Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 407 of 707

removes any traces of the sealing smut. Irregular shaped parts, castings, etc. are best finished by brushing with a Tampico brush or by tumbling with sawdust or other suitable media. PRETREATMENT Cleaning Proper and thorough cleaning of the aluminum surface prior to anodizing is one of the most important steps in the finishing process. Improperly cleaned mate- rial accounts for more reruns and rejected parts than any other single factor. It is essential that all machining oils, greases, body oils, and other surface con- taminants be removed prior to the continuation of the anodizing sequence. Both alkaline- and acid-based proprietary cleaners are available that will do an ade- quate job. If the oils or greases are specific in nature, some cleaners may need to be "customized" for adequate results. What is clean? Generally, we speak of a part being clean if it exhibits a "water- break-free" surface. This means that if the water rinses off of the metal surface in a continuous sheet, the work is considered to be clean. If, on the other hand, the water "beads" up or forms water breaks, the part still has foreign matter on the surface and con- tinued cleaning is necessary. Once the part has been determined to be clean, sub- sequent finishing steps can proceed. Etching Etching is the removal of some of the aluminum surface from a part using chemical solutions. There are a number of reasons for etching aluminum: 1.To impart a matte finish to the material (lower the specularity or gloss). 2.To remove surface contaminants. 3.To hide surface imperfections (scratches, die lines, etc.) 4.To produce an overall uniform finish. Chemical etching is accomplished using both alkaline and acid solutions. The most frequently used etch media is sodium hydroxide. Time, temperature, con- centration, and contaminant level will affect the type of finish possible in an etch bath. Many proprietary solutions are available from the chemical suppliers. Close attention to the technical information included with the chemicals is impor- tant. Probably one of the most abused steps in the finishing of aluminum is rinsing. Most anodizers practice some form of "water management," usually to the detriment of the other process tanks. Improper rinsing causes poor surface fin- ish due to cross reactions of chemicals left on the surface from previous processing tanks reacting with the chemicals in further processing tanks. Cross contami- nation of expensive solutions is another fallacy of "water management." Cascading rinses, spray rings, or just cleaner rinse tanks with adequate overflow will go a long way in reducing poor finish and cross contamination. Rinsing Deoxidizing/Desmutting After etching, a "smut" of residual metallic alloying materials is left on the alu- minum surface. This must be removed before further processing. The use of deox- idizer/desmutters will accomplish this, leaving the treated surface clean for sub- sequent finishing steps. 406

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