Metal Finishing Guide Book

2012-2013

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Table I. Spray Application Processes Figure No. Process Advantages Limitations 1. Air-atomization���most commonly used for industrial finishing. Complete pattern control. Finest atomization. Uses more air. Creates most fog. Low transfer efficiency. 1.1 Siphon fed���uses vacuum created at nozzle to draw material from cup, external atomization. Lowest cost. Less maintenance. Changes color quickly. Operator carries weight of material at gun. Sprays light materials only. One quart maximum. Spray position limited. 1.2 Gravity fed���material is fed to gun via attached cup. Ideal for most refinishing. Easy to clean. Improved atomization versus siphon fed. Same as 1.1 Unfanmiliar technology to some. 1.3 Pressure fed (external atomization)���uses external pressure source, tank, or pump to force material from nozzle; material and air mix outside of nozzle. Delivers more material than 1.1. Wide viscosity range. Sprays most materials. No air nozzle wear. Spray in any position. Independent control over air and fluid pressures. Consumes most air. Creates most fog. More controls to learn. 1.4 Pressure fed (internal atomization)���material and air mix inside nozzle. Less fog than 1.3. Larger patterns. Less air consumption. Coarse atomization. Fixed patterns. Nozzles wear. Fast drying coatings not recommended. 1.5 Pressure fed (external atomization) with heated material���same as 1.3 except material is heated to reduce viscosity. Better control. Reduced air and fluid pressures. Limits overspray and rebound. Finer atomization for a better finish. Better adhesion. More film build per coat. Reduced blushing. Reduced solvent use. Not all materials can be heated. Special paint formulations are required. Additional equipment to maintain. Equipment must be explosion proof (electrical). High electrical demand. Reduces pot life for catalyzed coatings. 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 High transfer efficiency, Sprays well into recesses and cavities. Complies with most air quality regulations. Atomization not as fine as that of air spray. Not recommended for some heavy materials or where high production is required. Some systems for generating HVLP air may be expensive. 2. Airless (hydraulic) atomization���atomization caused by release of high fluid pressure through small orifice. Most widely used by painting contractors and maintenance painters. High fluid capability. Large patterns. Fastest spray application process. Low air consumption. Limited fog and ���bounce-back���. Permits spraying into cavities. Potentially hazardous hydraulic injection. Higher rate of overspray. Sharp patterns; difficult to blend. Expensive nozzles may flood surface. Equipment requires top maintenance. 2.1 Airless atomization (heated)���same as 2 except with heat to reduce viscosity. Used by furniture manufacturers and industrial finishers. Better flow of material. Strict maintenance. Same Higher solids per pass of gun. limitations as 2. Viscosity control. Finer atomization than 2. 203

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