Cast-in heaters will provide long life and dependable, trouble-free service if properly installed, operated and maintained. This primer about the types of cast-in band heaters and suitable applications outlines the basics you need to know.

Cast-in band heaters are used on the barrels of plastic extruders instead of other types of band heaters where possible because they can withstand vibration, abuse and contamination. These characteristics give them an advantage in many industrial applications, but the method of manufacture and cooling mechanism selected can have an impact on the application. That said, this article describes certain variables related to cast-in heaters to help users select the type that is most suitable for specific applications.

The heat source in a cast-in heater is a tubular heating element. This type of heater typically has a nickel-chromium alloy resistance- wire coil assembly precisely stretched and centered in the element metal sheath, which is then filled with magnesium oxide powder (MgO). The filled tube then is compacted by a roll-reduction mill into a solid mass, permanently stabilizing the coil in the center of the tube while ensuring good heat transfer and electrical dielectric strength between the coil and the sheath.

The three methods of manufacturing cast-in heaters include low-pressure casting machines, tilt-pour gravity feed machines, and no-bake sand molds.

Three methods of manufacturing cast-in heaters are used, including low-pressure casting machines.

Low-Pressure Casting Machine. Used for large-volume quantities, a low-pressure casting machine is specifically suited for intricate and challenging geometric shapes and highly engineered applications such as pedestal cast-in heaters for semiconductor processing. It can produce near-net-shape quality castings with consistent dimensional accuracy and superior surface finish. Among its specifications are the following criteria:
  • Alloy: Aluminum.
  • Tooling: Requires a steel or cast iron permanent mold.
  • Machining: Minimum to no machining.
  • Weight Capacity: Up to 150 lb, depending on shape.

The second method of manufacturing cast-in heaters is tilt-pour gravity feed machines.

Tilt-Pour Gravity Feed. This method is used extensively for medium- to high-volume quantities. It will accommodate simple to some irregular shape castings, thus producing good dimensional accuracy and surface finish. Among its specifications are the following criteria:
  • Alloy: Aluminum.
  • Tooling: Requires a steel or cast iron permanent mold.
  • Machining: Moderate to extensive.
  • Weight Capacity: Up to 150 lb, depending on shape.

The third method of manufacturing cast-in heaters is no-bake sand molds.

No-Bake Sand Molds. This type is used for lower-volume quantities, prototypes, very large irregular shapes and thermal platens. Among its specifications are the following criteria:
  • Alloy: Aluminum, brass, bronze and iron.
  • Tooling: Requires a wood or plastic pattern.
  • Machining: Extensive.
  • Weight Capacity: Up to 600 lb.

A cast-in heater’s shape, finish, materials of construction and other specifications can determine the applications for which it is suitable.

The variety in the cast-in heater casting alloys allows variables in permissible maximum surface temperatures. For example, Aluminum 319 has a maximum surface temperature of 700oF (371oC); brass has a maximum surface temperature of 1,200oF (649oC); and bronze has a maximum surface temperature of 1,350oF (732oC).

Additional features and testing of cast-in heaters also can influence the selection of one type over another. Casting surface treatments are one such feature. Special surface finishes are required in some applications and may include electroless nickel plating, anodizing, Teflon or hard-coat anodizing. For example, an autoclave pure aluminum cast-in heater that is electroless nickel-plated is suitable for sterilizing dental instruments; by contrast, a bronze electroless nickel-plated cast-in heater is suitable for use in equipment that tests nuclear hazardous waste.

Sometimes, the heater’s shape can determine the suitable applications. For instance, certain shapes might be most suitable for use as part of the feed nozzle in a candy processing machine. Others might be suited for use in freeze protection for an outdoor valve assembly. Or, a heater that resembles a muffin tin might be made specifically for heating beakers in a laboratory environment.

Necessary laboratory services such as computerized infrared heating profiles, lifecycle testing, X-rays to confirm heating element location and casting density, and testing for heating and cooling ramp rates may be necessary. Of course, the results of these tests will influence cast-in heater choice as well.

Traditionally, liquid-cooled cast-in band heaters (left) have been the predominant method of controlling the melt temperature of extrusion barrels. Air-cooled cast-in heaters (right) require little maintenance.

Liquid Cooling vs. Air-Shroud Systems

The choice between liquid-cooled cast-in band heaters and air-cooling shroud systems for extruder barrels is another that must be considered. Up to now, liquid-cooled cast-in band heaters have been the predominant method of controlling the melt temperature of extrusion barrels. Although this is effective in removing heat from the extrusion process, there are a number of drawbacks that are primarily maintenance related.

Extruders using liquid cooled cast-in heaters can be subject to unpredictable and untimely failures of the cooling tube assemblies, resulting in downtime. Inherent maintenance problems include stress-corrosion cracks, linear thermal expansion of the heater body, and tube clogging due to the accumulation of mineral deposits. Additionally, liquid-cooled cast-in heaters require a cooling tower or heat exchange system, plumbing systems and installation labor.

Air-cooled systems have evolved to become more thermally efficient that early air-cooled designs as a result of geometric changes and implementation of sophisticated shrouding and airflow techniques. Air-cooled cast-in heaters require little maintenance and therefore, when properly installed and applied, have the capability to outlast and outperform liquid-cooled models. However, it is crucial to follow recommended installation techniques. These include:
  • Allow sufficient space for thermal expansion. The amount of space required depends upon the cast-in heater size and operating temperatures.
  • Keep the surface to be heated free of any foreign material. Likewise, to effectively employ an air-cooled shroud, the surface to be heated should have a smooth finish.
  • Make sure the casting is seated properly. The clamping devices used should be tightened down as much as possible.
  • After the initial heatup, retighten the clamping devices to ensure good surface contact.
  • Use thermal insulation to reduce heat losses, but be sure that the insulation does not come in contact with heaters.
  • Avoid mounting the heaters in an atmosphere containing combustible gases and vapors.
On cast-in heaters equipped with water-cooled jackets, the fittings must be securely tightened due to the high concentration of steam pressure buildup inside the cooling jacket. Flare type or braze seal fittings are recommended rather than compression-type fittings. Also, for both water- and air-cooled heaters, to prevent overheating and heater failure, adequate temperature controls should be installed.

In conclusion, while this primer about the variety of cast-in heater types can help with the initial step of heater selection, a manufacturer of cast-in heaters will be able to help with more detailed information regarding your specific application. With the range of designs and options, however, for certain, there is a cast-in heater suited to your needs.

Other Applications

If a cast-in heater cannot be made the conventional way for assembly into a machine part, the machine part can become an integral part of the casting.

In a typical process, a tubular heating element is attached to a steel roller and then is placed in a sand mold prior to casting. After casting, the roller outer diameter is machined to specifications. The finished heated roller could be used in a laminating web press, for example.