Plastic cutting secrets ?

Hi everyone

I have been doing a small project involving HDPE or UHMW can`t remember which.

I have to cut out a few shallow pockets 3 or 4 mm deep and then cut out the material whch is 15mm thick.
It has cut Ok and is accurate etc but my surface is not very nice.
Been using a single flute spiral endmill 3.175mm at 20,000rpm, 3000mm/min, pockets in 1 pass, cutting the 15mm depth in 4 passes. Pretty much what I use for birch ply. Tried the cutout with a 4mm 2 flute compression bit, same speeds, slightly better but not a nice finish.

The bottom of the pockets show a bit too much of the toolpath, not fluffy but not super flat either. The edges are a little bit rough.

My instinct is to move it faster and try deeper cuts, which I will try.

However, I am wondering if there is a really good specific bit for this plastic or a speeds and feeds setup that works nicely.

I am used to wood which is easy to sand etc but this plastic is hard to post process in the same way so I would like it clean straight off the machine if it possible.

Cheers

Ollie