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Old 06-12-2016, 10:25 AM
mrbanana101 mrbanana101 is offline
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Join Date: Jan 2007
Posts: 62
Casting silicone in a soft mold material - what should I use?

Hey all! Time flies, it's been many years since I last posted, wow...

I'm working on an ambitious project that merges digital and traditional art together but I've hit a bump. My requirement is that I need to make a mold (out of some material) of a plaster sculpture (the positive) and cast silicone in that mold. But I'm having an issue deciding what materials, if any, would be appropriate for this. I'm caught up on the fact that plaster is hard, so I'd need to make a mold out of a soft material like latex or silicone, but how would one cast silicone in a soft mold? So basically my question boils down to:

-can anyone recommend a SOFT mold making material that silicone can be cast into?
Is it possible to cast silicone in a silicone mold without the mold and cast sticking together permanently?
If not, how would you guys recommend making a mold of a plaster sculpture and then casting silicone in that mold?

Thanks in advance

EDIT: Gaaah... and there it happens again, just moments after posting I found cases of people successfully casting platinum silicone in a platinum silicone mold using a release agent. Will look into this some more, but still open for suggestions!
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Old 06-12-2016, 02:15 PM
Andrew Werby Andrew Werby is offline
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Join Date: Dec 2005
Location: Oakland, CA
Posts: 647
Re: Casting silicone in a soft mold material - what should I use?

If you trust your release agent, you can cast silicone rubber into silicone rubber, but it's possible to end up with a big lump of solid rubber.

It's safer (and cheaper) to make the mold from urethane rubber, and cast the silicone into that. You should still use a compatible release, but it's less critical.

Andrew Werby
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Old 06-14-2016, 08:35 AM
Mack Mack is offline
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Join Date: May 2008
Location: VA
Posts: 604
Re: Casting silicone in a soft mold material - what should I use?

PolyTek Corp. is a great source for all info on rubber and casting material
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Old 07-07-2016, 01:57 PM
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Chris_Johns Chris_Johns is offline
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Join Date: Sep 2011
Location: Warwickshire UK
Posts: 87
Re: Casting silicone in a soft mold material - what should I use?

A lot depends on how complex your pattern is, how much of its surface texture you want to preserve and how 'precious' the original is.

My general experience is that, unless the budget is really tight latex is more trouble than it is worth as a mould material. It takes ages to build up enough thickness and it shrinks a lot.

Assuming that you need a flexible mould PU rubber and silicone are your bet options. As mentioned in another answer PU has the advantage that it should be self releasing on a silicone cast. Although all else being equal silicone does have some advantages.

With either type of rubber you probably want to make a 'skin mould', with a rigid 'mother mould' or 'jacket'. There are two ways to do this. The first method is to add a thixotropic agent to your rubber and paint it onto your pattern. Once it has set you build up a second layer in some rigid material like plaster or fibreglass in as many parts as required.

The second approach is to cover your pattern in a layer or clay, Plasticine etc and cast your rigid mould around that . Then you strip the mould and pour your rubber into the void left by the clay. This method is a bit more complex but it allows for a thicker (and stronger) rubber layer and can make it easier to get good registration between the skin and the mother mould as you can add registration notches to the clay layer.

It is certainly possible to cast silicone in a silicone mould. Recently I forgot to put release agent on a 2 part silicone mould and it separated with no problem although it did have a bit of clay residue at the interface.

In general a combination of a wax release agent and a light dusting with talc should give you an adequate release but you obviously want to be reasonably thorough in applying the release agent and probably use a spray release to make sure you get good even coverage.

Another option is to use hot melt polyvinyl rubber.
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