Infrared (IR) energy can be used as a source of heat to cure a variety of industrial coatings. Such infrared curing applies energy to the coated part surface by direct transmission from an IR emitter, which can provide source temperatures of anywhere from 500 to 4,200°F (Figure 7-2 – IRED 2010). Some of the energy is reflected off the surface, some is absorbed into the coating, and some heats the substrate. In powder coating, this direct transfer of energy creates an immediate reaction in the polymer, and curing (called crosslinking) begins quickly once the surface is exposed to the emitter. For paints and other coatings, infrared energy enables drying (Figure 7-1 – IRED 2010).
In IR curing ovens, heat is very rapidly transferred by radiation directly to the coating. Infrared ovens can cure a coating much faster than convection ovens, since they directly heat the part surface that is coated and do not waste BTUs to heat the entire substrate or the surrounding air. Subsequent heat conduction may sometimes cure even areas of the part that are not completely exposed to direct IR heat; however, differences in part structure and mass will affect the uniformity of this cure unless the intensity of the infrared heat is adjusted for these differences.
Infrared light is located on the electromagnetic spectrum between visible light and microwaves, and is measured in microns. Temperature determines the wavelength of the source IR emitter, so its peak wavelength can be controlled by changing its temperature. However, although all emitters can be adjusted for wavelength in this way, not all heaters are designed to emit the complete spectrum of long, medium and short wavelengths.
Influence of Coating Characteristics
The rate of IR absorption, reflection or transmission by a coating depends on the coating’s spectral characteristics, which include its emissivity and color, as well as other surface characteristics. Emissivity is a measure of the material’s efficiency in emitting radiant energy and is defined as the fraction of energy being emitted relative to that emitted by a thermally black surface (blackbody). It is measured on a scale of 0 to 1, with an ideal blackbody having an emissivity of 1. Emissivity and absorptivity are closely related, so materials with a high emissivity value also readily absorb radiant energy. Highly reflective surfaces (e.g., polished metals) would have low emissivity/low absorptivity and would be difficult to heat with IR.