Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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mechanical surface preparation MECHANICAL SURFACE PREPARATION BLAST FINISHING BY DANIEL HERBERT EMPIRE ABRASIVE EQUIPMENT CO., LANGHORNE, PA.; WWW.EMPIRE-AIRBLAST.COM Blast finishing, with all its variations, is powerful enough to remove heavy mill scale and rust or gentle enough to take paint off delicate aircraft skins. Blasting is used for finishing, cleaning, coating removal, surface preparation, and surface treatment. Here are some common applications of this versatile process: Finishing: Add matte or satin finish, frost, decorate, remove glare, blend tool- ing marks and imperfections, hone and burnish, and mark identifications. Cleaning and removal: Rust/oxidation, coatings, paint, sealants/adhesives, car- bon deposits, excess brazing, casting medium, flash, and burrs. Surface preparation: Etch for bonding and adhesion of subsequent coatings, expose flaws for inspection, and remove hard cast surfaces for subsequent machining. Surface treatment: Shot peen for increased fatigue resistance, strengthen, increase wear properties, improve lubrication, reduce design weights, reduce susceptibility to corrosion, seal porous surfaces, and correct distortion. Several blasting methods and a variety of equipment options are available to do the job. Blast cabinets are self-contained units where the user is isolated from the process for safety. Cabinet enclosures are used for manual systems where an operator accesses the part through rubber gloves. Larger blast rooms require the operator to suit up to blast very large parts. Automated machinery uses an enclosure to protect passersby. Dust removal and grit reclamation are usual- ly integral to all blast systems. Media selection plays an important role in effective blasting. Many kinds of manufactured and natural abrasives, ranging from 12-gauge mesh to powders, can be used. Depending upon the amount of pressure exerted through the blast nozzle and the surface being processed, each type of media can achieve different results. The finishes produced by blasting are almost limitless. Change a few variables and the results can change dramatically. It is important, therefore, to "lock-in" the variables after the right combination has been found for consistent, high-qual- ity results. BLAST METHODS There are many ways to deliver the working medium to the surface being treat- ed, including compressed air, mechanical, and water slurry. The most popular is compressed air. Air Blast Air blast is categorized into two methods of media delivery: suction blast and pressure blast. Suction blast systems are selected for light to medium amounts of production and moderate budgets. Suction is not as efficient as pressure, so the range of appli- cations is more limited, but it often yields comparable results. Suction systems have the ability to blast continuously without stopping for media refills. They are also simpler to use and have fewer wear parts, making them inexpensive and easy 45

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