make sure you put the Vehicle Framework high on the mod load order + the research itself is called "mold casting", hope this helps

So yeah this was a little bit confusing bcz for some reason you can't track heavy casting via information tab. To craft Heavy Casting you need to open research tab and go to Heavy Industry > Mold Casting after you research it all you need to do go to your Art Bench, craft some molds, go to the smeltry craft Castings with mold and steel

... came here to look up this exact thing. the fact you cant backtrace castings in game makes it confusing and frustrating, along with everyting scattered amongst multiple benches leads to a lot of guesswork, mod could really use a guide in game.