Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 120 of 331

ization with small amounts of compressed air from the face and/or the horns of the air nozzle that they use. The result is a finely atomized spray pattern close- ly resembling that of a compressed air system. Newly designed low-pressure, air- assisted airless systems are also available. Some systems restrict the atomizing air pressure to comply with various EPA guidelines; as a result, these systems can be considered HVLP air-assisted airless. Advantages The primary advantage of air-assisted airless is its "soft" spray atomization. At- omization air pressures are usually low, and as a result, this system provides a fine finish with most coatings approaching that of compressed-air atomization. With air-assisted airless we can see an approximate 30% improvement in transfer efficiency over compressed air. This system allows us to increase flow rates, while also spray- ing into recesses and cavities, without excessive bounce-back of material. This means less booth maintenance and cleanup time. We also can expect a reduction in compressed-air supply requirements. Since fluid pressures generally range from 200 to 800 psi, less wear on the pump and tip is to be expected. Disadvantages Tip plugging may still be a problem with air-assisted airless. Many operators feel that air-assisted airless is too slow when compared to airless and that the fin- ish is still not as good as compressed-air atomization. Many operators tend to use excessive fluid and air pressures with this process. There are more controls to learn to use it correctly. ELECTROSTATIC ATOMIZATION During World War II, paints and solvents were in short supply or very expensive when available. Thus, to fill that need and to maximize the use of these materi- als, electrostatic atomization was developed. The coating is first atomized using either the compressed air, airless, or air-as- sisted airless methods previously discussed. Although these systems require equipment designed for electrostatic use, the atomization principles are the same as those for nonelectrostatic applications. The atomized paint particles, at this point, are provided with forward velocity and direction. The particles are made to pass through a cloud of free-floating electrons (neg- ative charges) produced by a high-voltage source such as an independent pow- er supply, cable, and electrode at the gun, or a turbine-operated generator lo- cated inside the spray gun. The basic principle of electrostatics is that "like" electrical charges repel one another and "unlike" electrical charges attract. Since each particle of material is negatively charged from 30–140 kV and 0–200 mA, it will want to seek the closest grounded object (positive) to com- plete the electrical circuit. If the product is sufficiently grounded, particles that would have missed that part will now be drawn back or attracted to it. This is com- monly referred to as the "wrap effect." Rotational Atomizers The first low-speed rotational atomizers premiered in the early 1950s. This type is sometimes referred to as a "true electrostatic" system. The atomization is cre- ated by the high voltage itself. The rotation of the electrically driven disk or bell evenly disperses the paint to the edge of the spinning disk or bell. When the unit is charged to approximately 120 kV DC, a current flow devel- 119

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