A CNC (numerically controlled) machine does the fine details, and can go for hours or even days as the mold is shaped to near perfection, and is then sometimes polished for even finer detail. Electronic measuring devices check the specifics of the size of the mold, and if any letters are needed in the cast, they are placed on in reverse. The final piece of the puzzle is the creation of cooling lines, where fluid can be injected to cool off the plastic.
CNC company in the United States (this company does fuel filters)
Injection molding images and further explanation