ISM is a method of melting metals in a segmented, water-cooled copper vessel, under vacuum or controlled atmosphere in an induction coil. This is done metal-to-metal, without a refractory lining.

The copper crucible is made up of water-cooled segments of “fingers”. The magnetic field produced by the coil, in effect, passes through the crucible to induce heat in the metal charge causing it to melt. The field also intensively stirs the liquid metal pool.

A thin layer of metal remains frozen against the bottom of the crucible forming a skull. The low velocity boundary layer in the liquid adjacent to the skull, together with the skull itself and the interface (joint) between the skull and the crucible all serve as thermal resistances, reducing the heat conducted from the hot liquid into the cold crucible.

When the crucible, the coil, the frequency, and the power level are all properly designed and matched, the sides of the liquid metal pool are pushed inward, away from the interior sidewalls of the crucible. In effect, the sides of the pool are supported magnetically. The absence of physical contact with the sidewalls prevents the crucible segments from electrical shorting and further reduces the heat loss into the crucible.

The process is most often used in casting with top of bottom pouring but it can also be used in ingot production. In this case, open-bottomed crucibles with a drawn down starter plug are used to slowly form an ingot from top fed charge material.