Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 197 of 331

Fig. 8. Monorail dry-off oven features. Recovery: Recovery systems have recently become very important to all users of energy. As fuel costs increase heat recovery systems become more practical. Radiant: This is particularly applicable to flat parts and batches processed on the basis of part shape and size. It is a clean, high-energy source of heat where high sur- face temperatures are easily obtained in short periods of time. Although expensive to control it can provide shorter cure times and minimal floor space. Features Knowledgeable plant operators want panels and the general construction to provide maximum strength and rigidity, minimum heat transfer or loss, and an attractive ap- pearance. The panels should be filled with insulation of full thickness, formed in batts to resist sagging and settling that can leave uninsulated areas. Tongue-and-groove con- struction mates into a strong, neat joint. Ovens should be able to be readily disassem- bled and moved if necessary. A typical oven system is shown in Fig. 8. Air seals: Ovens can be provided with a variety of seals to prevent escape of heat- ed air and fumes. The bottom entry oven, which must be elevated, has a natural type of seal because the heated air rises into the upper, sealed portion of the oven. Ex- haust-type seals,where conveyor openings into the oven are enclosed by a hood with an exhaust fan, are also available. In theory the most practical type of seal is the recirculating seal, which has been far from perfect in the past. Companies have invested considerable time and money in this seal and they are now producing recirculating seals that allow adjustment to com- pensate for various oven temperature losses. By providing a recirculating fan with ad- justable vents at least 80% of the normally escaping heat can be retained. Ovens can be designed as a high-velocity type or low-velocity type depending on the application. In most cases ovens are designed with both types in special zones to con- trol paint popping, blistering, or in the case of a powder oven, dry powder blow-off. Low-velocity or "quiet" zones allow the coating to cure slowly, allowing solvents to evaporate before the surface film is set, which minimizes pops and blisters. Ra- diant and infrared ovens can fit into this category. High-velocity zones allow direct impingement of the air to the work,which allows for quicker curing of the coating or evaporation of water. This air movement helps to obtain even temperatures throughout the oven as well. The air movement is induced by high-velocity nozzles designed to improve direct impingement of heated air on the work surface. Attempting to dry or cure a painted part too quickly is a notable cause of paint "skinning," resulting in bubbles, blemishes, and powder blow-off. Rolling air or high turnover rates: This induces an air movement that helps to obtain re- 196

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