What material will it be cast in? I have cast aluminum using blue extruded polystyrene (XPS) coated in drywall mix, then suspended in sand.
The foam can be joined with hot glue, and doesn't need to be removed from the plaster shell, it just burns out from the liquid metal.
I learned what I know from LFMC After all these years, it's a pleasant surprise this website is still online.
This website is what got me interested in CNC in the first place, building a cnc hotwire foam cutter for preparing the foam blanks.