Roughing endmills are only really useful when used with flood coolant, since the general idea is they make very small chips which are more easily washed away. You mentioned earlier 16000rpm at 650mm/min for a single flute cutter. If that's a 6mm cutter then the spindle speed is a bit fast, and setting the spindle speed too high does lead to excessive heat production. I use 12500-13000rpm with 6mm carbide cutters and 600mm/min for single flute, (almost) never more. The spindle speed is determined from the cutting speed of the material, so you will find the single flute and 2 flute carbide 6mm cutters require about the same spindle speed. As Jazz has said, using a finishing pass whereby you take off the full depth, 0.2mm wide, makes it fairly easy to get a good finish. Just make sure there isn't any residual swarf before starting the finishing pass. Whilst a lot of the heat can be extracted in the chips, on small parts like the one pictured they can still heat up if your bed has a very poor thermal conductivity, which is the case with HDPE.