I would probably never actually recommend using a pressure cooker for this task though I have never experienced worse than a blown lid gasket over several years use. Just as someone else earlier noted, paint pressure pots are an excellent and cost effective alternative, with most capable of handling pressures up to 80psi, though pressures this high are almost never required (never in my experience in fact), 40-60psi being more than adequate for the task.

Considering all that has been said here about vacuum casting, and having always used the technique for mold making I can offer the following observations to the discussion. First I have found that de-airing under vacuum works best when the surface area is large and the depth small. This isn’t to say that a deep item with small surface area can’t reliably be de-aired, only that the process’s effectiveness is reduced in these instances. Probably the simplest, most reliable things to be cast with this method of vacuum would be flat wall sections in open face molds, the walls being very shallow less material needs to be passed through to reach the castings surface. The increased surface area of something like a wall casting serves best to enhance the benefits of a shallow casting, or perhaps counteract some of the difficulties of a deep casting. Of course with resins possessed of an extended pot life these observations are probably of little actual use, and might not need to be considered at all.

In the end neither method is likely to be the best for all situations, and kinds of molds/castings. The only other observation I can make with confidence on this topic is that a well designed mold will always result in more reliable reproduction no matter which method (pressure or vacuum) is used. When considering pressure casting I would, as earlier alluded to, increase the mass of material above the proposed casting with a larger carrier, which of course needs to be incorporated into the mold design/manufacture. Molds intended for the vacuum process I would probably make in much the same way, though the increased carrier volume would not be filled with material when poring. This increased mold volume’s sole purpose would be to contain the casting material as it expanded during de-gassing, and would not be filled to capacity, either during poring, or after de-gassing.

 

[garyo37388]

Different resins develop different amounts of exothermic heat. And different mold material have different temp. tolerances.

I'd suggest you contact tech support for the materials you use. I've found the folks at Hyperlast ( Synair ) and Polytek to be superb in their desire to have their products work flawlessly.

In terms of noise, remember it's the compressor that determines noise level not the pot. So a big tank with ample volume will only come on occasionally. They be located outside or on rooftop as well. Or inside a sound insulated closet.

 

[garyo37388]

, the lid would swing up on simple hinges that has quite a bit of play in them, the lid would close on a rubber foam seal

Surgical tubing works really well, with RTV to join and seal the ends. The weight of the lid will begin seal. Vacuum easily causes the tubing to be smashed flat and create a perfect rubber seal.

 

[Jiver]

Here is from a few years moulding experience my opinion about vacuum and pressure.

If it comes to production, vacuum is way faster.Simply because your vacuum chamber can be continiuously used, where a pressure pot can not because it must be under pressure untill the resin is set.Given the fact that it's a wise step to take a resin with a longer potlife.It takes a little more time to set up and you don't want the resin beginning to set up before you have the chance to pressurize.The resin I use for vacuum casting has a 3 minute potlife.Not nearly enough for our pot.Unless you only have to fill 1 mould, but that's not much of a production,is it?
Has pressure no advantage over vacuum?Sure it does.It's more suitable for complicated pieces, let's say a car body for example.With vacuum, airbubbles need to find their way to the surface in order to disappear.For pressure, fysics clearly dictates that pressure is equily everywhere.So bubbles, where ever they are in the mould, will be compressed.Second advantage: almost no loss of resin.

A last personal note about the equipment: coocking pots are handy and cheap.Fixing up your face by plastic surgery is not cheap.It wouldn't be the first time I hear stories about the lit going for a walk to the moon.Spending $250-$300 for a pressure paintpot is way safer.

 

[Uruk-Hai]

Very interresting.

However, where can I find such a vacumchamber premade for my own use?
Or a detailed guide how to build me one?

Cheers

 

[Jiver]

Mine is a custome made metal case with a 1,5 inch thick plexi lit.But I let it made by a company, I didn't do it myself.
I'm afraid my set up isn't really something an ocasionnal caster should look for.The pump I use is quite expencive.

 

[Uruk-Hai]

Ok, perhaps other Figureteers knows where to turn to?

Cheers

 

[galdino]

I've made a vacuum chamber using a cooking pot:

It worked really good.
The vacuum pump was bought is stores that sell equipment to fix refrigerators.
The one I 've got is small and work well.
My friend joke saying that I was going to make "atomic beans".
The problem I had was already mentioned here:
The almost microscopical bubbles, when submited to vacuum, get bigger and visible.
I could not solve this problem yet.

 

[T50]

Check thi site:
http://www.abbess.com/vac/homepage-vac.html

 

[garyo37388]

I've made a vacuum chamber using a cooking pot:

It worked really good.
The vacuum pump was bought is stores that sell equipment to fix refrigerators.
The one I 've got is small and work well.
My friend joke saying that I was going to make "atomic beans".
The problem I had was already mentioned here:
The almost microscopical bubbles, when submited to vacuum, get bigger and visible.
I could not solve this problem yet.

I'm sure you're problem is in the gasket or the fittings. Remember that the lid on a pressure cooker is blown outward to create a partial seal ( a full seal is not necessary since it blows off at 30psi or less )

When you subject it to vacuum the lid is pulled downward in the opposite direction from which it was designed.

Pouring a soft RTV gasket into the lid's channel might fix minute leaks allowing you to pull full vacuum.

 

[galdino]

Thanks Gary. I'll work on it. The vacuum, actualy is not the problem. The increase of size of the bubbles is what I'm trying to fix.
T50, the equipment in the page that you indicated seems to be wonderfull but must worth a fortune!
For a while I'll have to use my "atomic beans" pot!