[3DTOPO] does a lot of metal casting (video link, embedded below). That’s obvious by the full and appropriate set of safety gear, a rarity on YouTube.
They had all the equipment to do it the normal way: craft or CNC out a master, produce a drag and a copy, make any necessary cores, and finally; pour the mold. This is a long and tedious process. It has a high rate of error, and there is a parting line.
Another set of methods are the lost ones. With these methods the master is produced out of a material like foam or wax. The master is surrounded by refractory and then melted, burned, or baked out of the mold. Finally the metal is poured in. Theoretically, a perfect reproduction is made without ever having to open the mold.
Their favorite method is a variation called the lost PLA method. In this method the master is 3D printed in PLA. PLA is happy to be burned or baked out of a mold. However, doing this does offer a chance for the mold to break, and the extra step takes time and fuel.
Recently they’ve discovered that simply printing the model with a single shell lets them pour the metal straight into the mold without the burning step. For most parts, there isn’t enough plastic to cause a serious off-gassing event, though they do recommend wearing the full and appropriate set of safety gear just in case. Video after the break.