Casting is a process by which molten metal flows via gravity into a mold where it solidifies in the shape of the mold cavity. Castings include ingots (large shaped metal intended to be roll formed or forged) and shapes. Shape castings have more complex geometries in regard to internal and external shapes. Castings have limitations on mechanical properties, dimensional accuracy, porosity and surface finish. Usually a secondary operation is needed, whether to harden it or bring it to net shape. Casting parts can go from weighing a few pounds to weighing tons, and ceramics and polymers can now be cast.