Machine mica band heater
Most mica band heaters do not actually “burn out”. Instead, it is often environmental factors that 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.
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 the failure and replacement rates. Most heaters fail from the severe lead wire damage caused by contamination not from contaminants finding their way inside the band.
Poor Contact Between Heater and Barrel
The second most common cause of premature failure is poor contact between the machine barrel and heater band. Without a tight fit, localized hot spots can be maximized on the band and cause the nickel-chrome resistance wire to fail.
Fewer Holes for Longer Life
Every time you add a hole to a mica band heater, you give another avenue for contaminates to enter the heater. You also add expense and often add to the delivery time. If the hole is for a thermocouple or other instrument, you can often order a heater with a larger gap to accommodate the thermocouple. Another option if the band heater has several holes along the circumference of the heater is to use two heaters instead of one. For instance if you are using a six inch wide heater with several one inch holes, you might try using two heaters that are 2.5” wide
Often, failing to understand a heater’s internal design leads to heater losses. For instance, stretching a one-piece mica band over a barrel during installation can damage the internal mica, resulting in a short circuit. Some manufactures will offer installation techniques inappropriate for a specific heater types and these will cause problems. So, in cases where one heater design will not work, for example, if a band cannot be installed over the end of a machine barrel without stretching the heater, use a heater that is better suited to application such as a two piece mica or a one piece expandable band. A two piece design minimizes the chance that air gaps will develop.
Another simple handling tip is to use two wrenches to install the wiring onto the band’s post terminations. This practice can eliminate failures because the wrench on the post’s lower nut acts as a strain relief. If this procedure is not followed, the post’s internal connection to the nickel-chrome resistance wire can be damaged and become a weak link within the heater.
Fine Tuning - The Tighter, The Better
Before installation, clean and smooth the machine barrel surface, removing any plastic residue. After proper actions are taken to assure there is no power going to the heater and that the heater will not cycle on, follow a strict installation and tightening procedure. 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 set point. Once at this temperature, cut the power and retighten the bands at 10ft/lb torque. Make a habit of checking the tightness of the band heater periodically and you will be surprised at how you can increase the life of your band heaters.