What you describe is essentially a 'centre cutting endmill' which is basically a slotdrill. The main problem with cutting plastics is heat generation through cutting. You want to keep the heat in the cutter and not in the material, so regardless of whether you use HSS or carbide cutters, supply a cold air feed to the cutting area, and don't be afraid of cutting quickly. Cut slowly and you run the risk of melting the plastic.

And as stated before, 2 flutes will clear your chips more easily.