That's how the make boat hulls & custom car body panels... It's only good for flat or slow curving planes, & requires several coats of mold release, usualy wax. Armor simply has to many curves & shaping to do with a hard mold.
The main rule is this- A soft mold for a hard casting, A hard mold for a soft casting. The only real exception to the rule is casting soft urethane ( the stuff used for batman suits) in silicone molds.