Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 405 of 707

imization of temperature variations within the tank. This type of acid movement assures one of better anodizing. Recently, the use of acid "spargers" in the bottom of the anodize tank has become popular. These spargers replace the more common air spargers now being used and give much better acid circulation and temperature control. Agitation To prevent localized high temperatures, some form of agitation is required in the bath. Low-pressure air, provided it is clean and oil-free, is often used. Mechanical agitation and pumping of the electrolyte through external heat exchangers are also used. Generally, compressed air is not recommended due to the presence of oils in the lines. Multiple filters in the air lines when using compressed air have not proven to be completely effective in keeping oil out of the anodize tank. Racks The two most common rack materials are aluminum and titanium. If aluminum is used, it should be of the same alloy as the work, or at least not be an alloy that contains copper (2xxx series). Alloys 6063 and 6061 are excellent rack materi- als. It must be remembered that aluminum racks will anodize along with the work and must be stripped before being used again. Titanium racks are more expensive, initially, but do not require stripping and are generally not attacked by the baths used in the anodizing process. Only commercially pure titanium can be used as rack material. Titanium racks are not suitable for low temperature anodizing (hardcoating) where high voltages are required. The lower conductivity of the metal causes heat- ing of the racks and eventual burning of the aluminum parts being anodized. Power Equipment For normal (Type II) sulfuric acid anodizing (68-72O capable of producing up to 35 V and 10 to 24 A/ft2 F), a DC-power source should be suitable. Some processes such as phosphoric acid, oxalic acid, hard coating, or integral color may require voltages as high as 150 V. Power supplies come with a variety of options. Such things as constant current control, constant voltage control, adjustable ramping, end-of-cycle timers/sig- nals/shut-offs, and a variety of other options make the anodizing process easier and more controllable. Power supplies for hardcoat anodizing require more stringent capabilities. Those used for Type III low temperature anodizing (28-32O approaching 90 V and amperages equivalent to 48 A/ft2 "room temperature" hardcoating (50-65O cient current to reach 36 to 46 A/ft2 F) will require voltages . Power supplies used for F) will require only 36 V and suffi- . SURFACE PREPARATION The type of surface preparation prior to anodizing gives the metal finisher a choice of effects. By combining mechanical techniques, such as scratch brushing or sand- blasting with buffing and bright dipping, interesting effects can be achieved. Sandblasting and shot peening have also been used to give interesting surface treatments. The beauty of dyed anodized aluminum can be further enhanced by color buff- ing the work after it is sealed and dried, using a lime-type composition, preferably containing some wax. In addition to actually polishing the coating, this step 404

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