Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 199 of 331

(40 ft long) or a four-pass oven (20 ft long).This you can determine from the floor space available. If floor space is a premium and the building height will allow it the oven can be hung from cross-beams supported from the floor or mounted on the roof. Available space usually dictates the position of the oven. The tunnel opening of the oven is normally the same dimensions as the wash- er except that usually 18 in. to 2 ft from the bottom of the tunnel opening to the floor is ample for ductwork. To sketch the oven layout you know that the maximum part width is 1½ ft and that they are spaced on 24-in. centers. This indicates that a 3-ft-diameter wheel turn will permit clearance of parts on a turn, so the proposed oven is outlined as shown in Fig. 9. PAINTING SYSTEMS The major types of paint systems fall into five categories: conventional solvent systems, water-reducible systems, high-solids systems, powder systems, and elec- trodeposition systems. Defining your criteria relative to operational characteristics, coating properties, initial capital expenditure, and operational costs will deter- mine the paint system. A powder system and electrodeposition system have been included to demonstrate layout. The following are component design considera- tions for an electrodeposition system. ELECTROCOAT (ELECTRODEPOSITION) PAINTING Coating Thickness Control The main factors controlling film thickness are the applied voltage and the film re- sistance. Increasing the coating voltage or lowering the specific film resistance caus- es an increase in film thickness. You simply dial the desired coating thickness. The electrocoating process will continue until the organic film deposited pro- vides an electrical insulating resistance, which prevents further current flow. When the coated parts are removed from the bath they are rinsed in permeate and deionized water to remove nondeposited paint particles. Tank Design Electrocoat tanks are designed for an immersion time of 1½ to 2 minutes. It is possi- ble to deposit approximately 1.0 mil organic coating in the first 15 seconds; however, for heavier film deposits a longer time is required. Tank equipment includes dual pumps with each pump able to maintain the bath and prevent the setting of paint solids. Plate-and-frame heat exchangers are used with chiller units to maintain proper tank temperature. Tank Design is Vital In the design of the electrocoat tanks some of the most important items are cir- culation rate, circulation flow, and density of the paint. With the paint solids nor- mally at 8 to 10% density a flow rate and pattern is determined to prevent settling. The flow rate in the average tank is accomplished by the use of headers with educ- tors. The flow pattern in the bottom of the tank is opposite that of the conveyor movement and with the conveyor at the top of the tank. The exit end of the tank is equipped with an overflow weir tank designed to prevent foaming without drop- ping or aerating the paint. The recirculating pump suctions are also connected to this tank. 198

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