I've seen the process for making an Army of Gromits, and think it would be very useful for making my characters.  I would like to melt Van Aken Clay, and pour it into a silicone mold with an armature already in the mold.

Can someone recommend a good silicone rubber that will last a long time that I can make the mold from.  It would be a two part mold.  I would most likely sculpt the original characters out of Monster Clay.  Since I do not have a way of degassing the silicone, it would have to just pour and cure type of situation.

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I've never cast clay in a silicone mold, but I have made a good few silicone molds and I'll at least get the ball rolling on the thread, if I can't give exactly the advice you're looking for. I usually use a silicone called Oomoo 25. The 2 components when mixed become purple, and it's a very nice firm formula. You can get it through Amazon - in fact here's a link: Oomoo 25 @ Amazon

There's also an Oomoo 30. The difference is in the pot life and cure time. I don't know if you're already familiar with these characteristics, but here's a quick breakdown - Pot life refers to how long the product stays workable after you've mixed the 2 components together to activate it. After a certain point it gets too thick and gummy and you can't do anything to it anymore - you've just got to let it sit and cure. 15 minutes should be plenty of time - all you need to do really is tap on the table for a few minutes to make any air bubbles rise to the top so they're not right up against your model to create voids. 

For Oomoo 25, Pot life is 15 minutes cure time is 75 minutes. 

For Oomoo 30, Pot life is 30 minutes cure time is 6 hours.

That's a pretty substantial difference in waiting time!

And of course cure time is how long it takes to cure so you can open the mold and use it. 

The only real reason I can think of why you might want a different product than Oomoo is viscosity - it's a pretty thick silicone, meaning a less viscous one would flow more readily around small surface details and be less likely to leave any voids. 

Another thing I like about Oomoo is it's a tin cure silicone, which is the friendlier type. You can use plasticine to sculpt the model and it won't stop it from curing. Platinum cure silicone is a lot less forgiving.

Oomoo is also very simple to use, being a 1:1 mix ratio by volume. That means you don't have to weigh the components precisely and calculate out how much of each to add - you just place 2 plastic cups side by side and fill them to the same level by eye. Simple as can be. 

If anyone knows of a thinner silicone that would work better,  or if I missed any necessary characteristics, then please post.  

Just out of curiosity Strider, If you don't use it for Clay what other purposes have you used silicone molds for?

Casting in urethane resin. I'll usually sculpt puppet heads or what have you from a polymer clay like super sculpey, then make one or sometimes several castings. If it's a big head I'll hollow-cast a thin skin and then backfill that with more resin that has microballoon filler in it to get ultra light weight. 

I probably should have added to the above post that Oomoo is a silicone for beginners, made to be really simple to use and almost completely foolproof, and that's why it's fairly thick. I'm sure there are much better silicones, but I don't know what they are (which is why I'm asking people to contribute). For my purposes I've never needed anything better, and really I haven't had many problems with voids or air bubbles. When I have I've usually I've been able to fix voids or bubbles by casting up a copy and just popping off any bubble shapes with an X-Acto knife or carving off any void shapes. This might not be ideal though if you need exact copies, like if you're wanting to make multiple identical heads for replacement animation for instance. 

I believe Monster Clay is sulphur free, so you can use either tin cure or platinum cure silicone (the fussy one) to make a mould from it.   But generally, tin cure silicone will work with most materials, and is the safer choice if you don't know what other things you might use the silicone for.   The brand I use is a local one I think, Pinkysil, so that's not much help to anyone outside Australia.

Thanks to both Strider and StopmoNick.  I had been looking at Smooth-On products for a while.  I guess I'll have to break down and buy some and I guess I will need the mold release too.  Little on the pricey side though, but probably worth it.

Just out of curiosity, has anyone tried these DIY Silicone molds, where you would use silicone caulk/sealant, soapy water or corn starch to make silicone putty.  There is one I saw that mixed paint thinner to make it pour-able.

I remember some people on the old board messing around with all those nasty toxic chemicals and I don't remember any of them ever saying it worked well. Usually they seemed to keep thinking that if they could find exactly the right way to do it they might be able to get acceptable results eventually. I don't recall any of them ever reporting that they had achieved it. In fact usually after a few attempts they got disgusted and decided to get some mold making silicone. 

You can buy silicone putty if that's what you want. I've used it and it was really neat stuff, but since what I use silicone for is making molds of sculptures (puppet heads etc) I need a thinner viscosity to flow into and around all the greeblies without voids and air bubbles. With putty you can make molds of very simple shapes like maybe an egg, but it wouldn't work very well for a head with eyes and a nose and a mouth that need to be faithfully captured. 

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