Hello
I want to cast a small spoiler for the trunk of my car and thought to do it in polyurethane

Which polyurethane should I cast the final spoiler in? 

I thought I would make the mold in silicone

Depends how thick you want to make it?  Solid PU for anything of any substance, other than a small lip spoiler, would be quite heavy and may need a couple of pours to avoid overheating and shrinkage.

However a typical small lip spoiler, (20mm by 20mm triangle type shape) can be done in any Fast Cast PU such as our Xencast P2 Fast Cast Polyurethane Casting Resin.  I would add some milled carbon fibre for strength then you can paint it once done and bond it into place.

No problem with a silicone mould, just ensure it has a nice support box around it so you know the resulting cast is straight and true relative to the boot lid edge.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant