Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 226 of 331

Fig.2. Electromagnetic energy scale for infrared radiation. into or through the coating. These peaks and valleys correspond to particular ranges of emitter temperatures. Most coatings have absorption peaks that fall be- tween wavelengths of 2.5 and 4.0 µm due to the common carbon-hydrogen bond in organic materials. Infrared radiation, with wavelengths expressed in microns, can be accurately measured, controlled, and applied to the product. Figure 2 illustrates the elec- tromagnetic energy scale in which infrared is centered. Any mass whose temperature is above absolute zero is radiating infrared en- ergy. It is not until that mass is heated well beyond the temperature of a differ- ent mass that the radiation from the hot mass to the cooler mass becomes mean- ingful. Generally, the amount of radiation available is determined from the dif- ference in absolute temperatures of the two masses raised to the fourth power mul- tiplied by a constant. Simply put, an infrared heater will exponentially produce more radiation as them emitter temperature is raised. Figure 3 shows the rela- tionship between emitter temperature in °F and radiation in Watts per square inch as well as wavelength of the radiation measured in microns µm) Long-, medium-, and short-wavelength infrared energy may all be used, in part or in whole, in the drying and curing processes, the most efficient type of energy being determined by the actual process requirements. Long-wavelength infrared en- ergy is emitted at wavelengths of greater than 4.0 µm, with energy densities of below 15 W/in.Medi- um-wavelength infrared energy is emitted at wavelengths between 2.0 and 4.0 µm, with energy den- sities of 15 to 80 W/in2 . Short- wavelength infrared energy, which is also known as high-in- tensity infrared, is emitted at wavelengths between 1.0 and 2.0 µm, with energy densities of up to 200 W/in2 . A heavy metal part that is to be coated with a high-solids coating or a powder coating may require long-, or medium- , or short-wavelength infrared energy, or some combination, to provide maximum efficiency and desired prod- uct quality. Long-wavelength infrared energy would be absorbed slowly by the coat- Fig.3. Relationship of emitter temperature (°F), infrared radiation (W/in2 and radiation wavelength (µm). ), 225

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