Airless Gun Spraying

Spray cleaning of parts with solvent, using an airless gun similar to a paint sprayer, is also practiced. Spray cleaning is typically performed in a ventilated fume hood so as to protect the worker from solvent fumes. In addition to the fume hood, an emissions control system is often required. Many local air quality agencies prohibit the spray cleaning of parts with solvent or require an appropriate emission control system to be in place. 

Brooks, W.R., "Advantages of New Airless Atomization Spray Gun Explained", Journal of Cellular Plastics, 1(3) 419 (July... 

Solventless, elastomeric coatings were reported by Bonk et al. (169). The sprayable compositions were based mainly on diethylene glycol adipates, Isonate 143-L, and various diamine chain extenders and curatives. Airless Binks spray guns were employed in the... 

Paint s sprayed by air or airless gun(s) for functional or decorative coatings. Especially good for large areas, uneven surfaces, or relief designs Masking used to achieve special effects. 

Airless electrostatic spray coating. The airless electrostatic spray coating method (70 to 80% TE) uses airless spray guns with the addition of a dc power source that electrostatically charges the coating droplets. 

Spraying Equipment, Spray Guns, Air Spray, Airless, Hot Spray... 

Inside coatings to protect both the metal and the contents are appUed to the can by an airless spray gun. After appUcation, the cans are baked in an oven to remove the solvent and cure the coating. 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

Airless spray uses hydrauHc pressure to deUver the paint. Paint is brought to the spray gun under 7—40 mPa (1000—6000 psi), where it is divided into small separate streams and forced through a very small orifice to produce the spray. Airless spray is faster, cleaner, and less wasteful than air atomization, but demands good technique because it deUvers paint very quickly. 

The principal factors affecting transfer efficiency are the size and shape of the object, the type of apphcation equipment, the air pressure to the spray gun, and the distance of the spray gun from the object. The transfer efficiency becomes lower as the object becomes smaller or more complex. The transfer efficiency increases when the spray gun is brought closer to the object and when the atomizing pressure is reduced. The transfer efficiency of different types of apphcation equipment in descending relative order is manual > electrostatic spray > airless spray > conventional atomized air spray. 

In conventional spraying paint is forced under pressure to the spray gun, where it mixes with air and, forced through a small orifice, atomises. Airless spray is created by forcing paint at extremely high pressures through an accurately designed small hole. Rapid expansion as it leaves the gun produces an extremely fine and very even spray pattern. No air is mixed with the paint before it leaves the gun, so avoiding dry spray . A wetter, heavier... 

Airless spray In this process, a high pressure (12-35 MN/m ) is applied to the paint to force it through a fine orifice in the spray gun. This process allows rapid transfer with reduced overspray. 

These requirements are usually met with two-pack paints based on hydroxyl-rich polyester or acrylic resins in the pigmented pack and aliphatic polyisocyanates in the activator pack. Cure with this type of finish is relatively fast and complete even at low ambient temperatures. An alternative finish is an acrylic lacquer, similar to the lacquer used for refinishing motor cars. These finishes are applied to the assembled aircraft by operators protected by air-fed hoods and using airless or conventional spray guns. High durability pigments are included. 

The rise time for airless-sprayed urethane foam is about 30 s. It can be refoamed for additional thickness after this period. The foam can be walked on after 3 to 4 minutes and reaches its full properties in 24 h. The airless spray gun is held about 30 in. (76 cm) from the surface and moved steadily over it. Dispensing rates of 4 to 6 lb (1.8 to 3.6 kg) per minute are generally considered optimum for most spray applications. The surface on which the foam is sprayed must be free of loose scale or grease. The adhesion of urethane foam to steel is essentially equal to the tensile strength of the foam, provided the surface is clean. Aluminum surfaces, on the other hand, do not provide a good bond unless a primer coat, such as vinyl wash, is used prior to spray foaming. 

The use of supercritical carbon dioxide (CO2) as a component in a solvent mixture is another ingenious technique to reduce VOC emission by 50 percent or more. This technique takes the advantage of the fact that CO2 is a supercritical fluid below its critical temperature (31.3°C) and critical pressure (7.4 MPa). Sohd coating and supercritical CO2 are metered into a proportioning spray gun in such a ratio so as to reduce the viscosity to the level needed for proper atomization. Airless spray guns are used. 

The rise time for airless sprayed urethane foam is about 30 seconds. It can be refoamed for additional thickness after this period. The foam can be walked on after 3-4 minutes and reaches its full properties in 24 hours. The airless spray gun is held about 30 inches (76 centimeters) from the surface and moved steadily over it (12). Dispens-... 

Pressure pot type guns and airless spray equipment enable much higher viscosity latexes, vinyl solutions, and plastisols to be sprayed. Hot airless spraying is desirable for very high solids or... 

Hydrogenated and modified castor oil is used as an additive to adjust the viscosity and facilitate application with a brush or spray gun (compressed air or airless) layer thicknesses of > 100 pm are thereby achieved [2.11]-[2.13]. 

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