PCD-coated bits are usually recommended for solid graphite. But be careful to patrol the dust really well; it's conductive and if it infiltrates any electric or electronic equipment, really bad things can happen. You might investigate an alternative process; making your mold parts positive in metal and burning them into the graphite with EDM. There's probably a shop that can do that part for you fairly cheaply, and the advantage (besides not having to machine graphite) is that duplicate graphite molds are easy to make off the same patterns, once you've burnt up the ones you made.