I mill UHMW alot.
Ditch the Endmill. Go with a single flute or 2 flute cutter made for plastic. I use either an Onsrud O-flute on a micro 100 made for plastic. I frequently cut (1 flute 1/2" cutter) .25" deep 60"/min 6000 rpm.
For Uhmw you need a razor sharp cutter.