Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 50 of 707

Table I. Pressure Blast Air Requirements (scfm) Pressure (psi) 1/8 3/16 1/4 5/16 3/8 -in. nozzle -in. nozzle -in. nozzle -in. nozzle -in. nozzle 20 6 15 27 42 55 5/16 7/16 -in. nozzle, -in. air jet -in. nozzle, -in. air jet -in. nozzle, -in. air jet -in. nozzle, -in. air jet 30 6 30 8 18 32 50 73 40 7 40 10 22 41 64 91 Table II. Suction Blast Air Requirements (scfm) Pressure (psi) 1/4 1/4 50 8 10 15 31 12 19 38 15 23 45 50 13 26 49 76 109 60 10 17 27 52 60 14 30 55 88 126 70 11 19 31 59 80 17 38 68 113 161 80 12 21 37 66 100 20 45 81 137 196 90 13 23 38 73 120 25 55 97 152 220 100 15 26 42 80 matter, deburr, frost or decorate glass, and letter on stone. Aluminum oxide is eco- nomical because it can be used over and over again. It is classified in various sizes for a wide selection of finishes. Silicon carbide is similar to aluminum oxide, but is especially useful for cleaning very hard surfaces, such as tungsten carbide. Silicon carbide is a sharp media that is extremely fast cutting. Garnet is manufactured from the natural mineral. It, too, is hard and fast cutting. It is used to remove heavy material such as rust and weld scale and leaves a uniform anchor pattern. Plastic media are relatively soft and gentle. They are most often used for paint removal from delicate substrates such as aircraft, fiberglass and automobiles. Plastic media are also used to deflash molded parts and for cleaning precision molds, dies, electronic connectors, and circuit boards. They can deburr soft materials such as aluminum. Agricultural media, such as walnut or pecan shells and corn cob, are soft enough to remove foreign matter without etching, scratching, or marring the cleaned areas. They find use cleaning molds, electric motors, and windings. Two newer media are wheat starch and sodium bicarbonate (baking soda). Wheat starch can replace plastic for paint removal. Sodium bicarbonate is a water-soluble medium that is convenient for cleaning contaminated surfaces as well as for strip- ping paint. Sodium bicarbonate requires a flow agent to work reliably, and the large volume of dust generated must be suppressed. Both are soft, low-aggression media that are unlikely to damage parts. Sand has lost favor to longer life, less dusty, and more versatile media. Silica sand dust has also been found to cause health problems such as silicosis. Alternative media should be explored for anyone still using sand abrasive. BLAST PRESSURE The correct blasting pressure (psi) and impact angle must be determined to achieve the best possible blasting results. Correct pressure selection will also make any blast- ing operation more cost efficient. See the air charts in Tables I and II. Direct pressure uses compressed air more efficiently, so anyone currently blast- ing with a suction gun at 100 psi may get the same results faster using 60 psi with direct pressure. As shown in the tables, less air volume (scfm) is used for the given unit of work produced, making direct pressure more economical in the long run. 49

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