Hey guys, I'm looking to order Mold Star 16 silicone mold as well as Smooth-cast 321 urethane plastic to make a basic one part mold for a simple project. I have used Mold Star 16 before and have an idea of how it works, but I have never used Smooth-cast 321. In the technical bulletin provided on the website, it recommends a release agent for casting. Found here: http://www.smooth-on.com/tb/files/Smoot ... 1,_322.pdf

My question is: Does anyone know if you need the release agent to cast with urethane plastic in silicone molds?

The reason I ask is because I haven't heard of using a release agent for casting plastic in silicone before, but I've never actually done it. So I'm just curious if anyone has any experience with this and would know.

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Urethane is a sticky plastic I have found, at least injection grade. Silicone is naturally releasable which is one of the reasons for it being a great at home mold material (I use silicone spray for a mold release on steal molds).
As to if one sticks to the other, it may be just precautionary so you don't ruin a mold and blame them for it. It wouldn't hurt. But you may want to see if there is a cheaper alternative to any product they suggest.

on the site you linked to, it says 'silicon molds usually don't require mold release unless casting silicone'. and also says it is to prolong mold life, so it is close to what I am saying here as well.

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http://www.sithious.webs.com

Thank you Sithious, that has helped me a lot. So basically I can use a mold release to help preserve the silicone mold, but it is not a "necessity" for casting. Smooth-on offers a universal mold release agent that isn't too expensive. I think I will add that to my order and keep it all from the same company. Again, thanks!

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