Been doing plastic signs lately and got a deal on some 1/2" extruded for the punch through letters.
First time I did extruded and sure had issues at first. I used single flute O flute and cut at 8k RPM and 1"/sec. No extra cooling. The bit was an up cut 3/16D (Onsrud). For cast cut with same bit but faster on RPM and speed. Up-cut fights holddown but gets more plastic out of the way.