Milling styrofoam board enquiry

[marbles]

Howdy,

I'm just running some test milling forms in blue styrofoam board. I am making lost foam masters for aluminium casting.
I'm running cut tests on a low relief model (150mmx150mmx40mm). I'm using a 8mm OD rasp bit to rough out and using a 2mm OD end mill to finish. I run both at a feed rate of about 2 cm/sec but the spindle on its slowest RPM (approx 11000rpm). This combination seems to give me the best results.

However I see that the final finish is still quite hairy and the rasp sometimes still gets too hot/ and or melts the styrene. Are there some know techniques or bits for working with styrene I should try? I think i'm pretty close but just wondering if there is anything I can tweak.

Thanks