For garage kit casting I use silicone to make my molds. This provides some flexibility that we need to make some items as we don't have the same luxury as steel molds with injection molding.
I use smush molds for small HIGH detail parts where a sprue system is impractical. Another thing, Injection molds are a 50/50 split. This can be problematic for garage kit casting. Steel molds don't give. They have no flashing. If you have flashing on your cast...your mold gave. That means your cast is now just a little bit thicker than it should be. If you are casting 5mm pegs this is now a serious problem as your pin is now bigger than 5mm and most likely out of round.
Silicone provides ways of working around this. Because silicone does have give...we can make molds using the flexibility of the silicone to help release cast parts. We can bend the silicone mold to release parts. Something not possible with steel injection molds. Hence the 50/50 mold split in injection molding.
Silicone molds are great, fun and easy. The hardest part, is determining just WHERE to make you mold split and what type of mold to make.