Chris - I've done a bit of "plastic" machining and though I'm no expert my results have been VERY different depending on the type of plastic. For example what I think is called "rolled acrylic" is hopeless - it just melts BUT "cast" acrylic machines a treat. I did get *some* success with "rolled" by slowing the cutter right down and I mean sloooooow just a few revs/sec but still wasn't brilliant.
Ian