Metal Finishing Guide Book

2013

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with the dust collector – and all particles pass through the separation device. Many (not all) are wear-protected and have adjustable "tune-ability" so that one style reclaimer can work with a variety of abrasives, from soft to harsh, coarse to fine. Media Make-Up As you'd imagine on a bell curve, grit size will skew towards fineness over time due to the constant breaking down of the impacted media. The "seasoned abrasive" will be doing the work for you, patterning the surface with the texture of that "working mix." For that reason, some operations charge the machine with a finer media and feed the make-up with coarser stock to draw the bell curve back into the intended range. Automated systems can be incorporated to constantly trickle in fresh media to "make-up" for the worn abrasive that's been pulled out of the working mix by the reclaimer – and these "automatic make-up systems" quickly pay for themselves. That's because, optimally, you should never have to empty the entire load of media for a fresh recharge. Surface quality will remain consistent, too. You can mimic the make-up system on simpler blast cabinets by emptying the dust collector and adding the poundage of dust taken away in fresh, new media – in frequent, small increments. With wet slurry blasting, blast media can retain its shape and size for long periods of time. One manufacturer of engines has processed more than 2 million small parts while consuming very little ceramic bead. Select the right pressure and angle What blast pressure should you use? Depends. With one-pass (disposable) media on durable parts you may wish to go all out and blast at the highest pressure6 in order to save labor costs. But you may find that blasting at 90 PSI gains little over blasting at 60 PSI. Blasting at the lower pressure will increase the service life of the equipment and your recycled media, keep the media's "working mix" bell curve centered, and save compressed air. Lower pressure blasting is also more forgiving and gentler on the parts. Experiment within the bounds of the blast specification. Higher operating pressures can cause rougher surface profiles. Excessive pressure is one of many reasons for "embedment" of abrasive shards, which might lead to coating failure. Embedment can also occur if the media is too friable, or if the wrong blasting method is used. What difference does it make if you blast at 90-degrees to the surface or some other angle? First, it changes the finish. You "dig" blasting straight on and "scuff " blasting at a low angle, and that affects finish. So keep a consistent angle when possible. In addition, you may find that low angle blasting is faster at removing a coating, since media plows beneath it to lift it off. When blasting at 90-degrees, incoming media is battling media rebounding off of the surface. Those collisions slow blasting rates and fracture media unnecessarily – leading to lower production, higher costs, high dust levels, and inconsistent finishes. 3. CLEAN UP! BLASTING AND "LEAN PROCESS" CAN BE COMPATIBLE No process can be considered "lean" when a part leaves the work cell, or worse, the factory, for blast processing. The extra handling and time lost in transport and waiting for parts adds unnecessary cost and complexity. With today's plating, painting, and adhesives, many specs do not allow parts to sit around for more than four hours, and some instruct you to plate within 30 minutes of blasting. You may not achieve that reliably with a blast lab in a remote part of the facility or off site. 13

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