Metal Finishing Guide Book


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finishing equipment & plant engineering DESIGN AND OPERATION OF CONVECTION DRYING AND CURING OVENS BY DAVID CARL GEORGE KOCH SONS INC., EVANSVILLE, IND. he three major processes at work in a finishing operation are the surface pretreatment, the coating application, and the drying and curing of the coating. There are several proven methods from which to choose. The processes are dependent upon each other and are subject to design considerations, such as coating specifications, substrates, factory space availability, capital budget, environmental concerns, and many others. Several options for the process are available. There are air-dry applications, low-temperature cures for woods, plastics, and even electrocoated parts, and the more traditional higher temperatures for solids and powders. The equipment required to properly dry and/or cure the coating is just as varied. Infrared (gas and electric), radiant wall, conventional convection, and high-velocity convection are but a few of the available options. Applications that combine methods are becoming increasingly popular. From the point of view of an equipment supplier, by far the most often applied process is the direct gas-fired conventional convection oven. Infrared or radiant wall designs are often incorporated for preheating; however, the completion of the cure still is accomplished by traditional means. The purpose of a drying and/or curing oven is to elevate the product and coating to a particular temperature and hold this temperature for a set period of time. The combination of time and temperature serves to drive off solvents and set the coating. The desired outcome is for the combination of pretreatment, application, and cure to produce a coating with specific physical and chemical properties. Understanding the operation of a convection oven requires the examination of the systems at work within the unit. There are five major components in an oven: the shell, the heater, the supply system, the recirculation system and the exhaust system. Each of these has an essential function, is comprised of several interlocking parts, and is subject to problems from misadjustment and misapplication. When they work together properly, they produce the process necessary for the successful cure of a coating. OVEN SHELL The purpose of an oven shell is to contain the environment necessary for the curing process. The shell consists of the supporting structure, insulating and sealing materials, and openings. It must be of proper dimensions to house the product and process equipment while exposing the product to the required times and temperatures. A steel structure supports the enclosure and the product-conveying equipment. Most often the structure is built using wide flange or tubular steel on 10 foot centers. For ease of construction, the steel is located within the enclosure, exposing it to the elevated temperatures and cycling of the oven environment. Expansion becomes a problem. The beams in an oven that is 40 feet wide, operating at 450��F, will grow about 1 in. as the oven temperature is elevated. Special slotted-hole connections must be used to allow the structure to compensate for the expansion. To contain the heat, the process must be enclosed with proper insulating materials. Panels that are 30 in. wide are used with the necessary fiber insulation (1 in. of 4lb density insulation for every 100��F) sandwiched between aluminized metal skins. The 840

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