Metal Finishing Guide Book

2012-2013

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A/ft2), so efficient heat exchange equipment is needed to keep the bath cool. HARDCOATING Hardcoating (Type III) is a name used to describe a special form of anodizing. The process, which usually employs higher acid concentrations, lower temperatures, and higher voltages and current densities is sometimes referred to as an ���engineering hardcoat.��� This is due to the fact that hardcoating imparts a very hard, dense, abrasion-resistant oxide on the surface of the aluminum. A dense oxide is formed due to the cooling effect of the cold electrolyte (usually 30-40OF). At these temperatures, the sulfuric acid does not attack the oxide as fast as at elevated temperatures. Because of the lower temperature, the voltages needed to maintain the higher current densities also help form smaller, more dense pores, thus accounting for the hardness and excellent abrasion resistance. Normal low temperature hardcoating is carried out under the following conditions: Acid concentration, 180-225 g/L Aluminum content, 4-15 g/L Temperature, 28-32OF There have been a number of organic additives developed in the past few years that allow the anodizer to hardcoat at elevated temperatures (50-70OF). 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 electrolytic coloring gives a more complete palette of colors from which to choose. Organic Dyes 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 140OF 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 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. 473

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