Metal Finishing Guide Book


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HOW TO SELECT BLAST MEDIA Step one in your goal of a cleaner, more effective, and profitable blast prep is selecting the best blast media for the job. As noted earlier in this article, ÒThe media that will give you the best results the fastest Ð and hold up the longest Ð will also be the cheapest to use, regardless of the initial cost.Ó Variety: With so much to choose from, different blasting media can handle any blast prep requirement. Various factors go into selecting the most appropriate media, including: Particle shape: Round media peen and sharp media etch. Think Òball-peen hammerÓ and ÒchiselÓ (on a micro, but broad, scale). The medium imparts its reverse image onto the substrate. For brittle coatings or removing light, burrs, etc., a round media will probably work best by ÒflexingÓ the coating loose, and it will leave the smoothest finish. To aggressively ÒcutÓ the surface and leave a profile of Òpeaks and valleysÓ a hard, sharp or blocky media will do best. A soft, blocky media can strip without etching. Hardness: Relative to the substrate and coating, pick an appropriate media hardness, which ranges from almost 1.0 (the rating for talc) to nearly 10.0 (the rating for diamond, on the logarithmic moh scale). Optimizing grit hardness An example of thoughtful hardness selection comes from blasting a fry pan made from an aluminum bottom fused to a stainless sidewall: A melamine plastic media at 4.0 will profile the softer aluminum so that Teflon® will anchor to it, all the while not etching the harder stainless portion of the pan. No masking required: how is that for "lean" manufacturing! Grit size: Often a too-coarse abrasive is selected, under the theory that larger media will last longer in the system and therefore cost less. However, a finer medium often does the job faster, whereas coarse media will not only take longer, but also require copious amount of primer to cover the rough peaks and 17

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