I too use plastic card (styrene) for 6mm vehicles & infantry, 1mm thick which I think is close to 40-thou in old money
I find it is very robust & easy to work (as edenviews says, score & snap, never had a single problem). I use black styrene, slightly harder to get hold of but less obtrusive for chips etc.
I have also used 2mm white styrene for 1:3000 ship models, needs a bit more brute force but still snaps very cleanly along score lines once I get my courage up to bend it hard enough
A while ago I made a large (4x5 foot) cover for boardgame maps out of an old piece of perspex, I had to saw it which was a bugger. Result was acceptable, but a long way from 'professional-looking'...
Cheers,
Pete