With different electrical termination styles, clamping mechanisms and the ability to accommodate holes and cutouts, mica-insulated band heaters are used successfully in many applications, particularly plastics.
Loss Prevention for Band Heaters
Most band heaters with heating plate do not “burn out”; instead, environmental factors create a short, cause hot spots to develop, or simply push the heater beyond its normal operating temperature. All of these factors cause a heater to fail prematurely and require replacement. Obviously, minimizing these environmental factors can reduce the frequency of replacing band heaters in your operation.
Contamination.By far, the most frequent culprit of band heater failure is contamination. Liquid plastic, hydraulic oil and moisture (often from high ambient humidity) are three main causes of premature failure from contamination. Obviously, keeping the heaters free of contaminants will reduce failure and replacement rates. In applications where liquid plastic and oil exposure are frequent and difficult to manage, the best solution may be to select a low cost band -- the heater will be replaced often, but the financial loss will be minimized. Alternatively, a band heater designed to resist contamination can be used. Keep in mind, though, that most heaters fail from lead wire damage caused by contamination -- not from contaminants finding their way inside the band -- and contaminant-resistant heaters will not prevent lead wire damage.
Poor Contact Between Heater and Barrel.The second most common cause of premature failure is poor contact between the machine barrel and the band heater. A tight fit is critical. Without it, localized hot spots can form on the band and cause the resistance wire to fail.
There are two steps you can take to ensure good contact with the machine barrel. First, make sure the machine barrel outer diameter (OD) measurement is accurate; then, order band heaters with that same measurement. The heater manufacturer will factor in a 0.25" gap, so there is no need to undersize the band’s dimensions.
Second, follow a strict installation and tightening procedure for mica, high watt density or extruded aluminum band heaters. Before installation, clean and smooth the machine barrel surface, removing any plastic residue. To install, tighten the heater snugly to the barrel using a clamping bolt torque of 10 ft/lb. Next, apply power to the heaters and allow them to reach halfway to setpoint temperature or approximately 300°F (149°C). Once at this temperature, cut the power and retighten the bands at 10 ft/lb torque. Retightening the band at an elevated temperature will account for the heater’s thermal expansion. (Remember, ceramic band heaters are radiant heaters and should not be tighten in this manner.)
