Can I cast Urethane plastic in a plaster of Paris mold?

[Magpie12345]

I have a project I’m working on where I’m making something really simple and I want to cast urethane plastic. I’m just worried if casting in a plaster mold is a bad idea. I know I’ll need a release agent but is there any reason I should consider buying silicone instead?

 

[mcusanelli]

Hi,
You would need to use Butchers Wax to coat the plaster, it’s a wood finishing product. But, you should try it on a scrap part to see if it works. You can also check out YouTube, there are a lot of videos showing how to use cheap silicone caulk to make molds too. Might be worth it if it saves you buying RTV

 

[udog]

I’m just worried if casting in a plaster mold is a bad idea

Not the best one indeed. As said above you will have to apply a release agent, wax. But also, if the plaster mold is recently made you might experience foaming as PU resin is sensitive to moisture. Also depending on the shape you are molding it could lock on the mold, might have to break it. It would be a waste mold actually. If so, you can use normal cheap plaster for that instead of plaster of Paris.

 

[udog]

cheap silicone caulk

It´s and option, just to add, caulk shrinks a lot.

 

[udog]

apply a release agent

Thinking of it twice, prior to applying a release you should seal the plaster of paris mold surface. You can use shellac or others. As plaster is porous it´s a candidate to stick to PU.
Also using a quick version of a PU resin could minimize the resin and moisture problem.