Use Silicone Rubber (if you have it available to you)
Ideally, silicone rubber molds should be used when casting polyurethane foam. Silicone is often preferred because release agent is not required during the casting process (adding a barrier coat or paint can extend mold life, however).
A silicone mold (TinSil® 80-30) being primed for foam casting.
Watch a foam casting tutorial (using a silicone mold) here: http://youtu.be/fR8WWkOo8aI
Perhaps polyurethane rubber is all that you have in-stock, or you don’t want to spent the extra money on silicone rubber. There is a solution that will make your polyurethane rubber mold act like a silicone rubber mold: PolyCoat Semi-Permanent Sealer & Release Agent.
PolyCoat Semi-Permanent Sealer & Release Agent
Create a Silicone “Coating” on Polyurethane Rubber Molds
PolyCoat Semi-Permanent Sealer & Release Agent is a low-viscosity, silicone-based liquid that can be applied to cured polyurethane or silicone molds (it can also be applied to original models before pouring liquid rubber over them during the mold making process).
PolyCoat can be brushed or sprayed onto the mold (use a Sure Shot Atomizer for spraying). Once applied, it takes about 5 minutes for the solvent carrier to evaporate and about one hour to cure, leaving behind a thin coating of silicone. The cure time of PolyCoat will depend greatly on temperature and humidity of the workspace and may take longer than one hour.
When casting polyurethane foam, it is recommended to apply at least two to three layers of PolyCoat to the cured polyurethane mold. Ensure that each layer is thoroughly dry before applying the next layer.
Once PolyCoat has been applied to the polyurethane mold, there is no need use any additional release agent. This is especially advantageous if the cast foam parts need to be painted. Paint will not adhere to cast parts when release agent is present and removing release agent from castings can be difficult; the application of PolyCoat eliminates these painting issues. PolyCoat even allows the application of a barrier coat or paint (after PolyCoat has been applied and is cured) to increase mold life and produce pre-primed parts right out of the mold.
Tests performed at Polytek have shown that over 100 rigid, pre-primed polyurethane foam castings can be made from a polyurethane mold (mold product: Poly 74-55) with a PolyCoat coating.
This polyurethane mold (on the left), coated in PolyCoat, rendered all of these rigid foam castings
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