The molds I made required up to 6 graphite plugs, multi part, immersed in a dielectric solution. At 440 volts, and more amps than I ever felt comfortable with, they would touch the hardened steel block, submerged in the dielectric solution, and pulverize the steel wherever the conductive graphite touched. You would hog out as much as you could with an end mill, then the block was sent out and hardened, which then made any further milling impossible. The final plug had the shape you wanted. You would work up to this final plug. This work is laborious, and very expensive. Some Tamiya model molds cost millions of dollars to make, but they can quintuple their money, if not more, with a popular model. The molds last a very long time, as long as they are kept in clean, oiled up, condition. Rust is their enemy. You would make the model sculpted out of the graphite block. That would leave the negative in the block of steel, then you had to make the inside, to allow for the thickness of the plastic, or white steel, depending on what your were making.
That job burnt me out, I also had a jerk boss who owned the company. He was so mad when I quit. HehHehHeh. My next job paid $2.00 dollars more an hour, with better benefits!