Understanding Infrared Line of Sight | Products Finish
An infrared boost oven can speed up the curing process so that less time in a curing oven is needed.
Photo credit: Trimac Industrial Systems LLC
Q: I am considering using infrared for my process and am concerned about line of sight issues. What exactly is it and is it a problem in a powder coating operation?
A: Excellent question! To understand line of sight, you must first understand infrared, which is essentially light energy. It is part of the electromagnetic scale just below visible light. When you stand in the sun, you feel the energy of heat coming your way. As you move into the shadows, you feel the line of sight energy being blocked. In the heating process, line of sight is the ability of infrared energy to see the part. When a part has a complex shape, interior parts can be shielded from light energy, which essentially prevents light from “seeing” all surfaces of the part, creating a line-of-sight challenge.
If light energy can’t see all areas of the room, does that mean infrared will only work on a flat room? In theory, this is correct because light cannot see around corners. However, in real-world applications, infrared can be used to heat and harden even complex welds.
To achieve the benefits of infrared on a complex part, another part of the physics of heat transfer, conduction, plays a huge role. Conduction is the transfer of thermal energy by direct contact with matter. If you place a metal poker in a fireplace and leave it, the heat will “conduct” the handle. Most often in powder coating, convective heat is used to harden complex shapes. The hot air from the oven also does not bend around the corners of the part, but the duration allows conduction to help the part harden. Heat is transferred by conduction to hidden areas to treat anything hot air can’t easily reach. With infrared, you can achieve the same result on complex parts, but also faster thanks to conduction. For example, a gas catalytic oven is typically 3 times faster than convection, but a complex weld can still be hardened with the help of conduction. With electric infrared the process can be up to 10 times faster than convection cures, however the speed reduces the time needed for conduction to play a part as the part is not in the oven long enough .
Another consideration is how do you plan to use infrared in your line? In a boost or freeze application, this is usually not a major issue. The purpose of a boost is to speed up the curing process so that less time in a curing oven is required. The only way to speed up conduction is to use higher temperatures, so a boost will conduct more heat into a room much faster than the convection oven can. Even though there are line-of-sight restrictions, the goal is to speed up conduction so these issues are usually resolved in the curing oven. A boost or gel oven, however, should always be designed to provide infrared energy coverage around the part to limit line-of-sight issues. The better your coverage, the less of a problem this will be. If you are considering a processing application that uses infrared, line of sight may pose a greater challenge. If the products are basically flat, you have no line-of-sight issues, so hardening is easily and quickly achieved. If your parts have a complex shape, your material manipulation can help, such as rotating the part into an overhead or chain-on-edge configuration. If the parts are not moving, you need conduction to play a role as well. You should still be able to harden the product in about a third of the time compared to convection and maybe faster.
Another consideration affecting line of sight is the design of the infrared heater. You would think that the hotter the radiator, the faster the process would work, right? Well, it’s a bit more complicated than that. No matter how much infrared energy you direct into a room, what really matters is how much energy is absorbed. Warmer is not always better because energy is more likely to be reflected from the part rather than absorbed by it. The infrared absorption rate of organic matter is in the medium to long wavelengths (300°F-1500°F). All infrared heaters emit infrared energy at their design temperature/wavelength (temperature is inversely proportional to wavelength). For example, catalytic gas emits in the range of 300°F to 1000°F (long wavelength) and electric infrared is generally designed to emit at 800°F-4000°F (long wavelength medium to short). At shorter wavelengths, the reflection of energy also increases. If the energy is only partially absorbed, the line of sight is even worse.
Bottom line, line of sight is an issue and should be a consideration when using infrared to speed up your powder coating operation. To understand how this might affect your parts, you need to work with an experienced infrared oven designer and have them perform tests in their lab. Using an oven temperature recorder, the impact of infrared energy on areas that may have line-of-sight issues can be probed for accurate results. With the data you will know how infrared can be successfully integrated into your line.
About the Author
Marty Sawyer is CEO of Trimac Industrial Systems LLC. Visit trimacsystems.com.