The main difference is that manufacturers who need local heating for their applications turn to the advantages of flexible heaters installed on components and equipment. These heaters can provide low or high levels of heat at different temperatures to provide proper heat transfer depending on the application.
Heater materials are configurable and customized to suit a variety of applications. Manufacturers will need to select a heating element that provides proper heat, balanced heat distribution, and the ability to place multiple power densities in a flexible heater. Two common heating elements include winding and etching foil.
Wirewound elements
The winding element consists of a resistance wire of a specified diameter. This wire can have single strands or consist of several strands that are woven together and placed on a fiberglass core. This wire is then vulcanized onto a neoprene substrate or nylon-reinforced silicone rubber, as the thickness of the material can range from 0.032 inches. The resistance wires are wound into a specific pattern so that they are evenly spaced throughout the substrate.
Example of manufacturing a wirewound element flexible heater.
Wirewound components have many benefits. These elements provide a higher level of physical strength, making them ideal for heating applications where friction forces can be present on flexible heaters. Flexibility is another major advantage of wirewound flexible heaters. They can easily handle small radius bends, as for applications that require larger heaters, the wire can be placed in long strips.
Wirewound elements are the original elements of flexible heaters and are a suitable choice for manufacturers looking for larger silicone heaters. Today, manufacturers still use this type of element in a variety of applications, including food heating equipment as well as heating pipes.
Etching foil elements
Etching foil is a newer technology than wirewound elements that have been used for the past 40 years. The main difference of etching foil is that it is not composed of wire. In contrast, much like a circuit board, it is a thin alloy metal with electrical resistance properties that is acid-etched into the substrate material. Aluminum foil can be as thin as 0.005 inches and look similar to aluminum foil. Manufacturers of flexible heaters will make the metal foil into a rectangular flat shape and place it in a pattern that is at least 0.10 inches wide.
Example of a manufactured etched foil element flexible heater.
Etching foil elements can offer special advantages to manufacturers looking for thinner flexible heaters in applications where size and weight are serious concerns. They can be used for short- and long-term production, as these elements enable higher power density and the ability to heat up quickly.
Although the etched foil requires a wider width to provide the same wattage and resistance as the winding elements, the elements can be placed closer together without contact and short circuits. Thus, the etched foil can be in a tighter space on the substrate. Finally, manufacturers can obtain more precise temperature control with complex heat distribution characteristics.
Alloys for heating elements
Certain metal alloys have both thermal conductivity and resistivity, making them suitable for use as heating elements. Alloys consist of base metals and other trace metal elements that are added in specific processes to alter the mechanical, structural, electrical, thermal, and electrical properties of the base metals. The inherent properties of the alloy may be improved, such as higher corrosion resistance or conductivity, or reduced impurities. Alloys commonly used in heating elements include stainless steel, nickel-chromium, copper-nickel or Inconel 600.
304 stainless steel
Stainless steel 304 is a metal alloy composed of chromium and nickel because it is also combined with carbon. It can be formed using a variety of techniques including hydroforming. The main advantages of stainless steel are excellent heat transfer ability, corrosion resistance and ease of manufacture. It has a heat capacity of 20°C and a maximum mechanical temperature of 420°C.
Nichrome
Nickel-chromium contains a percentage of nickel and chromium. The addition of chromium increases resistance as well as corrosion resistance to high temperatures, making the alloy suitable for wire-wound elements due to its ductility and strength. The maximum operating temperature of the alloy is 1100 °C and the heat capacity is about 20 °C.
Copper-nickel
Copper-nickel is incredibly resistant to corrosion as well as electrical resistance. This alloy is often sought after due to its good machinability. Although it is more suitable for heating elements at 400°C, its maximum mechanical temperature is 600°C. It also provides a heat capacity of 20°C.
Inconel 600
Inconel 600 alloy is best suited for applications requiring high corrosion and electrical resistance, and it is also non-magnetic, making the alloy ideal for use with flexible heater elements used with products susceptible to magnetic influences, outdoors and in wet environments. It contains a combination of nickel, chromium and some iron. It is also highly chemically resistant. The heat capacity of the alloy is 20 °C and the mechanical temperature is 1100 °C.
Use the right heating element for your application
The best way to select the right heating element for the application is to determine the required power density, the required heat transfer and preheating, the working environment, and the size of the heater required. If manufacturers need large, robust heaters with smaller bend radii and multiple watt densities throughout the process, wirewound heaters can do the job. For applications requiring a thin flexible heater with fast heating and good overall heat transfer capability, etched foil elements can be used.