Selecting the right flexible heater for your application requires careful consideration of several factors. The choices may seem endless with customization options in materials, heating elements, wattage, size, and shape. This guide will walk you through the key considerations when designing flexible heaters to ensure you get optimal performance and safety.
The Importance of Material Selection
The substrate material is a major factor impacting the heater's durability, flexibility, and operating temperature range. The three most common materials used are:
Polyimide (Kapton) - Known for its high heat resistance, Kapton heaters can operate at temperatures up to 500°F. It offers great mechanical strength and chemical resistance. Kapton is an excellent choice for applications requiring high temperatures.
Silicone Rubber - With a wide operating temperature range (-80°F to 500°F), silicone rubber offers great flexibility and conformability. It performs well in applications exposed to moisture and humidity. The material is fairly rugged but not suitable for extreme environments.
Polyester - An economical option, polyester heaters work for temperatures up to 300°F. The material is not as rugged or flexible as Kapton or silicone. Polyester heaters work well for general-purpose heating of flat surfaces.
Consider the temperature, environment, and application requirements when selecting the ideal substrate material for the flexible heater.
Understanding Heating Elements: Etched Foil vs. Wire Wound
The heating element type also significantly impacts the heater's durability and efficiency. The two main options are:
Etched Foil - A chemically etched foil is bonded to the substrate to form a uniform resistance heating element. Etched foil offers high reliability and an even temperature distribution. The bonded construction makes it prone to delamination issues.
Wire Wound - As the name suggests, a fine resistance wire is wound over an insulator and mechanically secured to the substrate. The durability of wire wound construction makes it suitable for high-flex applications. It is less efficient than etched foil with hot spots.
For most general applications, etched foil heaters provide better overall performance. Wire wound heaters are preferred when the heater is subjected to extreme flexing or vibration. When looking for the best flexible custom heaters, etched foil is typically the best option.
The Role of Output Wattage in Design
The wattage rating defines the heat output capacity of the flexible heater. Using Ohm's Law, the wattage can be calculated based on the heater's voltage and resistance:
Watts = Volts x Amps
Amps = Volts / Ohms
Therefore: Watts = Volts x (Volts / Ohms)
Key Factors Determining Wattage Needs:
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Operating temperature range
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Mass of the target object
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Specific heat of the material being heated
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Desired heat-up time
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Ambient environment temperatures
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Expected heat losses
Selecting the appropriate wattage is crucial for effective heating without damage. Higher wattage does not always mean better performance. Overpowered heaters can overheat the application and cause premature failure. Work closely with experts to determine ideal wattage.
Temperature Ranges: What You Need to Know
The operating temperature range depends on both the substrate material as well as heating element type. Here are some typical ranges:
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Polyimide heaters with etched foil elements can operate from cryogenic temperatures up to 500°F.
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Silicone rubber with wire wound elements performs well from -80°F to 500°F.
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Polyester heaters with etched foil work from ambient up to 300°F.
Keep in mind, that the higher the temperature capacity, the higher the watt density capability. Select a heater rated for your required temperature range with at least a 15-20% buffer.
Testing in Free Air: Is It Possible?
A common question is whether flexible heaters can be tested outside of the application by hanging in free air. The short answer is yes, with precautions.
Key Considerations for Open Air Testing:
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Without a proper heat sink, temperatures can quickly spike to damaging levels at full power. Power should be restricted to 25% or less of rated wattage.
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Smaller heaters with higher-watt densities require more precautions than larger, lower-watt-density heaters.
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Active airflow or cooling may be needed for high-watt density heaters to dissipate heat.
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The heater should be monitored closely with thermocouples to avoid over-temperature conditions.
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Mounting the heater horizontally allows for natural air convection vs. vertically.
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Element wires should not touch at any point during open-air testing.
Benefits of Free Air Testing
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Verifies heater functionality and integrity before installation.
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Allows testing of wiring, controllers, and diagnostics.
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Can validate thermal performance if properly instrumented.
With the proper precautions and restrictions, open-air testing can provide useful supplemental data to validate heaters. However, final performance testing should always be done in the actual application.
Shape and Size: Beyond the Square
One of the biggest advantages of flexible heaters is their customizable shapes and sizes. Here are some options to consider:
Shapes:
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Rectangles - The most common shape. Ideal for heating flat surfaces.
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Circles - Used for heating the outer diameter of cylinders and curved surfaces.
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Custom shapes - Heaters can be die-cut to fit around complex geometries. Consider added perforations for more flexibility.
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Cutouts - Strategic openings allow fitting over standoffs, fasteners, etc.
Sizes:
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Small sizes - Etched foil heaters can be made as small as 1 in2. Small heaters allow precision heating in tight spaces.
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Large sizes - Heaters over 60 in. in length are possible depending on the material. Large heaters can cover expansive surfaces.
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Narrow/thin - Flexible heaters down to 1/16 in. thickness provide contact heating across edges or small diameters.
Work closely with experts when designing non-rectangular or unusually sized heaters to optimize thermal patterns.
The Impact of Cutouts in Flexible Heaters
Strategically placed cutouts allow flexible heaters to fit over obstructions. The effects on heating performance must be considered:
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Small distributed cutouts have minimal effects.
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Large or clustered cutouts can create cool spots. Minimize individual cutout size.
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Additional wattage around the cutouts helps compensate for heat loss in those areas.
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Allow at least 1/2 in between cutouts for proper spacing and heat spreading.
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Cutouts should not exceed 20% of the total heater surface area.
Cutouts add flexibility but analysis is needed to optimize the design. FEA thermal modeling ensures proper cutout placement and sizing.
Frequently Asked Questions
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Can flexible heaters be used in wet or damp environments?
With proper sealing and insulation, flexible heaters can be used in humid environments. Silicone rubber and polyester substrates stand up well to moisture. Kapton heaters require extra care and insulation.
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How do I ensure uniform heat distribution in my custom flexible heater?
Work closely with design experts to optimize the heating element layout and watt density distribution. Strategies like peripheral watt loading help improve heat consistency across the heater surface.
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What are the limitations in terms of shape and size for custom flexible heaters?
There are few restrictions on shape with flexible heaters. Custom layouts help optimize complex geometries. For large heaters over 60", you may be limited to lower watt densities and temperatures depending on the material. Discuss your requirements with an expert.
Final Thoughts
Designing effective and reliable flexible heaters requires understanding all the material, elements, sizing, and shaping options. Work closely with experienced heating professionals who can translate your requirements into an optimized flexible heater design. From prototypes to full production, partnering with the right flexible heater manufacturer will ensure your project's success.
