Metal objects can be made in two distinct ways. Pressed metal is formed into a shape while it is still solid using pneumatic or hydraulic force and a mold. Cast metal objects are created by pouring molten metal onto or into a mold. In order to achieve this, metal first has to be melted. Most metals used in objects designed to withstand the elements have extremely high melting points. This means that specialist melting furnaces need to be employed to turn solid metal into its liquid form.
Background to casting
Metal casting has been carried out for over 7000 years. It is thought to have originated in Asia. The technology behind casting spread quickly around the world, with evidence of molten metal casting processes having been found in ancient Indus, Egypt, China, and Babylon. Copper was the most commonly cast material in the ancient world, with lead used to increase the fluidity of the metal in its molten form.
There are three main kinds of melting furnaces used in modern metal casting processes. Here is a quick guide to them all.
Induction
Induction furnaces are the most energy-efficient way of melting precious metals, iron, and steel for casting. They have largely replaced open hearth and cupola furnaces in high-tech foundries. A conductive coil is wrapped around the chamber of the furnace and electrical currents are passed through it. This produces an effect known as an eddy current in the charge – or metal – that needs to be melted. Eddy currents generate huge amounts of heat, allowing the charge to essentially ‘melt itself’. Check out cdocast.com for a more in-depth guide to how induction furnaces work.
Open Hearth
One-sixth of all steel produced worldwide is created in open hearth furnaces, which can also be used for the melting of metal ready for casting. Open hearth furnaces are largely being replaced in industrialized nations by induction or cupola designs that are more efficient.
Fuel and air are heated in chambers located below the main body of the furnace. After this fuel and air become sufficiently hot, a valve is opened: allowing this mixture to enter the chamber containing the metal that needs to be melted. Because the fuel and air are pre-combusted, it allows for immense heat to be swiftly generated inside the main chamber, creating a relatively efficient method of melting bulk amounts of metal.
Cupola
Cupola furnaces are commonly used for melting steel and iron. Cast iron is created by melting iron in a cupola furnace before dropping it into a cast. These are the furnaces you would see on a massive scale if you happened to visit an industrial foundry. These furnaces are large cylinders filled with alternating layers of metal and coke. Around the rim of the cylinder, small openings allow air to be pumped directly to the combusting materials. The combustion of the coke causes the metal to melt while separating ‘pure’ metal from impurities.