Finish on soft plastics

[magicniner]

The material is so soft that it's what you're doing with it and not the material causing the problem, have you tried disposable HSS 2 flute endmills (Carbide isn't as sharp unless you grind it razor sharp yourself and then it's a crap-shoot on anything but plastic so you have to reserve the tools for plastics), with lots of air from several directions to shift the chips and adequately cool the tool and material in all cutting directions?
Another alternative is the above plus a shallower cut using a ramped profile path.
Something as simple as ambient temperature differences can change what happens when you're cutting temperature sensitive materials and your basic set up isn't adequately controlling all the required environmental variables.
This stuff quite literally is Rocket Science ;-)

- Nick