To get into the Casting Room in The Casting of Frank Stone, you'll need to find the key earlier on. Since you can't go back, you'll need to find this key while you're still in the area underneath the Mill. But where exactly do you look? Here's where to get the Casting Shed Key in The Casting of Frank Stone.
Casting Shed Key Location
While walking underneath the Mill as Chris with the rest of your group, you'll head through various winding passages and have to make decisions about where you'll go next. At one point, you'll come to an intersection where you can either vault through an open grate, or keep walking forward. Vault through the gate and open the door at the end of the hallway.
In this large room, you'll find only one thing you can interact with: a box on a table on the far right of the area. Pick up and open the box to get the Casting Shed Key. There's nothing else in here nor can you progress further in this direction, so vault back over the grate and head down the hallway in the direction you didn't originally come from.
You'll use this key later on, to open the double doors once you get into the area where you're looking for the Machine Shop. This key opens up the Casting Room on the other side of this area.