The cartridge heater is a high-quality seamless metal tube, with electrothermal alloy wire (nichrome wire, ferrochrome wire) evenly twined around the the central axis in the tube. The gap in it is compactly filled with oxydic magnesia with fine insulativity and thermal conductivity, and the nozzle is sealed with silica gel or ceramics. This kind of metal armored electrothermal element has the function of heating gases, molds and a variety of liquids.