Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 412 of 707

Normal low temperature hardcoating is carried out under the following conditions: Acid concentration, 180-225 g/L Aluminum content, 4-15 g/L Temperature, 28-32O F There have been a number of organic additives developed in the past few years that allow the anodizer to hardcoat at elevated temperatures (50-70O F). These additives, by virtue of their chemical reaction in the oxide pores, help cool the material being anodized and retard acid dissolution of the coating. COLORING OF ANODIC COATINGS The coloring of anodic oxides is accomplished by using organic and inorganic dyes, electrolytic coloring, precipitation pigmentation, or combinations of organic dyeing and electrolytic coloring. After the anodizing step, the parts are simply immersed in the subject bath for coloring. The thickness of the anodic oxide can range from 0.1 mil for pastel shades up to 1.0 mil for very dark shades and blacks. Application of electrolytic coloring will be discussed below. Suffice it to say, the combination of organic dyeing and elec- trolytic coloring gives a more complete palette of colors from which to choose. The actual process of dyeing the aluminum oxide is very simple. A water solution of 0.025 to 1.0% of dyestuff at a temperature of 140O Organic Dyes F composes the dyebath. The aluminum, previously anodized, is simply immersed in this bath for a short period of time, usually 10 to 30 minutes, The work is then sealed and is resistant to further dyeing or staining. 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 main- taining a constant 140O F temperature and should be equipped with some form of agitation. Usual plant practice is to use air agitation; however, with proper fil- tration, 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 affect- ed 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 con- taining recesses, which are difficult to rinse, a neutralizing bath of sodium bicar- bonate 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 immediate- 411

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