Making Terrain - Foam and Silicone Moulds - Question

[Ethelred the Almost Ready]

I was wondering whether expanding foam can be used with rubber moulds for rocks.  I have some Noch model railroad rock moulds and thought expanding foam may work in them.  The advantages of foam is it will not chip like plaster of Paris and would be easier to cut.
Has anyone done this?
Would I need a releasing medium?

[FramFramson]

My suspicion is that the expanding foam wouldn't conform to the mould very well without some way of pressuring it into the mould. That said, I haven't actually tried it.

[katie]

Magister Militum used to cast buildings in expanding foam. This was great because they were light and cheap.

They now do them in normal resin -- I asked why and they said the expanding foam was really hard on the moulds. So that might be a thing to be careful of.

[Dolnikan]

For casting in a material like expanding foam, a more solid mould might be more suitable. Perhaps something like plaster or even harder. That way, the foam doesn't push the mould out of the way and instead is forced to conform to the mould.

[manic _miner]

 Fantasy Forge used to use a foam type material in thier buildings.Held detail really well and very light.They then moved to resin for them.

[SBRPearce]

With any sort of flexible mold + expanding material, you need a rigid outer jacket mold to keep it from radically distorting the flex mold. In the past (casting heads in expansion foam for a Grand Guinol theatre experience (IE, blood 'n' gore) we would fill the flex mold with plaster or something else rigid, then pour a solid jacket mold in plaster.

For a rock-face, which I presume is a one-sided mold and open to the 'back', you'll need a rigid support for the front and then some hard back for it to expand against. For cheap and easy, cover one face of a piece of cardstock with clear plastic packing tape. The foam shouldn't stick much to that. Use more tape to hold it against the combined flex mold/jacket mold piece.

Take care not to let the expansion foam contact the plaster jacket mold - they'll bond irretrievably, and you'll have to rip the foam or chip away parts of the jacket.