Flexible heaters are used in applications to keep systems, electronics, and products at the appropriate temperatures utilize a range of materials and thicknesses to achieve optimal heat output. Often when reaching out to a manufacturer, the customer will be asked to provide specifications regarding the application. The manufacturer will need to understand the wattage desired for the flexible heater and the temperature output that the flexible heater will provide.
Many customers wonder how heat output is achieved based on the size and thickness of the elements. Some common questions:
- Will it be necessary to have larger elements to get higher temperatures?
- What if the application requires a larger flexible heater?
- Will the thickness of the elements allow for uniform and stable temperatures throughout the flexible heater's surface?
In this blog post we will review how these specifications can affect the flexible heaters performance.
Voltage and Wattage
Understanding the size of the required heater and how it will interact with the application are necessary factors to consider. Yet manufacturers determine the thickness and spacing of the heating elements usually based on the voltage and wattage that is required for the application.
The advantages of flexible heaters are that similar heat output can be obtained between several different-sized heaters based on the thickness and spacing of the heating elements. Flexible heaters rely on the overall wattage that will be delivered as heat output to the application. Controlling this aspect requires understanding the watt density, which is normally calculated by the size of the heater divided by the overall wattage. So, for a heater that has a size of 4"x 4" and a required 100 wattage, the watt density will be 6.2W/in2.
For example, say that a customer requires 100 watts for two types of heaters that will be in separate locations. One heater may be larger than the other and there may be different voltages available. A manufacturer only requires the customer to provide the wattage for the heaters and the operating temperatures, as they will determine the thickness and spacing of the elements that will provide the same heat output that is required.
Thickness and Pattern of Elements
When elements are thicker, the heat can become more concentrated. Thinning out the elements can impact the electrical resistance and lower the heat output. However, both a thick element and a thin element can still provide the same wattage for the flexible heater. This aspect is accomplished based on the spacing of the elements. If the elements are laid in a tighter pattern yet with thinner elements, the flexible heater can be modified to achieve the same heat output as thicker elements that are spaced out more.
This factor is important since the size and thickness of the overall flexible heater itself may limit the amount of space that the elements can be laid in a pattern. A flexible heater requiring higher heat output may not be able to achieve the correct temperature if using thicker elements due to the spacing of the pattern. Instead, thinner elements placed in a tighter pattern would be able to reach the same wattage. This consideration comes into play when the flexible heater has to wrap around the application or must fit into tight spaces within the application while not hampering any moving parts.
Etched Foil and Wire Wound Heating Elements
The size of the heater material itself will also play an important factor in the flexible heater's performance. Flexible heaters can come in a range of sizes from 1/2" up to 30". The two most common materials that are used are polyimide (Kapton) and silicone rubber. Polyimide heaters come with etched foil elements while silicone rubber can have both etched foil and wire-wound elements.
Etched Foil Heating Elements
Polyimide heaters can come in extremely thin sizes as low as 0.005", as silicone heaters can come with thinness of 0.035". Both heater materials can be made with etched foil elements that consist of a thick metal foil that is acid etched into the substrate material. Heating output ranges can go up to 390°F for polyimide materials and up to 450°F for silicone rubber.
Etched foil elements require wider widths to achieve optimal heat output due to the thinness of the heater and the elements. So, to obtain the right resistance for heat output, the pattern will be modified so that the etched foil elements are placed closer together to generate more heat or spread out to generate less heat.
One thing with polyimide and silicone heaters with etched foil elements is that the overall heater size comes with limitations. Etched foil can only be used up to sizes of 18" x 24". When desiring larger sizes with greater heat output, silicone rubber materials with wire-wound elements are typically used.
Etched foil heating elements require wider widths to achieve optimal heat output.
Wire Wound Heating Elements
Etched foil elements are not suitable for all circumstances. For applications that have abrasive qualities or where more physical strength in a heater is required, then wire wound elements are used. Wire wound elements are also selected if a customer needs a flexible heater larger than 18" x 24".
Wire wound elements in silicone rubber heaters have a resistance wire that has a specific diameter. The wire will be made from metal alloys that offer both electrical resistance and thermal conductivity, such as stainless steel, copper nickel, Inconel, or nickel chromium.
Unlike etched foil elements that can be placed in tighter shorter patterns, wire wound elements can be placed in longer patterns, making them suitable for larger heaters. To control the heat output, the diameter of the wire wound elements and the spacing is modified. The average thickness of wire wound is 0.056".
Wire wound process during silicone flexible heater manufacturing.
Another way to control the temperature of wire wound is through the selection of the material used for the resistance wire. Some metal alloys have higher resistivity that is stable along the entire length of the wire pattern, which allows for greater maximum operating temperatures. Yet keep in mind the materials used to make the flexible heater and the adhesives will dictate how high temperatures can reach before malfunctioning. The polyimide and silicone rubber layers may begin to peel apart if going over the maximum limits.
Summary
When it comes time to designing and manufacturing a flexible heater, factors such as heating element types, thermal pattern positioning, the thickness of the elements, and metal alloys used will typically be decided by the manufacturer. After learning about the wattage and desired operating temperature for the flexible heater from the customer, the manufacturing engineers will determine the best ways to achieve these factors by controlling the spacing and element thickness within the size of the heater. The tighter spacing on the elements may generate greater amounts of heat output for heaters of smaller sizes while thinning out the size of the elements can lower the temperature output at specific spots.
These modifications can also be used for certain applications that may require multiple heating zones within one heater. By adjusting the pattern and element thickness, specific spots along the heater may generate greater heat output than other areas. This heating element design allows for the application to receive the right amount of heat at the right location. To obtain multi-zone heating within one heater, the customer must determine the differing wattages that are required for the specific locations. The use of multiple-input power lines may be offered, although zone heating control may be obtained with the use of one temperature controller.
