Polyimide flexible heaters are highly desired in industries such as aerospace, medical, food service, military, and others to provide controlled heat in specified areas in their applications. This heater type may be used in instrument panels inside aircraft to provide moisture-control properties and to prevent the systems from malfunctioning due to extreme cold temperatures at high altitudes. They may also be found in medical diagnostic devices, analytical test equipment, optoelectronic components, and many other applications.
A major benefit offered with a polyimide flexible heater is that it’s extremely thin profile can provide an even distribution of heat even when it is bent and curved around other components inside systems. Its exceptional tensile strength and dimensional stability provides the appropriate heat for irregular-shaped surfaces that offer limited space. These flexible heaters can be attached to the applications using pressure sensitive adhesive (PSA), mechanical clamping, shrink bands, self-fusing tapes, or liquid epoxy adhesives.
Temperature and Power Limits of Flexible Heaters
Polyimide flexible heaters have specific temperature and power ranges it provides to applications. When obtaining a custom design for a heater, a customer will want to fully examine the application to determine its minimum and maximum allowances. With this information, the customer can decide whether a flexible heater will be suitable or if they will have to select a different material to provide what is needed.
For flexible heaters, two layers of thin polyimide are used with stainless steel, 304, Inconel 600, or CUNI44 foil heating elements sandwiched inside. FEP adhesive is used to adhere the layers together.
The watt densities and temperature ranges for this material are:
- Watt densities: up to 50 W/in2 (7.75 W/cm2)
- Temperature ranges: -328°F to 392°F
- Resistance tolerance: ±10% or ±0.5Ω (whichever is greater)
Keep in mind that the temperature ranges are not only defined by what the watt density range is set at. The individual layers and the actual temperature that each layer can reach also factors in on the thermal limits of the flexible heater.
Flexible heater test comparison.
Issues with Power and Temperature Limits
Determining the correct watt density and temperature limits can be difficult for some applications. If the incorrect watt density is used, the flexible heater can overheat as it can experience failure. If the temperature exceeds what the flexible heater should provide, then the layers of the flex heater can experience delamination and the adhesive used to attach the heater to the application may fail.
While it is possible to design a polyimide flex heater to operate at the highest level of its material limitations, prolonged use at this range will likely cause a greater amount of wear to the heater. This factor will result in a shorter lifespan and a chance of experiencing mechanical failure sooner.
When designing a polyimide flex heater that will push its temperature limits, the application itself may allow this setup depending on several factors. The environmental temperatures and the startup temperature of the flexible heater may allow it to operate at the highest end of the material's limitations, since the environmental temperatures will be constantly pulling away the heat from the elements instead of allowing this heat to build up among the polyimide layers.
The application's materials may also act as a heat sink for the polyimide heater. If the heater is situated between heat sinks, it must provide a higher wattage range to be able to raise the temperatures to the desired level. However, the foil conductor should never be overpowered to where burn-out may result because the heat could not effectively dissipate away when the application's functions or conditions changed.
Test temperature: 140-150 degrees Celsius. Test voltage: 21.1V. After testing 2.5 hours, good appearance, no blackening.
Figuring Out Watt Density
Calculating the correct watt density for flexible heaters is determined by Ohms law. Ohm's law states that the current that passes through the conductor/resistor between two different points will be proportional to the voltage (current) across the same two points.
The wattage of power for the heater that will be emitted as heat will be created by the conductive medium materials that are used when the resistance (Ohms), voltage, and current are applied together. These conductive materials will consist of the polymers, foil, wires, and resistive materials. Any changes to these materials will result in a change to the wattage and temperatures that the heater can provide.
The total amount of power that will be required will be based on a value that is higher than these two variables: the warm-up power + the heat loss that is determined during warm up, and the process heat + the heat that is lost when in a steady state.
Once this factor is determined, then the power requirement is divided by the total area size of the heater that is required by the application. Many polyimide flexible heater manufacturers will provide watt density charts regarding the heater types that they provide, the clamping method that will be used, and the heat sink temperature, to help customers figure out the watts per square inch and the maximum allowable power.
Increase the test temperature to 200-210 degrees Celsius. Test voltage: 26.2V. After testing 2 hours, the appearance of the product starts to turn yellow and there is no blackening phenomenon.
Experimenting with Flexible Heater Samples
There are additional thermal requirements that should be evaluated when designing a polyimide flexible heater that will provide the right amount of heat without going over its limits. The overall environment, the maximum surface temperature that is desired in case someone touches the application, the material that needs to be heated, and thermal uniformity also play vital roles in determining the watt density.
One approach that customers can take in trying to figure out the correct wattage for their polyimide flexible heater is to use an experimental approach and then apply Ohms law. Using a laboratory setting, a customer can create a test model of the actual application and have an AC power supply that provides an adjustable voltage.
Once they figure out the operating voltage that they want to use to power the flexible heater, they can obtain heater samples that are in different sizes as well as different resistances along with a heater sample sheet. With these samples, the customer can pick out the heater that fits the size parameters of the application.
Measurements of the resistance of the heater should be taken. Then, wires can be soldered onto the heater that lead to the test application and the AC power supply. The voltage of the AC power supply should be slowly increased as the thermal performance is monitored. The speed of the voltage increase should also be noted to determine how slow or fast the thermal warmup should be as well as the desired surface temperature.
When reaching the desired thermal setting, check the voltage on the AC power supply and use Ohm's law to calculate the wattage based on the test. Then divide the wattage by the heater's area to get the watt density.
After testing 4 hours, the appearance of the product starts to turn black (test temperature: 200-210 degrees Celsius; voltage: 26.2V).
Getting the Most from Your Polyimide Flexible Heater
There are many advantages to using polyimide flexible heaters for an application. These heaters are suitable when working with applications that require lower heat specifications as they can be pushed up to 400°F. They provide both excellent dielectric properties when compared to other materials and contain resistive properties to prevent them from cracking or becoming damaged. In addition, they have a rapid thermal delivery response so that heat can be applied immediately without waiting. By providing uniform thermal distribution as well as thermal stability across a wide range of temperatures, these heaters can be used effectively in various applications across multiple industries.
If the power and temperature requirements for an application changes, using the same flexible heater and pushing it above its limits is not recommended. Instead, consider switching to a heater that can provide a higher watt density and thermal threshold such a silicon rubber, mica heaters or a high temperature polyimide. Adjusting other factors of the heater, such as its polyimide thickness and the thermal pattern, can also provide a customer with desired temperature and power results especially when there are different thermal areas that will be required for the same heater. The thickness and thinness of the metal foil that is used will also have a significant impact on the amount of wattage that is provided.
Summary
While engineers can perform estimates regarding the new watt density when there are changes to the application, using the basic approach with test samples of the flexible heater and a test model of the application can provide a more practical and accurate determination. The customer will be able to see how the heater will operate for the application and within certain environments that can impact the generated temperatures. Then, they can adjust before the final flexible heater design is built.
