[Kliopeion]

the 3D printer is a useful tool for pattern-making.

not only can it print a 'positive', for visualization purposes.

it can also print a 'negative', e.g. a mold impression.

typically the process is -

plastic mold (negative)
rubber mold (positive)
plaster of paris (good for aluminum) or investment (similar to plaster of paris, more porous) mold (good for brass, silver, gold)


or sometimes, in the "old days", the pattern-maker would start with a wood 'positive', then make plastic 'masters' (a negative impression) from that, then make working rubber patterns (positive), which were used to make plaster of paris or investment molds (negative mold impression), into which molten metal was poured.

either way, the metal casting can then have secondary machining to 'clean it up'.