Mizuno are one of the few companies to forge their wedges and the reason most are cast is that wedges have a harder life playing out of coarser materials such as sand and the grooves usually last longer when cast.
However like the Mizuno T7 wedges, the S18 are forged from a stronger Grain Flowed Forged 1025 Boron Steel that means they can strengthen the grooves whilst keeping the forged feel.