The Purpose of Cartridge Heaters
The purpose of a cartridge heater is the heat metal parts by inserting them into predetermined spaces of the parts. The most common use of cartridge heaters in the hot melt industry, is for heating all automatic applicators, all hand guns, and sometimes even melters. They are very easy to install as the heaters are made slightly undersized relative to their nominal diameter.
It is recommended that the watt density of a cartridge heater only be as high as needed. Under normal applications maximum watt density is not required. Safety margins usually are rated by using less than the maximum allowed. Rather than having the highest possible wattage per heater, heaters should be selected that give a more even heat pattern.
In the hot melt equipment industry, watt densities are usually high rather than medium. In machining the applicators that receive the heaters, holes are drilled and reamed to exact tolerances. In a high watt density application, a very close fit is important. This style cartridge heater should have a hole tolerance of plus or minus .003. After holes have been properly drilled and reamed they should be cleaned and degreased to remove any remaining cutting debris.
Watt density is the heat flow rate or surface loading of a cartridge heater. It is the number of watts per square inch of a heated surface area. Watt density calculation would be as follows.
Watt Density = wattage / (pi x diameter x heated length)
The heater cartridges in hot melt systems and their components are normally controlled by a master temperature controller on the hot melt units themselves. The temperature controllers on the hot melt units read RTD sensors to control the temperature of the heaters. RTD sensors are found in all components of a hot melt system including the tank, the heated hoses, and all heated applicators.
To find out more about our cartridge heaters please visit our website at www.universalsystemsse.com or call us at (561) 272-5442 or email us at sales@ussefl.com.
How Cartridge Heaters are Made and Why They Fail
There are two methods of manufacturing cartridge heaters to ensure a useful life and not have them fail prematurely. The first is a standard cartridge heater and the second is a swaged cartridge heater. Even though both cartridge heaters might look identical on the outside, they are constructed very differently. Standard cartridge heaters are the most commonly used, but if you need to operate at higher temperatures and vibration applications, then you will need a swagged cartridge heater.
A standard cartridge heater has nichrome wire heater coils that are weaved through holes in the ceramic tubing. Pure magnesium oxide filler is vibrated into the holes, heating the coils to allow maximum heat transfer to the stainless-steel sheath.
A swaged cartridge heater’s nichrome wire is wound tightly around a ceramic core, to situate it in close proximity to the heater sheath. Pure magnesium oxide is vibrated in and the heater is swaged to a specific diameter. This compresses the magnesium oxide so it becomes an improved conductor of heat from the wire while maintaining its dielectric properties. This is why it works so well with high temperatures.
The useful life of a cartridge heating element is determined by how quickly the heat generated in the resistance wire can be dissipated to the outside sheath. So why do cartridge heaters fail and sometimes prematurely? The reasons include improper fit, moisture, watt density is too high or if you are using the incorrect voltage.
Improper fit is the most common cause of cartridge heater failure. If the cartridge does not fit into the hole it is inserted, then the heater cannot dissipate the heat being generated by contact with the sheath. So, in effect, the temperature inside the heater will continue to rise until the magnesium oxide or resistance wire breaks down and the heater fails.
If moisture or impurities are present they can be drawn into the heater. Because magnesium oxide is hydroscopic, every time power to electric heaters is eliminated, an internal vacuum occurs which draws in air from the surrounding area. This vacuum can cause a short circuit.
Finally, make sure that the wattage density isn’t too high and that you are using the correct voltage. Either of these issues can a cartridge heater to fail.
If you have any other questions about your cartridge heaters, please call us at (561) 272-5442 and one of our technicians can help you diagnose any cartridge heater problems.