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Abstract

painting, airless cold spray. Hydraulic force rather than compressed air is used to produce atomization of paint material in airless spray painting. The coating material is pumped at high pressure through a hose to a spray nozzle or tip in an airless gun. As the material leaves the nozzle, the shearing action of the liquid passing at high velocity over the sharp edge of the nozzle orifice causes the material to atomize into tiny particles instantaneously. The momentum of each particle carries it to the surface being painted. Pumping pressure of the material is usually between 500 and 3000 psi (3.45 × 10⁴ and 2.1 × 10⁴ kPa) with most systems operating at about 2000 psi (1.4 × 10⁴ kPa). The degree of atomization and the spray pattern are determined by two variables: size and shape of the orifice in the spray nozzle and pressure of material at the nozzle. Spray nozzles are available in equivalent orifice diameters of from 0.007 to 0.109 in. (0.178 to 2.77 mm) and in spray angles ranging from 5° to 95°. Some nozzles are available with preorifices to obtain a better feather edge of the spray pattern and to help reduce the tendency of the material to spit when the gun is triggered. An airless gun is either full on or full off. Any adjustment to flow rate of material, fan pattern, or degree of atomization can only be made by changing the hydraulic pressure or by changing the spray nozzle. Spray nozzles are usually made of tungsten carbide and are very accurately machined to produce the desired spray pattern. If worn to any degree by abrasive pigments in the paint, spraying results will begin to deteriorate and cannot be corrected except by changing the spray nozzle. In contrast, compressed air spraying has infinite adjustment of fluid pressure and atomizing pressure to adjust for varying conditions in a manufacturing plant.