Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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to maintain. Suction systems work on the principle that air passing over an orifice will cre- ate a vacuum at that point. This action takes place in the hand-held suction gun, with a media hose connected from the vacuum area to a media storage hopper. Compressed air is piped into the back of the gun and causes the lifted media to be blown out of a nozzle on the front of the gun. Energy is expended indirectly to lift the media and then mix it with the compressed air, making suction less effi- cient than a pressure system. Pressure blasting feeds media into the compressed air stream at a pressurized storage vessel. The media then accelerates in the air stream as it is routed by a blast hose to the nozzle. Resulting media velocity is often several times that of a suc- tion system, resulting in a common fourfold increase in production. Direct pressure uses force, rather than suction, so it offers much more control at very high and very low operating pressure. Low pressure is used for delicate or fragile substrates, such as removing carbon from aluminum pistons or flash from integrated circuits. High pressure may be necessary for removing a tight mill scale. Direct pressure systems are especially useful for finishing hard-to-reach recessed areas and odd shapes, and in the case of very demanding applications (such as removing tight mill scale), they may be the only choice. Options for Air Blast Systems A variety of options is available for suction and pressure blasting systems. Options can tailor the system to your needs for increased productivity, material handling, longevity, and ease of use. Many of these will come as standard equipment with the cabinet, and most can be added after the fact without difficulty . Media reclaimers remove useless dust and debris from otherwise reusable media and are generally included in production blasting systems. The reclaimer aids economy by reducing media waste, keeping blasting speed constant, and improv- ing finish consistency by reentering media particles in the proper size range only. Spent media, dust, and debris are conveyed pneumatically from the bottom of the blast cabinet to the reclaimer inlet. Heavier particles are thrown against the reclaimer wall, where there is less air movement due to laminar flow, and are pulled down to the storage hopper by gravity. Debris is screened off there. Lighter particles and dust enter a counter vortex in the center of the reclaimer and are sucked off to a dust collector. Dust collectors filter dust-laden air from the blast cabinet or reclaimer, if so equipped. A dust collector will allow plant air to be recycled back into the plant, sav- ing heat or air conditioning costs. Many states now mandate and regulate dust col- lector use. There are two general types of dust collectors used for dry blasting: bag and car- tridge. Traditional bag collectors trap dust on a cloth filter, usually cotton. Cleaning these bags is accomplished with a rapping mechanism that can be automated. Cartridge collectors are generally more efficient and are typically self-cleaning but are more expensive. Extended wear packages protect vulnerable surfaces inside the system from wear. A typical package includes rubber curtains for the cabinet walls, heavy duty conveying hose, reclaimer wear plates, and carbide nozzles. Air dryers and moisture separators condition the compressed air by removing mois- ture that can cake media. Aerated regulators and vibrating screens keep fine and lightweight media flowing smoothly through the system. Magnetic separators remove ferrous particles that may harm the workpiece. Manual turntables facilitate handling of heavy, bulky, or delicate parts. Stationary low-profile designs make it easier to access the full height of the workpiece as it 46

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