Like warren says, use a good release agent, silicone is selfreleasing (which is nice if you want to cast a lot of PU parts) 

Thanks that's great help.

There is no problem with using rigid moulds.  You would be best to skim the polystyrene with a layer of fibreglass to give a bit more durability.

The big difference is you will need to use a mould release agent to stop the polyurethane sticking. We would recommend our Easy-Lease Chemical Release Agent when moulding PU off an epoxy mould surface.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Hi,

Im fairly new to this. Whenever I see polyurethane cast, it appears that silicone molds are used. Is there any alternative because I am trying to cast a large shape that is only 1cm thick. The overall PU content would be 2litres, but the volume the mold would take up would be 86litres (which makes using silicone very expensive!!). I can machine the molds from polystyrene. Could this be covered in epoxy and then used as a PU mold?

Cheers,

Markie