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| The Skinny on Thick Film Heaters |
Thick Film heaters are a compact way to quickly and precisely put heat where it is needed.
-Thomas Lyons, Thick film heaters product manager
For years, manufacturers in the life-sciences industry struggled to get the most out of their tubular and silicone-rubber heaters. Although these older devices have been improved, they still burden designers with many of the problems and limitations that have been hampering product development for years. Today's medical devices need higher watt densities, custom distributed wattages, faster response times, and lower profiles. And the older devices are falling short.
Thick film heaters provide an effective alternative. Introduced for new equipment applications in 1997, they deliver heat with fast response rates, uniform heat densities, and take up little space. Thin film heaters are also available but their power ratings are much lower. |
Building in layers
Thick film resistance heaters are built in layers. The base substrate can be either 430 stainless steel, aluminum oxide, aluminum nitride, or quartz with layers of ceramic-metal films sintered on at high temperatures. The layers are a sandwich of a glassy dielectric material, a metal resistor, and an overcoat dielectric layer on top. One advantage of this technology is that designers can vary the heat output across an entire working surface, so they can correct most temperature-uniformity problems generated by conventional heaters.
Despite their name, thick film heaters have a low profile that is useful when
space is a premium. And because the substrates can be thin, they offer superior heat transfer with response rates as fast as 45°F/sec, depending on application.
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| Thick films are a relatively new heater technology, so manufacturing processes available typically are not conducive to generating the lowest cost heater option at higher manufacturing volumes. Fortunately, thick film heaters often offset higher costs by replacing several thermal components with one heated part.
The new heaters are used in life-science applications ranging from pharmaceutical manufacturing, where thick film designs help more tightly control process variables when making drugs to refining biological samples for analytical testing. Determining whether or not thick film heaters are suitable for a particular application depends on several variables.
The heaters are best suited for applications that require uniform heat across a surface. The flexibility of printing heating circuits in many thick film arrangements lets designers use individual or multiple zones to distribute heat. |
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For example, an analytical-equipment manufacturer needed a better heater for a biological sterilizer. The sterilizer originally used a Kapton heater glued to the equipment. The heaters occasionally came partially unglued because of high temperatures. As a result, heat transfer wasn't uniform and drove the heat to failure. To solve the problem, the manufacturer replaced the Kapton heaters with parts that had the thick-film heater built right onto equipment surfaces.
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| The switch significantly increased the sterilizer's mean time between failure. In addition, because thick film heaters more uniformly distribute power, the sterilizers' temperature uniformity improved from 3.6°F to less than 0.9°F, which is critical to accuracy.
The new heaters also work well in limited space. With substrates as thin as 0.035 in. and a sintered glass thickness of 0.002 in., thick film heaters are thinner than most other designs, yet can be safely operated at Watt densities double that of silicone rubber and Kapton heaters. Available Watt densities range from 5 to 125 W/in.² depending on application and temperature. And the maximum operating temperature is 1,022°F.
Applications requiring fast response rates can also benefit from thick film heaters. The base or substrate for thick film parts is usually 430 stainless steel, aluminum oxide, aluminum nitride, or quartz. The heaters can have 2D shapes as small as 0.5-in. wide and as large as 48 in. Cylindrical units can have IDs ranging from 0.25 to 2.5 in., with lengths from 0.5 to 5.0 in. The surface area available to each heater determines wattage specifications.
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A few applications present challenges. For example, thick film may not be best for immersion heaters. Isolating the heating surface and its terminations creates concern over possible electrical hazards. However, thick film designs may be applied in a way that eliminates the need for direct immersion.
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The right materials
Substrate materials are usually selected based on processing concerns, operating temperature, cycle rates, and the environment that will house the heater. Stainless steel, for example, is a good, inexpensive thermal conductor that provides a low-profile heater custom fit to most any 2D shape. Parts are fabricated using precision laser cutting or conventional machining techniques. The 430-grade stainless steel is thermally stable and has sufficiently uniform thicknesses above 20 gage on cylindershaped products. A few high-temperature digesters, for example, are fitted with thick film heaters on stainless steel. The substrate's uniform temperature ensures that test vials in the digester are all heated to exactly the same temperature and will therefore yield valid and repeatable results. |
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| Ceramics are an alternative to stainless steel. Ceramics have relatively low thermal expansions, high dielectric strength and dimensional stability, and tolerate high temperatures, making them a preferred substrate for more demanding applications such as gas chromatography.
Alumina is the most widely used substrate due to its chemical resistance, relatively low cost, and stable physical properties. It's easy to fabricate into a range of shapes and will remain strong at high temperatures. Aluminum nitride, another ceramic, has high thermal conductivity, making it an excellent choice for devices needing fast responses or precise and uniform temperatures. On the downside, custom shapes in aluminum nitride are expensive and difficult to fabricate.
Into the future
Several other substrate materials are in development. Two of the most promising includes aluminum and 304 stainless steel. And a new polymer-based film is in beta tests. The film can be applied to aluminum and copper, which will transfer heat exceptionally well, meaning less temperature differentiation across a heated part. They are also easily machined.
When available as a substrate, 304 stainless steel will provide a lower cost alternative. This material is less expensive than 430 stainless steel and is more readily available, making it a good choice for lower-cost applications. |
430 stainless steel
Radiant
Conduction
Immersion |
35
75
175
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1,022
1,022
302
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Aluminum oxide
Radiant
Conduction |
23
75
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1,022
1,022
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Aluminum nitride
Conduction |
105
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572
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Quartz
Radiant
Conduction
Immersion |
20
30
100
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752
752
302
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