Metal Finishing Guide Book


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Table 1. Common Automated Coating Processes Process Dip coating Flow coating Electrodeposition Spray coating Primers Top Coats Yes Yes Yes Yes Noa Noa Yes Yes a Unless quality and appearance of finish are not critical before they achieve their optimum properties. Plural-component paints include the curing agent as one of the components. These include two-part epoxies and polyurethanes. Some coatings cure when they are exposed to special high-energy ultraviolet light or electron beam sources. These coatings cure very quickly (1–5 sec) when exposed to the light of an ultraviolet lamp. Screen inks for printed circuit boards are a typical application, as are other substrates, which are heat sensitive. Coatings can also be classified as air dry or bake. Air dry coatings will cure at room temperature. When heat is applied, usually less than194°F, they are termed force air dried. Baked coatings require the use of an oven, and are usually cured at temperatures in excess of 250°F. There are three major types of ovens. 1. Impingement ovens are used for forced-air drying of flat surfaces, or parts. Coated parts are passed along a conveyor belt and hot air is blown over them. 2. Forced convection ovens are versatile devices, which consist of an enclosure with means to circulate heated air. They can handle a wide variety of shapes and sizes. The desired temperature for curing can be selected and a uniform temperature maintained. Batch or continuous systems can be devised. These ovens are described in a separate section of this Guidebook. 3. Radiant heat ovens use infrared lamps with reflectors arranged around the pieces. The primary feature of these lamps is that they provide rapid heating. Their main disadvantage is line of sight limitations. In other words, only coated surfaces that are exposed to the infrared rays benefit from the process. Coated areas, such as in recesses shaded from the light, do not cure as rapidly. A separate section of this Guidebook provides details on infrared ovens. CONSIDERATION FOR AUTOMATION In facilities that coat large volumes of metal, automated coating application is common. The application methods can vary depending on the size and shape of the parts, the number of parts being coated per hour, whether or not there are long runs of one part geometry, and other factors. When conditions favor automation, or when the primer does not need to have a high appearance finish, any of the methods listed in Table I can be used for pri mer application. Automated spray guns can be in the fixed position or can be mounted on reciprocators or robots. Topcoat application is usually fairly demanding, particularly when the final finish is expected to have a high-quality appearance. Under such circumstances dip and flow coating are less likely to be used, although electrodeposition, which produces a superior finish, remains a viable option. For the most part topcoats are applied by spray. The decision to use manual or automated spray application depends largely 158

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