Carefully matching a heater to the application will help extend the life of the unit.

In a split-sheath heater, external grooves allow installation and extraction of the needle-type thermocouple.  


Mae West said, “It’s not the men in my life that count, it’s the life in my men.” Nearly every maintenance person maintains a similar idea about cartridge heaters. The life in a cartridge heater, however, depends upon on factors related to the application and the manner in which the heater is used.

Cartridge heaters are manufactured in a range of styles, including low density (non-compacted), conventional with ceramic cores, split sheath and multiple cell. Different manufacturers use an array of materials and manufacturing techniques to produce cartridge heaters of different levels of quality, performance, potential and price.

Examples of these varying factors are both seen and unseen by the user. For example, if the heater design relies on the pressure of the coil on the terminal pin to maintain contact, the heater will develop a high resistance joint much faster than a spot-welded junction. As another example, if the cartridge heater design employs a crimped transition to the braid or hose lead wire protection, the heater is more likely to fail in applications with repeated flexing much faster than a heater design that uses a welded transition.

Different applications place different demands on cartridge heaters. For instance, the amount of space available for the heaters in a process application ranges from roomy to cramped. Both conditions, and the gamut between them, affect the heating system designer’s ability to satisfy process demands effectively. Having suitable space in an application will allow a sufficient number of cartridge heaters to meet the watt density (wattage dissipated per square inch of the heated surface of the sheath) requirements. A sealing platen will accommodate a number of heaters while a sealing bar may allow only one. Lowering the watt density of a cartridge heater will generally extend its life.

The amount of heat required will decidedly impact the life and performance of a cartridge heater. Lower temperature applications such as plastic molding and packaging sealing are not generally tough on a heater, and most heaters will do the job. Higher temperature applications such as metal die-casting and friction pad manufacturing will require heaters with higher watt densities and the best manufacturing techniques.

The highest temperature requirements, from research applications to metallic plastic-state forming dies (operating at temperatures to 1,800oF [980oC]), require very high performance heaters and particular design considerations. Achieving the highest temperatures and maintaining wattage in a process of this nature demands the use of multiple heaters, operating at a lower watt density than a single heater might, to maximize heater life.

At extreme temperatures, the failure of a cartridge heater usually is due to work-hardening of the resistance coil. Over the life of a heater, the chromium in the NiCr resistance coil will diffuse out to the surface of the wire, where it oxidizes. The higher concentration of nickel in the wire will cause embrittlement of the coil, and it will fail under the stress created by thermal cycling of the heater. Very high temperatures will accelerate this process, so it is particularly important for the heater manufacturer to maximize coil design and specification.

Other process demands that impact the life of a heater are application related. Constant cycling of the heater can lead to premature failure. Applications that employ heaters with crimped (rather than welded or brazed) wire, braid or hose junction that also repeatedly flex or stretch the leads may experience premature failure. Or, if the heater is exposed to repeated jarring, quality manufacturing techniques become particularly important.

Contamination can leach into the cartridge and insulation, causing premature failure.

Contamination

Contamination is the number one enemy of a cartridge heater. The termination area of the heater is particularly susceptible to dripping oil, machine wash-down and process-related contamination. The heater manufacturer must be made aware of the environment in which the heater will operate to effectively protect it against contamination. Protection techniques include RTV rubber potting, ceramic potting, crimped-in high density plugs, welded header protection and even a hermetically sealed terminal end with rigid tubes over the wires.

From a thermodynamics standpoint, conductive heat transfer is optimum for cartridge heater performance and life. Radiant heat transfer can cause hot-spotting on the heater sheath and limits the ability of the heater to transfer its heat. Hot spots on the sheath also will create areas inside the heater where accelerated chromium diffusion takes place on the resistance coil as previously discussed.

The issue of fit in the bore becomes the overall factor for best achieving the setpoint temperature. Fit is simply the difference between the outside diameter of the heater and the inside diameter of the bore. An interference fit between cartridge and bore, under conditions of heated sheath grain growth, will result in total seizure of the heater in the work piece. In all cases, the heater will be slightly downsized from the specification dimension by 0.002 to 0.005"; in other words, a heater with a nominal diameter of 0.5" will be manufactured at 0.498" to 0.495". This allows the heater to be freely inserted into the bore.

Many manufacturers include a graph in their brochures that shows the recommended maximum watt density at different operating temperatures and fits. If used properly, this can be a useful chart when attempting to understand the impact of fit on a heater. At a particular fit and temperature, the maximum tolerable (or warrantable) watt density can be determined. As the fit tightens up at a given temperature, the application can tolerate a higher watt density.

The only danger of this chart is a reverse extrapolation; that is, the user takes a watt density at a given temperature and determines that the heater will tolerate a fit in a substantially oversized bore. It is necessary to understand that the chart is configured to determine the maximum watt density and not the maximum allowable fit.

An oversized bore will cause problems with a solid-sheath heater. The area of the heater exposed to the gap will overheat due to the inefficiency of radiant heat transfer as compared to wall-to-wall conduction. This results in a weakening of the insulation value at that point of overheating. Eventually, the current in the resistance coil will arc through the insulation to ground at the sheath. When it does so, it is in a catastrophic manner that will blow a hole in the heater sheath and arc weld the heater into the bore.

Solid-sheath manufacturers often recommend the use of heat transfer compounds that are either magnesium oxide (MgO) or metallic based. Care must be taken during the application of these substances to avoid letting them reach the heater leads and terminal. Some heater manufacturers apply a nonstick coating to the heater.

Split sheath cartridge heaters offer distinct advantages with respect to fit and elimination of bore seizure. The independent halves of the heater expand bilaterally to make wall-to-wall contact with the bore. This affects a maximized heat transfer to the heat sink so the heater coil will run cooler, resulting in longer heater life. Conversely, when the split sheath cartridge is de-energized, it will contract, facilitating ease of removal from the bore. Oversized and inconsistent diameter bores can be most effectively addressed with this type of heater.

Multiple cell cartridge heaters approach bore fit differently. They are manufactured by ganging two to six tubular heaters with low watt densities and swaging them in place in a solid tube sheath. They are manufactured with diameters of 0.065" under the bore diameter and depend on the high emissivity of an oxidized sheath to provide radiant heat transfer. Watt densities must be carefully controlled. Another advantage of the multiple cell cartridge is that it can be designed with independently controlled zones for greater control of individual areas of a large platen into which they are inserted.

Designing a high-temperature platen with multiple heaters help keeps individual heater watt density at a reasonable level.

Temperature Control

The system chosen to control the heater can be an important contributor to the length of life of the cartridge. An analog controller that slams full power to the heater will limit its life. Ramping controls and digital controllers treat the heater kinder. Powering the heater with small bits of current, digital controllers adjust the delivered power, correcting for system response and temperature over/undershoot, with self-diagnostic capabilities. The kindest control for a cartridge is a simple variac which supplies a constant voltage to the heater, an effective heating scheme as long as the process demand is fairly constant.

Finally, thermocouple placement is critical to the life of a heater. A thermocouple located in the heat sink should be located near the heater to reduce response to changes in temperature generated by the heater. Solid-sheath heaters can be supplied with internal thermocouples, which measure the internal coil temperature of the heater. Unfortunately, failure of the thermocouple requires replacement of the entire assembly.

Split-sheath heaters accommodate thermocouples in a unique fashion. A groove is swaged down the seam of the split, into which a needle-type thermocouple can be inserted. Temperature is monitored at the sheath, which provides accurate indication of the heat being transferred. The thermocouple can be inserted to any depth in the bore, and it is independently replaceable from the heater, saving the cost of the heater for each replacement.

So, if it’s not the cartridge heaters in your life, but rather the life in your cartridge heater that concerns you, take certain considerations -- your process demands, the proposed heater environment, and the intended temperature control system -- into account when selecting a heater for your application. When properly matched to the application, the life of the cartridge heater will certainly be extended.

Links