I just finished up my first HDPE carve. I made a Batman vs Superman sign for my son. It came out nice:
I used a Dual Flute Straight Flute End Mill from the Starter Set ( https://www.inventables.com/technologies/end-mill-starter-set-18-in-shank ), and some of the Two-Color HDPE - Black on Yellow ( Two-Color HDPE - Black on Yellow ). I’m gonna try to clean up the flats with come sandpaper, but the cut was extremely clean.