Need help with latex shrinkage in a silicone mold

Evewatson

New Member
Hi! I'm trying to slush cast latex in a single-sided silicone mold using Monstermakers' RD-407 but I'm running into a problem. I've successfully done a latex slush cast in this mold for a prototype in their "bat black" latex and hadn't experienced any shrinking issues, but with a new jar of their latex my pours are shrinking badly when they dry! I suspect it's due to this particular batch of their white latex being watery and thinner than their black, but I'm not aware of any way to sufficiently thicken the latex prior to pouring. What can I do to work with this one out?

Thanks!
 
Hi! I'm trying to slush cast latex in a single-sided silicone mold using Monstermakers' RD-407 but I'm running into a problem. I've successfully done a latex slush cast in this mold for a prototype in their "bat black" latex and hadn't experienced any shrinking issues, but with a new jar of their latex my pours are shrinking badly when they dry! I suspect it's due to this particular batch of their white latex being watery and thinner than their black, but I'm not aware of any way to sufficiently thicken the latex prior to pouring. What can I do to work with this one out?

Thanks!
If you have a bad batch, the only real remedy is a different, good batch. Maybe try a different silicone. Make sure any release agent it thoroughly dried.
Also make sure the mold isnt moving/shifting when you do the slush-cast-dance. I always have a rigid, crunchy, outer shell on the mold.

Pics would help
 
Some batches may shrink more than others but latex always shrinks as the moisture evaporates out of it.

Silicone is generally not used in latex casting applications because of its non-porous nature. If you were to apply a thickened latex and have the outside surface skin over, it would be the equivalent of closing a lid on it, the latex under the skin against the non-porous silicone may never be able to fully dry. Latex is typically cast in stone molds which are able to wick the moisture and allow the latex to dry from both the side exposed to air, and the side in contact with the mold.

You can try stippling in thin coats and drying thoroughly before applying the next layer, which will help with drying and also help minimize shrinkage. It may be worth looking into making a clay press of the piece and molding it with stone such as hydrocal, ultracal, pottery plaster, etc if possible to make the latex casting process easier. Without knowing the exact details of your project it’s hard to give much more specific recommendations.
 
Yeah, I've never heard of people using silicone molds for latex for the exact reasons above. That's a first for me. I'm surprised that it's not at all lead to any inhibition problems from the latex and silicone.
 
If the silicone is already catalyzed, there should be no problem with latex.

But as others have pointed out, your latex will not dry because the silicone won't pull the moisture out. You'll have to do a lot of small thin layers.
 
As said above, latex in a silicone mold is not a good choice and latex shrinks quite a lot by itself.
One thing you can try is spraying vinegar on the mold prior to applying or slushing latex. Vinegar will drastically change the PH and latex will coagulate.
Also, latex suppliers should sell latex thickener (will loose stretchiness).
Another thing you could try is applying a first coat of 1 part latex and 1 part water based paint (mix both). And when that dries, go on with plain latex. Stretchiness will be reduced a little too.
As also said above it will depend on what the project is.
Maybe that black latex has some filler in it that could be reducing shrinkage. In fact I recall using some black pigments (powder) many years ago that thickened or coagulated latex
Without knowing what you´re doing, another option to reduce shrinkage could be using latex hardener (a filler at the end) or reinforcement materials. Again, it will loose properties (stiffer).
But latex shrinks, it´s part of it. As said above too, your best option is pulling out a copy with silicone and foam for example and mold that in stone plaster, hydrocal or similar. Besides, a master copy is always a good thing to have.
 
You could try casting a rigid positive in the silicone mold and then spray the latex on top of the positive. Yes, you may lose detail, but you are already starting from a deficit.

You could also just cast in silicone. There are numerous brush and spray on types with little to no shrinkage.

Be sure to release VERY well.
 

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