Recycled plastic moulds

Hi Dave,

I’m new to your site and would like a little advice. I’ve melted (not burned) some plastic and was intrigued by the possibilities. However, I found the plastic was rigid when it cooled off. I would like to make moulds for benches into which I could pour concrete. The benches would have the texture of bark and look as if they were made from real log slabs. The bark would have small undercuts so the plastic mould would have be capable of flexing enough for the removal of the casting. What type of plastic would be best suited for melting to make the mould for repeated use? Thanks for any help you can provide.