Heater bands just burn out, right? Wrong. Usually, environmental factors create a short, cause hot spots to develop, or force band heaters beyond their normal operating temperatures, causing premature failures and the need for frequent replacements, according to some specialists.
Problem #1: Contamination
Contamination is the most frequent cause of band heater failure. Plastic, hydraulic oil, and moisture are the three main contaminants. Most band heaters fail from lead wire damage caused by contamination, not from contaminants inside the band. Contaminant-resistant heaters donât prevent lead wire damage.
1. Buy low-cost band heaters and plan to replace them frequently.
2. Make sure the heater is installed over a machine barrel that is free from grit and scale. Scale can cause a hot spot on the heater, altering its heat-conduction path.
Problem #2: Poor contact
Mica, high watt density, and extruded aluminum bands are conductive heaters, so a tight fit is critical. If the fit isnât tight enough, localized hot spots can develop and cause resistance wire failures. The higher the working temperatures, the more critical the fit. A tight fit is not as critical for a ceramic band. Ceramic band housings have serrated edges, allowing full expansion during installation.
1. Check the machine barrelâs OD and order band heaters with the same measurement. 2. Strictly follow installation and tightening procedures.
3. When possible, specify strap-style clamping devices. They have a lower CFTE rate than the heater and can help hold the heater tightly against the barrel during operation.
4. Snug up the heater, rotate it radially on the barrel seating, and then tighten the clamps in an alternate fashion at ambient temperatures and retighten at setpoint temperature. If the setpoint temperature is high, tighten at ambient, tighten again halfway to setpoint, and again at setpoint.
Problem #3: Watt density
Heaters are often sized to bring the press up to heat as quickly as possible, and the band heater wattage is far in excess of operational requirements. This drives up internal temperatures during molding operations and can shorten heater life.
1. Choose a watt density that is as close to the load requirements as possible, factoring in an appropriate safety factor.
2. Your heater band supplier can help you in properly sizing wattage for a given application.
If conductive heaters have a watt density thatâs pushing their designed maximum, blankets can cause them to run hotter, putting additional stress on the dielectric materials and the element wire.
1. Check with your heater supplier to identify maximum watt density levels.
2. Reduce the designed watt density of the band by at least 25% if youâre using blankets. In fast-cycling machines this may not always be possible since the heater must supply enough make-up heat to run the process correctly.
3. Keep the terminal connections and lead wires going to the band as cool as possible. If using a blanket, try to exit the leads as soon as possible from the blanket.
Problem #5: Temperature control
Runaway temperature commands usually occur when a sensor (a thermocouple or RTD) becomes loose or disconnected, and doesnât make solid contact with the surface to be measured. Faulty input received by the control device results in full heater output, even though the process is already up to appropriate temperature. Temperature overshooting is another problem.
1. Check sensor contact.
2. Consider a control that provides a soft or ramped startup. A soft-start control will cycle the heater on and off to allow for the wattage generated to soak into the barrel. (Note: Soft-start controls are designed to avoid a short to ground caused by moisture infiltration inside the heater. Some will slowly increase the percent of voltage going to the heater.)
3. When designing the system, match the total wattage applied to the actual wattage required to decrease cycling frequency and temperature overshoots.
Big Chief Inc.
Watlow, St. Louis, MO