In this moldmaking instructional video from Smooth-On, Milo shows how you can make your own mold knife using readily available, affordable blades.Mold knives create a key in your mold when you cut the mold, which helps the mold register and align properly when casting resin, rubber, foam, or other materials. A mold that registers well will give the least amount of flashing when making props, sculptures, or other types of pieces.