The Joule effect of the current is used to convert electrical energy into thermal energy to heat the object. Usually divided into direct resistance heating and direct resistance heating. The former's power supply voltage is directly applied to the object to be heated. When the current flow expires, the heated object itself heats up the ironing machine to generate heat. An object that can be directly resistively heated needs to be a conductor, but has a higher resistivity. Because heat occurs in the heated object itself, due to internal heating, the thermal efficiency is high. Direct resistance heating needs to be made of special alloy data or non-metal materials to generate heating elements. The heat generated by the heating elements is transmitted to the heated object by radiation, convection and conduction. Because the heated object and the heating element are divided into two parts, the type of the object to be heated is usually unconstrained and the operation is simple.
The data used for the heating element of direct resistance heating usually requires a large resistivity and a small temperature coefficient of resistance, and is small in deformation at high temperature and is not easily embrittled. Commonly used are metal materials such as iron-aluminum alloy and nickel-chromium alloy, and non-metal materials such as silicon carbide and molybdenum disilicide. The maximum operating temperature of metal heating elements can reach 1000-1500 °C according to the data type; the maximum operating temperature of non-metal heating elements can reach 1500~1700 °C. The latter equipment is convenient and can be replaced by a hot furnace, but it requires pressure regulating equipment during operation, and its life is shorter than that of alloy heating elements. It is usually used in local furnaces and special occasions where high temperature furnaces, temperature exceeding metal data heating elements allow maximum operating temperature.