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Casting/mold making

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I've got some questions about casting silicone and urethane. This can be a general too if you so wish.

So I'm making a small creature for a scupture, that I'd like to cast as a single part. Preferably in skinlike silicone.

>What would be a good sculpting medium if I want to take a urethane mold from it?
>how would you deal with casting limbs in a complex pose (my creature is supposed to be in a fetal position, and while it would be much easier to cast laying flat, I don't know how I'd get it to curl up into a ball...

It'd be great to hear about your projects, whether it's plastic or metal.
Selfish bump.

If I were just building a silicone model, I would cast a wire armature in to make the thing poseable... but I hope to work my way up to casting some light animatronics into the thing--just a servo or two to make it twitch a limb.
Don't bump your own thread...
...everything you need to know about casting is available at www.smooth-on.com
I'm pretty sure u need to make model or texture of exactly you would like it to look like then you need to make a cast of that with silicone just make a wooden box to fit your model in to then pour in the silicone I know at my work we use a chamber and vaccume pump to get the air out of the silicone it makes it more dense therefor lasting longer then after its done curing take the silicone out of the box and carefully cut in half making sure to bypass the sculpture inside after that you have your mold I would use latex for skin and silicone for textures
At ease soldier, at least he's not bumping every 5 minutes like some of the sperglords we get here.
Their website is horribly organized... and I'm looking for more detailed advice on prop building, I know how to cast static shapes and use the materials.
You will want the initial sculpt to be in the same position as it will spend most of its time in. Sculpt it in fetal position, laying down would require some impressive level of internel joint articulation that I don't think wires alone would achieve.
Work your way up to the more technical stuff till you fully understand the basics.
And if you have a vacuum degasser, make sure your rubbers have a long pot life to work out their bubbles. You can get some amazing casts with one, but if that shit sets up while the product is at a foamy stage, you'll run into some troubles. Just had a resin cast come out as a brittle sponge--but amazing detail was captured. Too bad key features were crushed in removing the piece, or I could have used it as a one-time-use master for another mold.

I am working on casting urethane rubber and shitty resins to make some custom 1/6 scale figures, and while I already know what I'm doing for the most part, I'd love this topic to take off. It's a very interesting subject to me. Fun stuff.

I will post some pictures when I get home from work today.
I'd like to see some shots of your work, too, OP.
And as for what sculpting medium to use for a urethane rubber mold, urethane generally seems a little more rigid and much less elastic than doing a silicone mold.
I usually see it used in two part molds of rigid, symetrical things with very few or no undercuts. To get a figure as complicated as a generally human form in a fetal position would require some creative thinking and likely more than 2 parts to the mold (increasing the opportunity for flashing in more places).

The sculpt would need to be made of something durable and rigid to survive the common process of using clay walls to section off the mold areas. I would say a baked polymer clay would probably stand up to it if you're gentle and there aren't any particularly thin sections.

Is there any reason why you specifically want the mold to be urethane? It would probably be easiest to make the mold silicone, and use the proper release agents to also cast silicone into it.
Pictured here are the ghetto as fuck molds I made out of some extra silicone that was going bad.
In front of them are resin casts I pulled and painted. A beret and an AK.

The original beret was quickly modeled in Sculpey polymer clay, didn't bother baking it to harden. In retrospect, I should have baked it, because I did a lot of damage to the mold digging bits of clay out later.
The AK was a 1/6 scale model from China or some other azn shithole.

The beret does great casts, with a few defects (because the mold material was going bad, there's spots it never cured). The ak mold gives me problems, probably because I had never done a two-part mold, and just made up a plan as I went along. In addition to the imperfections from uncured mold material, it has little support in important stress locations, and likes to separate into its two halves when filled with casting material.
It gives passable results in cast resin, but has LOTS of room for improvement if I ever try again.
Here's the top of the molds, where you feed the casting material.
Tonight, the plan was to experiment with casting urethane rubber in them to see if I could get better results. I added extentions using hot glue, electrical tape and bits of paper plates. The extensions will give the urethane somewhere to foam up and then feed back into the molds when I degas it. The material can foam up quite a bit, so make sure you give it enough room to expand.

Degassing has two effects. It draws out any unnoticed air trapped in the material, which will 1)aid in getting a bubble free casting and 2)reduce the volume of the material.
1) is a good thing
2) is a side effect that you will have to account for by pouring extra material into the mold, because when you let off the negative pressure, its volume will shrink down to a smaller size than when you first poured it.
Here's both of them in the degassing chamber. My urethane is naturally a translucent off white color, but I tinted this stuff flesh tone because I felt like it.
You'll notice the beret mold is foaming pretty well.

This type of urethane has a pot life of 50 minutes, so I let it sit in the chamber 45 minutes.
After the 45 minutes, I SLOWLY release the negative pressure. If you just open the valve and let the air suck in as fast as it can, you could introduce more air back into the material.

It's important to let the casting material resume a normal atmosphere before it completely sets, otherwise you run the risk of it curing into a spongy substance.

You can see in this picture that the bottom of the chamber is filled with the urethane. The AK mold wasn't able to handle the density of the rubber, and separated, spilling shit everywhere. Odds are that's a bad cast, but we'll find out tomorrow.
Demolding time is 24 hours, and I will update with the results.
Feel free to ask if anyone has any questions about this process, or other aspects of molding and/or casting rubbers, resins, plasters and similar stuff. While I haven't worked with everything, I have done quite a bit of research on a good deal of this stuff.
If I don't have the answer, it'll be a good reason for further research or maybe someone else might have the answer.

At some point I threw a second cast-in-progress in there. I had the extra room, and wanted to try a different experiment.
As expected, the AK was a complete failure. I'll eventually fix the mold design when I have more silicone.
The beret came out pretty good. I can demel off any defects that the cast filled in.
However, an issue I didn't think of till *after* I pulled the cast, is that the flexible nature of this urethane rubber in this thin a cast may not lend itself very well to the acrylic paint I had intended to use to color it. I guess I'll find out.
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