Re: using Polypropylene for cnc structure?
Don't go there.
PP is not rigid, not dimensionally stable and temperature sensitive. It is a recipe for disaster.
Well, it may be possible to build a good router out of PP somehow, but it will take a huge effort and that could better be used for something else.
If you are worried by humidity, use mdf or better, HDF and epoxy to build a torsion box router, covering every bit of mdf with epoxy in the process.
Sven
http://www.puresven.com/?q=building-cnc-router