Metal Finishing Guide Book

2012-2013

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be too slow. Fluid deliveries that exceed 20 oz/min. may sacrifice finish quality for speed. AIRLESS ATOMIZATION In the 1960s, attention turned to another process of atomization known today as airless spraying. Airless spraying is a method of spray application that does not directly use compressed air to atomize the coating material. Hydraulic pressure is used to atomize the fluid by pumping it at high pressure (500���4,500 psi) through a small orifice (0.007���0.072 in.) spray nozzle tip located at the front of the airless gun. As the fluid is released at these high pressures, it is separated into small droplets, resulting in a finely atomized spray. The fluid is discharged at such a high velocity that it tears itself apart and sufficient momentum remains to carry the minute particles to the surface. The spray pattern size, or fan angle (3���21 in.) and orifice are usually preselected, but different spray angles deposit the same amount of paint over a different area. A good rule is to determine the largest fan angle and the smallest orifice that is practical for your needs. Advantages The main benefit of airless atomization is speed of application. It is the fastest method of spray atomization. This process can deliver twice the amount of material as a compressed air system. Other advantages include improve transfer efficiency due to a reduction in fog and overspray, the ability to spray into recesses and cavities with a minimum of material bounce-back, and reduced spray booth maintenance. Disadvantages The airless process has had to cope with several limitations over the years and one is coarse atomization. Although some users, such as maintenance painters, ship builders, and highway stripers, find airless atomization acceptable, others may not. Automotive finishers and refinishers, for example, would not select airless atomization. Other limitations include less control of spray pattern when compared to air spray or HVLP. An airless gun is either on or off; there is no ���feathering��� capability. The tendency is to flood the surface, causing runs or sags if the spray gun movement is momentarily delayed. Also, one must provide strict maintenance with this system. Any foreign object in the fluid supply that is larger than the spray-tip orifice will cause the system to block or shut off. Equipment maintenance on pumps is also high because of the high pressures used. Nozzle tips will wear and may prove costly to replace. Finally, the high velocity of the fluid stream and spray pattern, as it immediately exits the gun or hose, is a potential hazard. Never allow any part of your body to come into close contact with this high-pressure material. Failure to keep several inches away from the coating as it exits the gun or hose will cause serious injury. AIR-ASSISTED AIRLESS ATOMIZATION In the 1970s, various combinations of spraying systems emerged. Air-assisted airless was one that developed to fill a need that resulted from escalated material costs and newly enacted Environmental Protection Agency (EPA) regulations. Air-assisted airless, under ideal conditions, combines the best features of air spray along with the best of airless. Air-assisted airless spray guns first partially atomize the fluid with a special fluid nozzle tip similar to a standard airless tip. Second, they complete the atom209

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