Silicone election for small detailed casting and pressure pot problems.

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udog

Active Member
Hi
I´m into molding and casting a pair of babies (reborn kind, 8-12cm). I´ve already molded them with PlatSil 73-15.
And am casting both with Smooth On´s Colormatch 325 Slow and a FormX resin called Rhino (slow version).
Silicone was degassed prior to pouring, but not pressured potted. Resins are pressure potted when casted.
I´m running into several problems. 50% of the castings have to be discarded due to different reasons And it´s starting to drive me nuts.
For what I´ve read here in the forum molds that will be used in a pressure pot should have been put in a pressure pot too or you can get distortion when pressure pot casting. Well yes, I´m experiencing these distortions.
Although I degassed the silicone (molds of 1 and 2kg each, powerful pump), most probably there are still bubbles left and therefore the distortion when casting in the pressure pot. I mixed the silicone with a mechanical mixer, which introduces much air in the mix, I have to try mixing by hand I guess. Would that help?.
Is it a must to put the silicone in a pressure pot then?. It´s a narrow one, I could have problems fitting the big mold in.
The other thing is that Platsil 73/15 is shore D15 hardness, might be too soft for the pressure. Anyone uses it?. Should I go for a D20 shore?. The problem there is that it´s viscosity is higher and I need it to be as low as possible. Any suggestions?.
And also, the figures have lots of details that in the negative end in zero, peaks in the mold etc. which tear off after little castings. I´m in the dilema of using a harder silicone, but not sure it would stop happening or if it would be worse...and the viscosity issue remains.
What silicones could you recommend for this kind of small detailed castings?. I can get my hands on most Smooth On and Polytek products here in Spain. But can´t find anything better than what I´m using.
Thanks in advance.
 

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sjanish

Sr Member
RPF PREMIUM MEMBER
Smooth-On tech support told me it not necessary to cure the mold under pressure. They recommend degassing in a vac chamber and pouring in one spot, let the silicone find it level, etc.

I normally use Plat-sil, different numbers over the years. The hardness I have use have been 35, 40 and 50. All have performed great making small detailed parts under pressure. When I have problems it is normally due to a boneheaded mistake I made in the mold design. The silicone you are using is much softer, so maybe it is different. My thinking is that softer stuff should still work, but you need to make the mold walls thicker (bigger mold box, more RTV, more expensive) and/or make an outer support shell of some kind.

Read the Smooth-On technical data sheets to see what they say about degassing and curing. I call their tech support (Reynolds Advanced Materials in Dallas) if I have questions and they have always given good advice. If calling isn't an option for you maybe you could find a tech support email your questions.
 

udog

Active Member
Thanks!
Well, it´s obvious I´m getting the distortion issue. Probably the silicone is too soft, and also it may still have some air in, though degassed. As I said I have to try mixing by hand to try to minimize the air that gets in.
So it looks like a good idea to try for a higher shore. Will have to do some research to find a low viscosity one. But I´m having trouble to find what I think I need. The first test molds where made with a silicone very similar to Dragon Skin, higher viscosity, and got some bubbles in the mold. I solved this with lower viscosity and brushing some silicone prior to pouring, with the 73-15.
Also I went through the translucent route, these are block molds with a slit cut in the back, so I could see what I was doing. Probably not totally necessary now that I know where to cut.
I´ll also take a close look to Tech Bulletins as you say, and will write Smooth On. They have always answered when I´ve done so.
As for the mold design, I´ve been working with 3-4 cm thickness minimum. Yes, bigger will raise budget. I would prefer not having a case for the mold. It will slow down the process as I see it. Do you think that could help the distortion issue?. Not sure, as the pressure should happen not only from outside, but also from inside?.

Yes, the mold design is another part of this. To solve trapped air problems I had to put in breathers...too many. Before starting I saw some pics from a studio molding the same kind of babies. I know not two figures will be the same mold designing wise, but the only breathers I saw in their pics were for the ears. Which surprised me, but I decided to give it a go that way. With bad results, lot´s of trapped air bubbles. So I had to go for a breather everywhere they appeared. But this was before I started pressure (I also got some pinhole mixing bubbles in the casting) pot casting and slower resins. I don´t know if pressure and slower resins would get rid of trapped air, my learning has always been to place breathers to solve this. I think I should open a new thread for this part.
 

udog

Active Member
I´ll make some tests with what I have. New molds, different casting process and will post results. Any further feedback will be much much appreciated.
 

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Duncanator

Sr Member
I suspect the softness of your silicone may be the main culprit for the distortion. I usually use 30 or firmer for mine. Also I always use a box or mother mold to support the silicone to keep it from distorting, especially with soft silicones. Rubber bands or clamps to hold the mold shut will squeeze the mold unevenly without a mother mold.

I find that proper vacuum degassing is sufficient for molding. I actually don't recall ever pressure potting my molds to cure. And I do pressurize my castings.

Pictures of your molds would be helpful to diagnose. Perhaps you need a few more vents?

Just curious why you are wanting very runny silicone? In my experience, even thicker silicones will flow into all the nooks and crannies by the time it cures if it has been degassed. Vacuumed silicone almost seems to suck itself in on its own. If I have a particularly detailed master, I will sometimes degas the silicone before I pour it, and then degas the poured up mold, then remove from vacuum and let it cure at ambient air pressure. If you do this, make sure your mold walls are tall enough to not overflow, and that your master won't collapse under vacuum.
 
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JPH

Sr Member
Sorta Clear by Smooth On is pretty rigid, and works well when degassed for picking up details.

I have also used the Eco-flex 00-50, for molds and castings.

No need for pressure pot. If you suck the gas out, forcing it back in is a does not compute.

Pour a thin stream to the lowest point, brace yourself against something, and just take your time. It is a boring, non glamourous way to do it, but great results with Ecoflex and Sorta-clear when I do that
 

udog

Active Member
Thanks for all the feedback, very useful info.

Sorta Clear by Smooth On
That´s the one I spoted for now. A bit out of budget but looks like the one I need. It would be great to use a polytek one as they´re a bit cheaper here. But the Polytek supplier is not bringing polytek´s clear platinum silicones higher than 20D shore regulary, and I can´t wait for a special order.
I could find non translucent ones though. What do you think?. Now that I´ve cut open five or six molds I know where to do it.
Just curious why you are wanting very runny silicone?
Well, because I got trapped air bubbles in some parts. So the first idea was to go for a less viscous one to make degassing easier. But it could be the pot life too. One good thing about Sorta Clear 40 is it´s 60 minutes working time, which I guess will give more time for air to go. I´ve been working with 20 min pot life silicones, probably a mistake, too little. Also I tried brushing in silicone in the problematic spots. But one of them is so "deep" it won´t get in every time I brush in.
In any case they look like trapped air bubbles, not mixing ones.

Pictures of your molds would be helpful to diagnose. Perhaps you need a few more vents?

Yes, I´m casting some tests. For now I can say that distortion is not as much as I was thinking at a first glance . I will be starting new molds on Wednesday hopefully so I´ll try to be back by then and post pics of the model etc. and the results of my tests.
That way it will be easier to explain.


Thanks all!
 
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udog

Active Member
I just found out I have 10 kg of unopened T-4 silicone (Xiameter, before Dow Corning) in the studio. Purchased to a company called Notcutt (UK) years ago. It´s a very viscous plat silicone, it´s thick uncathalized, we used for some shark teeth or something similar.
It´s goes more liquid when you add the cathalyst, which is totaly liquid.
Very thick (35000 cps), but 60 minutes of working time (that´s Mouldlife´s, Dow says 90 min).
Anyone used this one for a detailed model?. I think I will be testing this one too.
One concern is that it might be too hard making demolding difficult, But if it works I am sure very lucky. Besides it should be used before it goes off.

How does it sound to you guys?
 

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JPH

Sr Member
Somuch of these quest are the mini-quest to discover what "works." I used sorta clear for some molds that had thin spots and didnt work so well but are great at holding shape for long flat parts. I used Ecoflex for thin molds with multiple concavities and it was sweet. Sad that we have to spen 30-40$ per trial, but when it works, it is worth it.
 

udog

Active Member
Ok, I´m going to try this T4 Xiameter 40D shore silicone. I´ll be lucky if it works fine as I already have it. My concern with this product is how much will the thin parts of the mold, such as the areas where arms meet the body for example (very thin section), will last. With the 15d shore polytek 73-15 they start tearing pretty soon, which gives some sanding in the casting. Not sure what will happen with a 40D sure like the T4. They could tear off in bigger pieces. But as JPH says, one has to test to know.
I attached some pics with the models, breathers and defects.
Now I´m pretty sure that distortion due to pressurizing is not the main problem I´d say. There where some bubbles in the mold, not easy to see in the translucent material. Probably the pressure pushed the resin out a little, but I casted some without the pressure pot and the difference is minimal. But these pimples looked weird in first instance, not what I expected for a bubble. That confused me.
The models
BIG.jpg
ymail-tmp-4753603884224060011.jpg

Pimple distrotion
DEFECTS.jpeg

Breathers
MOLD.jpeg

The breathers work ok for trapped air, what bugs me is this image I found before strarting. It´s a mold prepared by a studio that casts this kind of pieces.

https://www.modelarte.net/wp-conten...ente-para-bebe-reborn-miniatura-1030x1030.jpg

They only have two breathers set, in the ears. It´s not the same position but it´s similar to mine, the small one. Initialy I made a pair of molds this way, though I´m used to solve trapped air with breathers I though "if they did it it should work"...well, no, I didn´t. Lots of trapped air in the castings (ears, toes and others). Haven´t tried them with the pressure pot, but some were big bubbles, don't think the pressure would hide them. Any feedback with this?. Taking out the respirators in the casting takes time, and gives some sanding too.
Also, sometimes, when the breathers break demolding, they break "too much" leaving a small indentation inwards that needs sanding to hide it...more fixing. Any tip to aid a better "break" of the breathers?
This is my casting process:
Spray Ease Release (will sart with polytek 2500, never tried it before) in the mold and spread it with clean hands. Its closed and not easy to get in with a brush.
When the spray is dry, talc powder and remove excess.
Close mold with pins in the seam and duct tape, trying not to tighten too much so there´s no distortion.
Mix my resin and tint (so-strong). I don´t want to premix all the resin at once, as I´d have to shake it...more air in the product.
Inject with syringe...¿good?...any other method.
When it´s almost full I rotate the mold a little.
Once filled I stick in the same "toothpick" I used for the breathers in the breathes holes to make sure any air comes out and that the breather is full ok.
I squish the mold a little to help air out again.
Mold goes into the pressure pot. At 5, started with 6 but lowered it when I thought there might be distortion. The small one slightly tilted forward to avoid a small bubble in the nose.
Once gelled, out of the pressure pot and into a heat tunnel to speed up hardening. I need to pull two castings a day, I didn´t expect using such slow resins. Dumb me.
I´m aware the heat tunnel might help to shorten the mold´s life, but deadlines...
 
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sjanish

Sr Member
RPF PREMIUM MEMBER
MOLD.jpeg

THIS image bugs me! It looks like something out of a Vlad the Impaler movie. :eek:;)

Did you sculpt that? If so, very nice. I could never do that.

Remember that everything is happening in the negative. It does look like you have some tiny bubbles, and some bigger, close to the surface (baby master) of the mold. When the pressure compresses them, the resin flows in. The only advice I can offer is to make sure you pour the silicone according to the directions. I have had this problem if I try to pressure cast using a mold that I did not degas. I have never had it happen when I degassed the silicone. Pick a spot away from your master and pour slowly. You may try leaning the mold box to get a better spot if space is tight. Try to stay over the same spot, holding the bucket high up so the silicone makes a thin stream and stretches(pops) any small bubbles. Bubbles that don't pop stay right on the surface. Air is lighter than silicone so they should rise with the level of the silicone if you pour consitantly in the same spot to avoid covering them under a thick layer of silicone. If they don't end up popping at least they are away from the master's skin so they won't matter.

Edited to Add: Curing your mold under pressure should get rid of these tiny bubbles. You will need to make sure your equipment - pressure pot and air compressor are reliable. You will need to keep your pot pressurized for the full cure time of the silicone. You may have to rely on regulators to keep your pot pressurized unless you want to sit there and monitor it the whole time. I normally use silicone that takes 24 hours to cure. My pot holds fine for half hour cure resin, but would be at zero within an hour or two. Unless you have a high quality pressure pot you will most likely lose some pressure over time, defeating the purpose. Be careful, pressure pots scare me. Never lose respect for them.

Venting and pouring, IMO, is an art as well. Sometimes you have to learn how to use a mold. It takes some trial and error, like you are doing. Of course it is best if you can see it all in your mind as you design the mold - before you pour the silicone. It starts to make sense as you get more experience. If you are doing it right, the resin will always be at the same level as you pour, i.e. with the horizon. Just like when you fill a cup of water. That is how I think of it when planning my vents. Think about where air can get trapped based on the resin rising at the same level the whole time. If air cannot get trapped by the level of the resin, you may not need a vent there. Does that make sense?

As for the vents breaking during demold, that is a problem. Especially after several pours. You may try to carefully open the mold and snip the vent before just pulling the casting out. Also, you may try using something thinner where it actually touches the master. Any damage would be smaller, and it would be easier to snip if you can get to it during demolding. The air wants to escape. It will leave through a small vent just like a larger one.
 
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udog

Active Member
Did you sculpt that?
No, not mine. I was asked to mold and cast it. I´m also a sculptor but never sculpted something that small and detailed. Yes, very nice models, if I get to know her name when this is finished I´l credit her her. If I knew I would have already done. It´s super sculpey by the way, a bit fragile.
THIS image bugs me! It looks like something out of a Vlad the Impaler movie
Haha!, yeah, I´ve thinking the same these last weeks. It´s macabre.

Thanks, I´ll try to pour as you say. Will be doing it in half an hour. The silicone is 12 hours cure time, probably little for what you say. And also, temps are rising here and I made a small silicone test and it is going faster. I´ve sticked it in the fridge to see if I can slow it down. The resins are also going faster, bad thing. Probably they´ll go to the fridge too.
The vents are doing their job, the only thing is the breaking problem I mentioned. I used these large toothpicks. The first molds with the point almost like they come (thin), but sometimes the air or resin wouldn't come out so in succesive molds I filed the point a little more depending on the place. Will try to make them thinner in the next one (I have some molds to prepare yet).

¿Did you get to see the link I posted?
MODELARTE - ESCULTURA, MOLDES, RÉPLICAS Y FICTICIOS
Can´t imagine how they cast with no breathers but the ears..if they get to do so, which looks like. It would be great to avoid as many bleeders as possible.
I can snip two or three vents, before totally demolding but not more. I don´t have good access to them. Some of them just break when I open the back part.

Think about where air can get trapped based on the resin rising at the same level the whole time. If air cannot get trapped by the level of the resin, you may not need a vent there. Does that make sense?
Yes it does. That´s my concept too, I will review what I´ve done to see if vents can be reduced.

About molding with the pressure pot, it would be great. But I fear my pressure pot is not big enough. I use a pressure pot designed for sandblasting, its not very wide and I´m sure the bigger mold won´t fit. I´ll think of fitting the smaller one, but looks difficult.
I use a 50L belt compressor. It should stand for some hours, but don´t know if 24 hours. Besides I fear the pressure pot lid has distorted a little. The exhaust valve didn´t work once (must have been clogged) time, and the pressure was set too high when experimenting. I had to create a silicone seam to prevent leaks, but not 100%.
So I´m not very confident in being able to pressurize the molds, but will try with the small one if possible.
Thanks!
I´ll post my progress as it comes up. Fingers crossed, I hope this T4 works, if not, ready to spend some more money.
 

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sjanish

Sr Member
RPF PREMIUM MEMBER
Another thing I forgot to say earlier. You mentioned shaking your resin. I have run into this issue too. I use Smooth-On Smooth Cast 305. They say to shake well. I have tried two things. Pour the resin into A and B mixing cups and degas each. Don't put them into the vac chamber with each other as you don't want the A and B gasses mixing and start curing before you are ready to mix. The other thing, and this is what I have settled on is to shake the resin really good an hour or two before I will be pouring. That give the bubbles lots of time to rise out of the liquid. When I mix A&B try to do it thoroughly but not hard or fast. Any small bubbles get squashed by the pressure.

I did look at the picture of the other guys mold. I don't know how he does it. Somehow he makes it work. It must be possible, but I'm not that good. Most of my molds are two parts. I've never tried to cut one like you are doing. The one thing I think I see is that the baby is not oriented straight up and down. I think the orientation allows the resin to flow in such a way as to not trap air. Again, pressure squishes the small bits of trapped air.

On one of my earliest, poorly designed molds I had some vents that were causing problems with demolding. I used styrene rods to make the vents so I got the same rod and coated it in mold release. I would pour the resin into the mold and then insert the rods into the vents, forcing the resin back into the casting. This mold is too big from my pressure pot, so once I see the resin kick I pull the styrene rods out. If I leave them to full cure sometimes they would break off causing the same problem. It doesn't always work perfectly, but fortunately the vent is on the underside of the casting and is never seen once the casting is assembled into place.

On some of my more recent molds I have inserted steel or brass rods when using RTV to make the void would cause demolding problems. I coat the rod with mold release, and seal it into the mold. Once the resin is cured I carefully remove it from the cured resin.
 
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robn1

Master Member
...No need for pressure pot. If you suck the gas out, forcing it back in is a does not compute.
Curing a mold under pressure does not force air into the silicone. It squeezes it out, the same as it does with resin. I cure all of my molds with pressure, without degassing, and they are always perfect. There are two caveats to this technique:
1. Your mold box must fit in your pressure pot.
2. Your master has to be able to withstand pressure. A solid model like resin, plaster, wood etc. is fine, a hollow vinyl doll head won't work. You must either fill the model with something solid prior to molding, or add an air vent in the mold box bottom to allow the pressure to equalize.

...Igot suckered into using Omo (something like that) worse than trying to use wet clay. Dont buy Omoo
It also shrinks too much, I don't use anything that's more than 1% shrinkage.

View attachment 1311169
...Edited to Add: Curing your mold under pressure should get rid of these tiny bubbles. You will need to make sure your equipment - pressure pot and air compressor are reliable. You will need to keep your pot pressurized for the full cure time of the silicone. You may have to rely on regulators to keep your pot pressurized unless you want to sit there and monitor it the whole time. I normally use silicone that takes 24 hours to cure. My pot holds fine for half hour cure resin, but would be at zero within an hour or two. Unless you have a high quality pressure pot you will most likely lose some pressure over time, defeating the purpose. Be careful, pressure pots scare me. Never lose respect for them.
I use a 24 hour silicone and pressure leaks are a problem. I use two auxiliary air tanks, 12 gallons each, to supply extra air. I also have a regulator on the pot set to my desired pressure. The regulator and extra air tanks keep the pot at pressure long enough for the silicone to cure.
 

sjanish

Sr Member
RPF PREMIUM MEMBER
It is kind of nit picking...vacuum degassing the silicone sucks the air bubbles out. Curing the silicone under pressure compresses the bubbles so small that they are unnoticeable to our eyes. Pressure does the same thing to the air bubbles in resin, makes them really small. When you take the cured-under- pressure mold out of the pressure pot the teeny-tiny bubbles will distort, you just won't notice it. When you fill it with resin and re-pressurize the bubbles go back to the state of "not being there" as far as the skin of your casting is concerned. They will not show.

I just don't trust my equipment enough to leave it under pressure for long periods of time. I did it, once. I was nervous the whole time my regulator and safety valve would fail and my pressure pot explode. My wife would be PISSED! Then I figured out I get the same results if I vacuum degas and skip curing under pressure. I am no where near as scared of my vacuum chamber as I am of my pressure pot.

You can also pour the silicone in the mold box and vacuum degas again if your mold box is big enough. I have done this a couple of times if my master has really small negative details. Sometimes the silicone flows over them and traps air. I pour just enough to cover the details, degas to suck out the trapped air, and then continue pouring the rest of the silicone. You have to be careful, if your master is porous silicone will be sucked into it making the mold a total failure and trash your master at the same time.
 

robn1

Master Member
It is kind of nit picking...vacuum degassing the silicone sucks the air bubbles out. Curing the silicone under pressure compresses the bubbles so small that they are unnoticeable to our eyes. Pressure does the same thing to the air bubbles in resin, makes them really small. When you take the cured-under- pressure mold out of the pressure pot the teeny-tiny bubbles will distort, you just won't notice it. When you fill it with resin and re-pressurize the bubbles go back to the state of "not being there" as far as the skin of your casting is concerned. They will not show.
As I said I make all my molds this way and they're always perfect, with no sign of bubbles expanding under the surface. I've even sliced some of them open and there is never a sign of bubbles, it's just solid silicone all the way through.
 

sjanish

Sr Member
RPF PREMIUM MEMBER
It isn't my chosen method, but I agree, it works. I am just picking nits becaue I enjoy the discussion. Any bubbles that might be there are so small from curing under pressure that they will be unnoticeable to the naked eye.
 
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