Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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preparation for adhesive bonding and as a base for electroplating). These conventional anodizing processes require a DC power supply similar in nature to those found in electroplating, except that the voltages typically used in conventional anodizing (18-50 V) are higher than those commonly found in plat- ing (6-18 V). Otherwise, the design of the rectifiers for DC electroplating and DC anodizing is basically the same. Hard-coat anodizing is often employed in applications where a more abrasive or corrosion-resistant oxide layer than that obtained with conventional anodiz- ing is desired. Hard-coat anodizing processes typically demand voltages between 50 and 150 V, and in many cases, pulse power supplies are utilized to obtain spe- cific results. As in electroplating, the pulse rectifiers are very similar in design, options, and usage. Color Anodizing Many architectural aluminum anodizing applications require that color be applied to the finished product. Colored finishes are obtained through the use of dyeing, integral, or electrolytic color processes. Dyeing is a simple process. A dye bath is composed of water and dyeing mate- rial, and the anodized aluminum is placed in the dye bath for some minutes. After removal from the dye bath, the aluminum is then rinsed and sealed in a normal manner. Integral color is a process by which the color is produced during the conventional anodizing process. Organic acids are added to the anodizing bath, and these acids produce a color, ranging from amber through black, in the aluminum oxide. Standard DC rectifiers are used, though at a voltage approximately three times that found in sulfuric acid anodizing. The electrolytic or two-step process begins by conventional sulfuric acid anodizing using DC power. The parts are then placed into a coloring solution con- sisting of salts of various metals such as tin, nickel, and cobalt, and AC power is applied. The AC current causes the deposition of metallic particles in the pores of the anodic coating. By varying the relative amplitudes and times of the posi- tive and negative half cycles of the AC output, numerous colors and finish char- acteristics can be obtained. The electrolytic coloring processes have become popular as they require less energy than competing methods. An ideal power supply for the two-step process will provide the opportunity to adjust the voltage and on-and-off times of the positive and negative por- tions of the output independently. This provides the maximum amount of flex- ibility to generate the broad range of colors available through electrolytic coloring. COMPUTERIZATION In the 1970s many metal finishers investigated modifications that would be required to upgrade their rectifiers to computer control. At that time, however, the price and risk of automation was too high for most companies, forcing them to continue using manual control. Today, the importance of incorporating some degree of automation into the metal-finishing processes is becoming more evident. For example, smaller firms find themselves at a disadvantage when competing against larger, more automated companies, especially for jobs where the finished parts require precise coating thickness and consistent finish qualities. Additionally, certain plating applications require multiple layer applications to achieve the desired coating thickness and surface quality. These multilayer processes demand extremely accurate and 639

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