The calculator assumes best possible cooling. That same goes for plastics as well. Low melting point plastics should be either machined with coolant or cutting speed considerably reduced. How much I do not know since I never machined aluminum or plastic dry. There is just too many variables to calculate at which maximum cutting speed a given plastic will melt. All "other calculators" do is just decrease cutting speed by like 30% and be done with it. It is a patchwork solution IMO. There is admittedly a flaw I my calc because it currently applies Metal cutting principles (carbide giving higher cutting speed than HSS. Etc.) to plastics. A proper solution is in the works but will not be available for another month or so. I can't say why your 2 mm Chinese mills broke. I need to see the screenshot of your calculation. But if you get the flutes clogged because of bad chip evacuation it will break no matter where deflection slider is. If the cutter is not designed for wood or plastic it will likely clog, melt or burn. Regards.