12mm at least in my opinion.
A coarse toothed circular saw or jigsaw on slow should do the job...... go too fast and the stuff melts.

http://www.sheffieldplastics.com/web_docs/PDS062.pdfhttp://www.sheffieldplastics.com/web_docs/PDS062.pdf

(Message edited by woody on April 30, 2007)