I would be amazed if the cutters were actually dulling from cutting ABS. They will load up if you cut too fast. With plastics, you have to either cut slowly, so the cutter doesn't heat up, or use coolant (air will generally do). You have to keep the RPMs down, as that's what causes heat buildup, which is what causes the cutter to clog. Also, you should be using 2-flute cutters, to reduce clogging.