Metal Finishing Guide Book


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valleys it leaves behind. The reason finer media often works faster is that, for instance, a fine #10 glass bead (100-170 mesh) has over 300 million particles per pound compared to a coarser #5 glass bead (40-50 mesh) at "only" 7.9 million beads per pound. So, you see, many more particles hit every square inch every second. Fine media won't work all the time. In circumstances when you need to "power through" a tough coating or severely rusted surface, the momentum of a coarse, dense media particle is the only way to "cut it." Also, medical implants such as titanium knees may require a very coarse profile for the human body's bones to "knit" onto successfully. So every application requires a thoughtful analysis of grit size needs. And, of course, your blasting spec may dictate your mesh size. Density: A denser media will carry more of its kinetic energy onto the surface for more power when needed. You can actually save some energy by lowering the PSI when upgrading to a denser media. And, you can safely work on many delicate substrates with low pressure, and soft, low density media such as plastic, walnut, corn cob or bicarb. Durability – Friability: A friable medium is more likely to fall apart upon impact. This can be a problem or you can use it to your benefit, depending upon your aim. Dry ice blasting is popular for specific applications, and it takes friability to an extreme – it literally explodes upon contact with the surface, with the expanding volumetric energy getting underneath and lifting lightly adhered contaminants, such as hot plastic deposits inside injection mold cavities – as the media transitions to gas. (Thermal shock of CO2 at -107 degrees F against a hot residue also helps in these cases.) Generally, durable media is the best choice. And media that keeps its shape gives the most consistent blasting results and quality finishes over time. Ceramic bead and cut wire are fine examples of high-yield blasting media. Cost: Your selection of media directly affects blasting cost. Remember, the best media is fastest and cleanest, and you'll have your priorities right – and be on the road to more profitable blasting. Divergent needs: There's no two ways about it. Those shops that need to produce a variety of finishes will have to keep several kinds of media on hand. If production or scale warrants, you may find that you dedicate one machine to aluminum oxide, one to glass bead, etc. Seek expertise: Buying media from a distant catalog source that offers no expertise is no way to purchase media. Seek out your local blasting supply house for expertise. And use their local test lab, which should be well stocked with modern blast equipment and a variety of different media. CONCLUSION Abrasive blasting can be a cleaner, faster, lower maintenance-intensive process. Labor costs dearly, so reduce it through thoughtful selection of equipment and 18

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