Acrylic as a very narrow chip load range and thats why it melts so easliy. I'd be going for higher feed rate first as 400mm/min is way to slow. Try 18000rpm and 900-1200mm/min then play around with both. Thou i'd be going higher on feed not slower. You'll definatly be wanting to at least double your original 400mm/min. I'd also cut with more DOC start with 50% cutter Diameter but it's not unusual to cut at 100% cutter dia.

The type of acrylic also plays a big part to how it cuts.! Extruded can be a pig to cut and Cast tends to be better.