As maintenance and engineering personnel's duties have expanded, the time available to spend on any given production need has shrunk, and the need to eliminate potential production problems at the source has grown. Band heater problems -- and the resulting downtime -- must be kept to a minimum if maintenance and engineering personnel are to satisfy all of the other demands on their time. At the same time, market forces are driving molders to deliver products on time with zero defects -- other factors that demand trouble-free band heater operation. Understanding band heater options, design considerations, performance expectations and methods of loss prevention is critical for a smooth-running operation.
Four band heater styles are used in the plastics industry: mica, ceramic insulated, high watt density and extruded aluminum. Each style has merits and detriments, and all are available with various termination and clamping options. Choosing the right style for your application is a matter of matching your process and budget requirements with the heater that best meets those specifications.
Mica Bands. Mica band heaters are designed with nickel-chrome resistance wire precisely wound around a mica sheet, which is then placed between two additional mica sheets. Mica is used because it provides good thermal conductivity and dielectric strength. The nickel-chrome resistance wire and mica sheets then are surrounded by a corrosion-resistant outer steel sheath.
Because mica bands are conductive heaters, intimate contact with the surface to be heated is important to ensure long life. Operating sheath temperatures should not exceed 850oF (454oC). (Note that this rating is for the heater's sheath -- not the process.) Maximum watt density varies by manufacturer, but the average is 35 W/in2. On smaller mica bands, the watt density usually can go slightly higher. If the application requires higher temperatures or watt densities, another style band should be selected.
Ceramic Bands. Radically different in design from the mica band, ceramic insulated band heaters have a helically wound nickel-chrome resistance wire strung through small ceramic blocks. The series of ceramic blocks then is laid inside a stainless steel outer housing lined with ceramic fiber insulation. The housing has serrated edges to allow the band to fully expand during installation.
Unique to the group, ceramic bands utilize radiant rather than conductive heat transfer. Therefore, ceramic bands do not require a tight fit on the barrel; in fact, a very tight fit may crack the ceramic pieces as they compress on the barrel. The radiant heat transfer also allows ceramic bands to be used more effectively on large diameter barrels, as long as the heater is not exposed to oil or melted plastic. Substances such as these can contaminate the heater due to its open design.
Maximum watt density is 45 W/in2. Unlike other designs, a ceramic band's built-in insulation can be provided in single- or multi-layer designs, improving efficiency and safety while protecting the product from ambient temperature changes that can affect heat profile.
High Watt Density Bands. With newer plastic resins requiring higher work temperatures, there is a growing demand for heaters that can provide more wattage within a given area. When the wattage required exceeds the range for mica and ceramic bands, a high watt density band should be used.
Like mica bands, high watt density bands are conductive heaters. However, their internal construction allows for much higher work temperatures and watt density ratings: Sheath temperatures can reach 1,400oF (760oC) while watt densities range from 80 to 150 W/in2, depending on the band's physical characteristics. Generally, smaller diameter bands can operate at higher watt densities than larger diameter units.
Extruded Aluminum Bands. Extruded aluminum bands are designed with tubular heating elements that run in a precise grove on an extruded aluminum segment. With a temperature rating of 600oF (316oC) and watt densities from 35 to 40 W/in2, these heaters are durable and can withstand contamination from plastic, oil and other materials. One restriction is band widths, which are limited 0.75, 1.5, 2.5, 3 and 4". By contrast, mica, ceramic and high watt density bands generally do not have width limitations. When environmental conditions are volatile, an extruded aluminum band is a good option.
Loss Prevention For Band HeatersMost 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.
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 the failure and replacement rates. In applications where liquid plastic and oil exposure is 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. Alterna-tively, a band heater designed to resist contamination can be used. Keep in mind, though, that most heaters fail from the severe leadwire damage caused by contamination -- not from contaminants finding their way inside the band -- and contaminant-resistant heaters will not prevent leadwire damage.
Poor Contact Between Heater and Barrel. The second most common cause of premature failure is poor contact between the machine barrel and heater. Because mica, high watt density and extruded aluminum bands are conductive heaters, a tight fit is critical. Without a tight fit, localized hot spots can develop on the band and cause the nickel-chrome resistance wire to fail. As a rule, the higher the work temperatures, the more critical a tight fit becomes.
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 300oF [149oC]). 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 bands are radiant heaters and should not be tightened in this manner.)
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.
Runaway Temperatures. Heaters are extremely obedient entities. If a controller tells them to produce temperatures beyond their limitations, they will do so -- until their demise. Runaway temperature commands often occur when the thermocouple or RTD does not make solid contact with the surface to be measured. If the sensor becomes loose or disconnected from the surface, its readings may be hundreds of degrees lower than the process or barrel's actual temperature. This faulty input then is received by the control device, which calls for full output from the heaters when in fact the process is already up to appropriate temperature.
Given the range of products plant maintenance or engineering personnel encounter, is not realistic for them to become experts on every piece of equipment used. This article only touches the surface of band heater design, options, performance expectations and loss prevention. Users should link up with a qualified supplier, who can help design a new system or perform a design analysis on an existing system, then make recommendations to ensure the best performance for the given application. Systems arising from a good supplier/user partnership will extend equipment life and allow critical production schedules to be reached.