Metal Finishing Guide Book


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Table I. Spray Application Processes Figure No. 1. 1.1 1.2 1.3 1.4 Process Advantages Limitations Air-atomization–most commonly used for industrial finishing. Siphon fed–uses vacuum created at nozzle to draw material from cup, external atomization. Gravity fed–material is fed to gun via attached cup. Complete pattern control. Finest atomization. Uses more air. Creates most fog. Low transfer efficiency. Pressure fed (external atomization)–uses external pressure source, tank, or pump to force material from nozzle; material and air mix outside of nozzle. Pressure fed (internal atomization)–material and air mix inside nozzle. 1.5 Pressure fed (external atomization) with heated material–same as 1.3 except material is heated to reduce viscosity. 1.6 High-volume, low-pressure atomization (HVLP)– available in systems as discussed in 1.1, 1.2, 1.3, 2.2, and 2.4 2. Airless (hydraulic) atomization–atomization caused by release of high fluid pressure through small orifice. Most widely used by painting contractors and maintenance painters. Airless atomization (heated)–same as 2 except with heat to reduce viscosity. Used by furniture manufacturers and industrial finishers. 2.1 Lowest cost. Less Operator carries weight of maintenance. Changes color material at gun. Sprays light quickly. materials only. One quart maximum. Spray position limited. Ideal for most refinishing. Same as 1.1 Unfanmiliar Easy to clean. Improved ato- technology to some. mization versus siphon fed. Delivers more material than Consumes most air. Creates 1.1. Wide viscosity range. most fog. More controls to Sprays most materials. No learn. air nozzle wear. Spray in any position. Independent control over air and fluid pressures. Less fog than 1.3. Larger Coarse atomization. Fixed patterns. Less air patterns. Nozzles wear. consumption. Fast drying coatings not recommended. Better control. Reduced air Not all materials can be and fluid pressures. Limits heated. Special paint formuoverspray and rebound. lations are required. Additional Finer atomization for a equipment to maintain. better finish. Better adhesion. Equipment must be explosion More film build per coat. proof (electrical). High electrical Reduced blushing. Reduced demand. Reduces pot life for solvent use. catalyzed coatings. High transfer efficiency, Atomization not as fine as that Sprays well into recesses and of air spray. Not recommended cavities. Complies with most for some heavy materials or air quality regulations. where high production is required. Some systems for generating HVLP air may be expensive. High fluid capability. Large Potentially hazardous patterns. Fastest spray hydraulic injection. Higher rate application process. Low air of overspray. Sharp patterns; consumption. Limited fog difficult to blend. Expensive and "bounce-back". Permits nozzles may flood surface. spraying into cavities. Equipment requires top maintenance. Better flow of material. Strict maintenance. Same Higher solids per pass of gun. limitations as 2. Viscosity control. Finer atomization than 2. 169

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