Cartridge heaters or pencil heaters are used in applications that require a localized heat source, often in restricted work areas. They are used to heat up metal dies, rubber molding patterns, tools for the manufacture of plastic components etc.
They can also be used in applications where it is not possible to use other types of heating elements. Cartridge heaters are typically designed to produce a large heat source in relation to the overall surface area of
the heater, and as such must radiate as much heat away from itself, as quick as possible to eliminate the possibility of the resistance wire within the heater overheating and thus burning out. This would be the same effect as turning on an electric kettle with no water inside the kettle. The kettle element would burn out in seconds, simply because the water would normally act as a medium in which the heat from the element is distributed, if there is no water the heat cannot be radiated away from the element and the temperature of the resistance wire inside the element will keep climbing up until it reaches its maximum operating temperature and subsequently burn out.
A cartridge heater is manufactured by winding an 80% Nickle (Ni) and 20% Chrome (Cr) resistance wire around a ceramic core. The resistance wire together with the ceramic core is encapsulated within a metal sheath, and filled with an electrical grade magnesium oxide (MgO) powder as an insulation medium and also isolates the resistance wire from the sheath. The filled metal sheath is then swagged down in diameter by approximately 15%. This compacts the MgO powder preventing the resistance wire from creeping or expanding when hot and possibly touching the metal sheath causing a short. The sheath is then put through a center-less grinder which grinds the cylindrical surface down to within extremely tight tolerances. In some applications the resistance wire is spiraled around a mandrill to form a coil which is then feed through
holes in the ceramic core.
It is extremely important, when purchasing a cartridge heater to specify the correct hole diameter into which it is to be used, taking into account the very tight tolerances required, anywhere from 0,1 to 0,01mm depending upon the relationship between the surface area and the wattage of the heater. This relationship is referred to as the watts density, and is calculated by dividing the wattage of a heater by the surface area in centimeters, to give a factor referred to as the watts density or watts/cm². The higher the watts density the more heat will be radiated from the heater. A black heat element in which the element itself does not change colour but still radiates a small amount of heat would result in a low watts density, and on the other end of the scale would be a red heat element whereby the element sheath is red/orange in colour and radiates a large amount of heat i.e. a high watts/density element. Cartridge heaters typically operate within the high to very high watts/density range and as there is a need for a higher watts/density element the corresponding tolerance between the sheath of the heater and the surface of the material it is to heat up must be less and less. Too large an air gap between the two and the heater can burn out.
When ordering a cartridge heater please specify the: DIAMETER, VOLTAGE, WATTAGE and TERMINATION TYPE.
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For the different types of Terminations and End Shape of Heater please refer to the drawing below:
