Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 227 of 331

ing, at nearly the same rate as the transfer of heat in a convective oven, and would eventually heat the whole product mass. Radiant wall ovens work in this fashion. When medium-wavelength infrared energy is used, the coating will di- rectly absorb the majority of the available energy, with small losses by conduc- tion to the metal substrate and some losses by convection from the coating to the surrounding air. Short-wavelength in- frared energy would tend to rapidly pen- etrate the coating and heat the sub- strate directly, which in turn would heat the coating by con- duction (Fig. 4). Assuming that none of the ap- proaches will cause bubbling, blistering, or surface skinning, Fig.4. Selection of proper infrared wavelength ensures the most efficient process. the medium-wavelength system that heats the coating directly will generally re- quire less operating energy because the mass of the substrate will not act as a heat sink; however, if the coating is prone to blistering and surface skinning, air must be introduced with the medium-wavelength infrared system, possibly reducing the efficiency, or the heaters can be separated to reduce the overall energy den- sity at the product while maintaining the proper wavelength. Alternatively, a short-wavelength system will penetrate the coating, possibly preventing surface skinning, heating from the inside out, resulting in the desired finish quality. Long ovens utilizing long-wavelength, low-temperature heaters or radiant wall systems are sometimes the only alternative to safely process a product. Different infrared heat sources produce different effects; therefore, the process should be carefully planned and specified. Part shape, how the parts are racked, and the base material of the part are all considerations when selecting infrared. A part that has hidden areas with a base material that has low thermal conductivity may be difficult to be processed uniformly with infrared, where the same shape part with a high thermal conductivity base material may be a good candidate. Infrared used as a preheat prior to an existing oven can bring the parts to temperature more ef- ficiently, leaving the convective section to hold the parts for the cure. Choosing the wrong infrared system for a particular product can result in a system that wastes production floor space, demands excessive operating energy, produces inconsistent quality, and causes defects in the product finish. TYPES OF HEATERS The various infrared heater products manufactured today comprise many different categories; however, three product groupings will be used here to make com- parisons easier. Short-Wavelength/High-Intensity Heaters These heaters consist of quartz tubes with tungsten filaments in a halogen atmos- phere. They have normal internal filament temperatures of up to 4,000°F at emis- 226

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