I find that; as Jason says; Working with much less than 3.5 - 4 oz. is difficult and problematic. Once I get below a full injector; uneven heating and air bubbles become issues. I try to get several molds clamped up that will utilize the "last injectors" worth of plastic. The heal, sprues, and runners; once cooled; get zip locked for adding to another batch as reheats.
