Metal Finishing Guide Book


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Page 427 of 843

The equipment required, in addition to that needed for the actual anodizing operation, consists of rinse tanks with clean, flowing water; a dye tank for each color desired; and a sealing bath preferably equipped with continuous filtration. The dye tanks must be of stainless steel, plastic, fiberglass, or some other inert substance; never of copper or steel. They must be supplied with means of maintaining a constant 140OF temperature and should be equipped with some form of agitation. Usual plant practice is to use air agitation; however, with proper filtration, the filter itself can be used as the source of agitation. With air agitation the use of water and oil traps, plus a filter on the air supply, is necessary to prevent contamination of the dye solution. A few drops of oil spread on the surface of the dyebath is very often the cause of streaked and spotted work. Typically, the use of blower air agitation is preferred over compressed air. Rinsing after anodizing, followed by immediate dyeing, is of prime importance. Since some dyes will not dye aluminum in the presence of sulfate ion, poor rinsing can cause streaks and discolorations. Even in the case of dyes not affected by sulfates, any carry-over of acid causes a lowering of the pH of the dyebath, which means shade variations in succeeding batches of work. In the design of parts to be color anodized, care must be taken to avoid the use of closed heads or seams, which are impossible to rinse. In the case of parts containing recesses, which are difficult to rinse, a neutralizing bath of sodium bicarbonate is of value. In working with coated racks, care must be taken that the rack coating does not separate, thereby forming pockets that can entrap sulfuric acid, later allowing it to seep out into the dyebath. Work must not be allowed to stand in the rinse tanks between anodizing and dyeing, but should be dyed immediately, following a thorough rinsing. For most effective rinsing, three tanks should be used. In this way the final tank, usually deionized water, will remain relatively free of acid. The variables in the dyebath are time, temperature, concentration, and pH. Time and temperature are readily controlled in plant practice; however, regulation of concentration presents some difficulties. Fortunately, in the case of most single component dyes, concentration control is not very critical, a variation of 100% causing little change in depth of shade. The usual dyebath concentration for full shades is 2 g/L except for black, which requires from 6 to 10 g/L. In the case of pastel shades concentrations of considerably less than 2 g/L may be required in order that the shade does not become too deep. This reduction in concentration will have a negative effect on the dye lightfastness. Control of pH is important and a daily check (more often in smaller tanks or where high volume is a factor) should be made. The pH range between 6.0 and 7.0 gives the best results with the majority of dyes; however, a few are more effective at values close to 5.0. Initial adjustments should always be made since it is not practical for the manufacturer to standardize the dyes with respect to the pH of their solutions. These adjustments are made by addition of small amounts of acetic acid to lower the pH value and dilute sodium hydroxide or acetate to raise it. Solutions may be buffered against possible carry-in of sulfuric acid by adding 1 g/L of sodium acetate and adding sufficient acetic acid to reduce the pH to the desired value. 420

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