Infrared preheating of powder coatings on complex components Compact, energy-saving oven designs
Powder coatings are melted using heat and are generally cured at temperatures of around 180 °C either in convection ovens or using i nfrared heaters. Hot air and infrared radiation have different heating properties. Infrared is a popular heat source because of its ability to heat the parts quickly and because of the small amount of space needed for infrared ovens. However, these systems must be designed carefully to accommodate parts with complex shapes. Increasingly, combinations of the two heat sources are used for powder coatings. The following examples show how the powder curing process can be improved with infrared heaters.
1. Different heat sources
Convection ovens transfer heat using the medium of air. The higher the temperature and the speed of the air, the greater the heat transfer. Because of the risk of the powder being displaced at high fan speeds, convection ovens in the preheating zone generally operate at very low air speeds. In addition, it is not possible to increase the oven temperature to a very high setting, as there is a risk of the parts being overbaked (yellowing) if the conveyor stops. As a result, the heat transfer values of convection curing ovens are relatively low. This means that large ovens are needed, which take up a great deal of space Infrared systems transfer heat without the need for a contact medium. They emit electromagnetic waves with similar properties to light, which are only transformed into heat when they meet the material. Infrared radiation has a significantly higher heat transfer capacity than convection. Another benefit of infrared systems is their short response time. Short-wave quartz heaters and medium-wave carbon heaters, in particular, respond within a few seconds. Therefore, infrared ovens can be switched on and off and adjusted to suit new products very quickly.
Like all plastics, powder coatings absorb infrared rays very effectively. The intense radiation melts the powder and brings it to curing temperature very quickly. As there is no air movement, there is no risk of dust contamination and the powder is not subjected to turbulence or displaced from the product.
Because infrared heat is transferred rapidly and with a high output, in most cases a much shorter oven can be used or the production rate can be increased. Infrared heaters have very short response times. For example, Heraeus Noblelight’s short wave and carbon heaters respond within one to three seconds. As a result, the heat is easily adjustable and this, together with temperature measurements of the object being coated, helps to prevent materials from overheating. In addition, it is possible to switch quickly between different types of coating with different baking temperatures in the same oven. Energy savings can also be made because the heat source is only switched on when it is actually needed.
2. Factors that influence infrared heating
The extent to which the energy emitted by infrared heaters heats the object and the temperature reached by the lacquer depend on:
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angle of impact of the radiation distance between the heaters component, the pigment of the powder heat conductivity of the component material thickness of the component walls.
Two-dimensional parts with a uniform wall thickness, such as powder-coated aluminium or steel panels, are the simplest to heat using infrared radiation. Highly three dimensional products are difficult to heat with infrared heaters because specific areas may be in shadow where the infrared rays do not reach them. Metals, and in particular aluminium and steel, have high levels of heat conductivity. This allows even complex three-dimensional parts to be heated with infrared heaters, as the temperature differences balance out after a short period. In addition, infrared heaters can be arranged in different positions in the oven with regard to their angle of impact and distance from the parts and can also be used across different zones. For example, in the case of very tall components, the heaters higher up in the oven can be operated on relatively low power settings because the natural convection process will help to ensure that the upper parts of the components are heated.
Less power is needed to maintain the temperature of the oven than to heat a component. Therefore, in the waiting area of an infrared oven fewer heaters or heaters with a lower specific output are used. In practice, infrared systems often allow the throughput of a powder coating line to be increased. Where there is not enough space available to enable the existing curing oven to be extended, an infrared booster can be the ideal solution. This is an infrared heating unit installed before or in the entrance to the oven which ensures that the powder melts quickly. Any temperature differences in complex components are balanced out during the subsequent convection heating process. If infrared heaters that can be switched on and off quickly are installed, they can be shut off when the conveyor stops or during breaks in the production process in order to save energy.
3. Applications for infrared heaters are described below
3.1 Aluminum frames A highly water-resistant coating on faces of buildings and window frames made of aluminum needs to be cured 5 minutes longer than usual coatings. It was not possible to extend the existing oven. Finally the company decided to install an infrared booster with two emitters positioned opposite to each other. The system could be installed in front of the entrance of the oven. The modules inside can be moved electromotively according to different frame thicknesses. The distance between the emitters can be adjusted to different lengths form 0,2 meters to 0,5 meters. The oven is 0,7 meters long and 7,5 meters high. In total it consists of 36 carbon emitters. They are positioned in 12 movable zones, each side with 6 zones aligned vertically. So the upper emitters can run at lower power. Furthermore the emitters can be adjusted to different wall thicknesses of the frames. In case of line stoppage or breaks during transportation the emitters can be shut off. The power of the emitters can be changed very quickly which ensures a save heating process and a great deal of energy savings. Four pyrometers measure the temperature of the object without getting in contact with the material. For the company this is an easy way of process controlling.
3.2 Steel cylinders In a plant for coating gas bottles, replacing a gas convection oven with an infrared oven allowed the oven footprint to be reduced by 80% and the gelling and curing time to be cut by up to 80%. The existing convection oven was 30 m long and took around 60 minutes to cure the powder coating onto metal gas bottles. Powder turbulence also resulted in quality problems. The move to an infrared drying unit produced significant process improvements. The new infrared oven is only 6 m long and, depending on the bottle size, the powder coating is gelled and cured on at a maximum temperature of 200°C in just 12 to 18 minutes. In order to ensure even heat distribution, the heaters have been installed in the infrared oven in such a way that the surface coverage is adapted to the bottle shape and wall thickness. More heat is generated in thicker areas, such as the bottle base and neck, than in the thinner walls.
3.3 Curing powder coatings on compressors using infrared heaters. A range of compressors used in small refrigeration units are around 600 mm high and have a circumference of up to 280 mm. They weigh up to 250 kg and are coated with a black powder coating. The powder coating is melted and cured in a 10-metre long infrared oven with long-wave infrared heaters. The problem for the company using the oven was that the curing temperature was too low and the temperature distribution was also uneven. In order to resolve these problems and to increase the processing speed, the compressor manufacturer replaced the existing long-wave heaters with medium-wave quartz heaters with gold reflectors. This simple solution allowed the processing speed to be increased by a third and the curing temperature to be raised
significantly. However, the problem of uneven temperature distribution was not resolved to the compressor manufacturer‘s satisfaction. The next step towards further increasing productivity was to install a one-meter long infrared booster at the entrance to the infrared oven. The booster has vertical, short-wave, twin-tube heaters (with gold reflectors) which make it suitable for use with the angled chain conveyor. It has an electric power input of 108 kW. The heaters can be turned on and off quickly. When no compressors are approaching the oven, the control system turns the heaters off. Fitting the short-wave heaters resulted in the production speed doubling, when compared with original system. In addition, no parts of the product are left at too low a temperature and the oven reaches the required curing temperature of 220 °C.