My initial design used just 3d printed PLA to act as a concrete mold. While this was a good Idea I soon found out that Concrete and Grout Stick to everything and are difficult to separate. To remedy this I decided to go with making a silicone master mold.
To do this I start out with making a PLA version of the Knob with a PLA Ring to act as a mold box
Once printed I spray painted using several coats of High Fill Primer and sand the Knob down for a smooth finish.
I then poured Moldstar 30 Silicone into the cast. I waited 2 days for a full cure.
Once demoted simply pour concrete and wait 1 day for concrete to cure.