Thermite mixtures which are used to cover open risers.
They contain magnesium and aluminum as combustible substances, Fe2O3, NaNO3, KNO3, SiO2 and others as materials which contain and give off oxygen and chamotte and pearlite as filling materials which delay the reaction. The binder used is usually water glass.
Specific components play a particular role for the reaction process. For example, nitrates make it easier to ignite the mixture and accelerate burnout. Although fluorides reduce the ignition temperature, they are not longer desirable in exothermic mixtures today due to problems with disposal of the used sand.
Exothermic riser covers (s. a. Exothermic sleeves) are put on top of the melt in the open riser after casting. When they come into contact with the melt, they create heat and thereby prevent the riser freezing prematurely. The heat transfer occurs due to an aluminothermic or similar transfer reaction of the heating compound, for example according to the following equation (Eq. 1):
The heat created does not heat the liquid metal in the riser but simply compensates for the heat loss. Another form of use for exothermic mixtures is for exothermic heating pads. The main function of these mold inserts made of moldable exothermic riser heating compounds is to keep the melt warm at particular places of the casting, e.g. below the risers.
Insulating riser sleeve