"plastic to metal"
two methods:
a lost wax style casting, where-in the master is incinerated...but a brass master is created out of a one use mold.
Direct casting: A spin mold is created from a pourable rubber that act's as a regular spin mold. from this, a master set for another mold is created (and used when mold one is past it's use)
A third method that involves quickly casting a resin sub master than can withstand the heat and pressure of making a standard spin mold...
Also, just as easy to make the part out of metal and greenstuff to begin with...