Polyurethane vacuum casting involves a multi-step process.

First a master mold is made of a durable material. Master molds can be customer-supplied or made using traditional prototyping methods including CNC machining, Metal 3D printing, stereolithography (SLA) or selective laser sintering (SLS).

The master mold is placed inside of a casting box, which is then filled with liquid silicone rubber. Once the silicone is cured, it is split open and the master removed. There is now an empty cavity conforming to a reverse image of the master. This cavity is then filled with the casting polyurethane (PU) of your choice. A vacuum is applied to the silicone mold, which effectively pulls the PU into any voids inside the mold.
Silicone molds are usually durable enough to make up to 20 or more copies of the original with excellent fidelity.