I have had quite good results with 2 flute fine grain carbide endmills – these intended for straight plunge into the material.
The one flute is larger than the other, ie, the flutes are not symmetrical around the axis of the cutter, so the end of the cutter does not have a 'dead' center and cuts fully as it plunges. I will take a photo of the cutter and post for you tomorrow. 4 Flutes worked poorly – insufficient room as the swarf expands, is trapped and heats up rapidly.
I used a cutter 12mm diameter, flutes 80mm tall. The flute makes a spiral almost 2 times in the 80mm length. It was used to rough route composite wing molds in HDPE, depth at max was 25mm @ 140mm/s feed, cutter RPM = 3800. I found feeds less than 20mm/s left very poor side wall finish , I suppose due to rubbing rather than cutting.
Joe