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“Grit can be used by itself or in a blend


[with shot] to remove burn-in or burn- on,” he said. “It can be more aggressive in removing some minor defects, such as veining. It can also be used to provide a texture or textured fi nish. It makes the surface look homogenous.” For surface fi nish and defect re-


moval applications, the larger the grit size, the more aggressive the etching action. When selecting a grit size for a coating application, the buyer of the material must match the thickness of the


“Cycle times are money. If you have long times, you are spending too much.”—Nick D’Alessandro, Astech Inc.


coating with the profi le of the etching that will be produced. Depending on the size of the grit material used, the


peaks and valleys created on the surface of the casting will vary. These peaks and valleys should be slightly smaller than the thickness of the coating to be applied to the surface. This optimizes the adherence of the coating to the surface while not allowing the peaks of the casting surface profi le to extend beyond the thickness of the coating. “You measure the profile—the


peaks and valleys of the product you’re edging—using a profi lometer,” D’Alessandro said. “In the coating or painting, you want to avoid peaking. You fi ll the void or the etch with paint, but if the profi le gets above the peak, it can cause rusting.”


The Other Guys In addition to steel shot and grit, a


variety of blasting materials are available for use in specialty applications. Some metalcasters looking to limit the amount of ferrous residue transferred to the work piece by the shot, for example, use zinc or aluminum shot as an alternative to stainless steel. However, aluminum shot is soft and in some cases does not do an adequate job cleaning castings. Glass bead shot, ceramics and organ-


ics like walnut shells are used in facili- ties like Prima Die Casting to touch up parts that don’t require sand removal. “Those are used more in the auto-


motive and medical fi eld, where they don’t want to beat up the surface,” D’Alessandro said. Cut wire, typically made of carbon


steel, can be used in peening applica- tions (where the physical reaction be- tween the shot and the work material produces elevated mechanical prop- erties). Zinc wire is used for blasting magnesium. In this application, the zinc gives the metalcaster the benefi t of not creating a spark in the blasting cabinet, as steel shot does. These sparks have been known to ignite magnesium dust. In the end, metalcasters looking for a


magic bullet for selecting the right shot material for their plant are out of luck. The selection of the material only can be performed on a case-by-case basis. “The industry has a lot of variables,


and two [plants] doing much the same things can use different shot because of the material or equipment,” Rhodaberger said. “It is application-dependent.” MC


Visit www.moderncasting.com for an audiocast with Bill Rhoda- berger, Ervin Industries.


36 MODERN CASTING / January 2011


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