Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 639 of 707

Table I. Pulse Technology Comparison Type of Power Supply SCR SCR with filter Linear Switcher Switcher with filter Ripple High Low Lowest Highest Low SCR, silicon-controlled rectifier. an SCR phase-controlled power supply technically is a switcher, practical con- siderations usually limit pulse repetition rates to 12 times line frequency. Functionally, a switcher will typically start by rectifying the incoming line direct- ly. This raw DC will then be chopped by a variable pulse width modulator, feed- ing the primary of a high-frequency transformer. The high-frequency trans- former performs the desired voltage/current transformation. The output from the secondary of the transformer is then rectified and filtered. Switchers have a number of advantages over the other designs. Because of the higher frequencies, both transformer and filter inductor sizes and weights can be reduced, resulting in a more compact unit. Additionally, switchers have efficiencies comparable to that of phase-con- trol systems. This is due to the fact that the semiconductors are either fully on (saturated) or off, as opposed to the linear supplies, where the semiconductors are biased in the active region. Table I illustrates the relative merits of each design when considering ripple effi- ciency, bandwidth, physical size, and initial cost. The configuration that is most suited to your application will depend on factors such as those. Contact your pow- er-supply manufacturer for additional information. ANODIZING Direct Current Anodizing As in the case of electroplating, there is a wide variety of anodizing processes cur- rently in use. Electroplating deposits a metal layer onto a substrate, which may be a metal itself or some nonmetallic material such as plastic. Anodizing, on the other hand, is the conversion of the surface layer of a metal to an oxide. The met- al most commonly anodized is aluminum, but other metals, such as magnesium and titanium, can also be successfully anodized. Aluminum will naturally form an oxide layer when exposed to oxygen, but this is a relatively thin layer. Anodizing provides a much thicker coating. Anodized fin- ishes exhibit a number of desirable properties. They are capable of being processed further to modify the appearance of the aluminum. For example, colored finishes are easily obtained by such techniques as dyeing or color anodizing. Anodizing also improves the wearability of aluminum. An anodized finish is much more resis- tant to abrasion than the base metal. Anodizing is also extensively used in envi- ronments where corrosion is a problem. A number of anodizing processes are employed for aluminum. The most common is the sulfuric acid anodizing process. This provides a coating typical- ly 0.1 to 1.0 mil. thick and lends itself to further color processing. Other con- ventional aluminum anodizing processes are those utilizing chromic acid (found in marine and aircraft applications) and phosphoric acid (used as a surface 638 Efficiency Highest High Lowest High High Bandwidth Low Lowest Highest Mid Mid Size Mid Largest Largest Lowest Small Cost Lowest Low Highest Mid Mid

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