A metal must be heated to the correct pouring temperature which is slightly higher than its melting point before it can be cast. There are several ways in which a metal can be reduced to the liquid state.
Cupola Furnace – The first and usually the cheapest method of melting iron is in a cupola furnace. The cupola is a melting furnace which is fired with coke to produce the high temperature required for melting iron. In addition to the charge of pig iron, other materials such as lime stone and prepared fluxes are added to each lot of metal. The lime stone and flux aid in separating the impurities from the pig iron.
Gas Fired Crucible Furnace – A second method of melting metals in a foundry is a gas fired crucible furnace. In this type of furnace, the burning gas does not come into direct contact with the metal charge, but heats a crucible in which the metal, in turn is melted.
Electric Arc Furnace – In some foundries the electric furnace is found to be the best equipment for melting metal. There are two main types of electric furnaces for this purpose. One furnace makes use of the carbon arc principle. Carbon electrodes are moved close to the metal charge to produce an electric arc which, in turn, develops sufficient heat to melt the metal. The carbon electrodes become shorter as the intense arc continues; therefore, as the melting progresses in the furnace, the electrodes are slowly moved down automatically to maintain a continuous arc. Fluxes are added to clean the metal as in the cupola furnace. This type of furnace is used primarily for melting steel where a relatively large quantity of high quality metal, is required.
Induction Furnace – Another type of electric furnace is the induction furnace. This furnace has a steel jacket that is encircled by a coil through which a high frequency electric current is passed. The current in this coil develops an intense heat in the metal charge. The heat thus produced is more than sufficient to melt most metals. The principle of this furnace is that when a high frequency current is passed through an induction coil a very high temperature is developed within the area encircled by the coil. The induction furnace can be used to melt either a large or small quantity of metal.