finally, the casting must be designed to produce directional solidification, which sweeps from the extremities of the mold cavity toward the riser(s). in this way, the riser can feed molten metal continuously to part of the casting that is solidifying. one part of achieving this end is by placing the riser near the thickest and largest part of the casting, as that part of the casting will cool and solidify last. if this type of solidification is not possible, multiple risers that feed various sections of the casting or chills may be necessary.