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.)
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