Spraying Airless spray

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. 

Air-Atomized Spray Airless Spray Pressure-Atomized Electrostatic Spray... 

A method for atomizing and spraying a liquid, suspension, or emulsion by high pressure, without using compressed gas at the spray nozzle. Also termed hydraulic spraying . Airless spraying is used for paints and urethanes among others. 

Hydrostatic Spraying (Airless Spraying) Electrostatic Spraying Powder Coating... 

See also coating spray, airless spray, electrostatic spray coating, flame spray coating and plasma-spray coating. 

There are several methods of spraying [9]. AU techniques can be manual or automatic. Three common types of spraying include air spray, airless spray, hot spray, and hot-melt spray. In the air spray technique, the adhesive is pumped under low pressure, generally 1 bar, to the gun. Pressurized air (from 3 to 7 bars) arrives also at the bottom of the gun. Inside the spray head, the pressurized air causes the atomization of the adhesives into small particles (less than 1 mm diameter), which are sprayed on to the substrate to be coated. 

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. 

An aerosol container can be considered a special appHcation of airless atomization (see Aerosols). The pressure is usually suppHed by a Hquefied gas in the container at its equiUbrium pressure. The material being sprayed has a very low viscosity to provide easy material flow through the feed tube and to permit fine atomization. 

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. 

Acrylated rubber These are based on styrene butadiene and have become commercially available only relatively recently. They are manufactured in several grades but most have the advantage over other materials in this class of being based on white spirit solvent rather than the stronger and more obnoxious xylol. In other respects, they are similar to chlorinated rubber and cost approximately the same, although they are easier to airless spray and the dried film contains less pores. They are considered to have superior weather resistance to chlorinated rubber and vinyl. 

Suitable skilled labor may not be available, at a particular site, to use complicated equipment (e.g. hot airless spray). On a foreign site or in remote areas even conventional equipment may be unfamiliar or unobtainable. 

In considering design the first two can be reviewed together. Conventional and Airless 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... 

In the industrial world of manufacturing plants, power stations and the like, more than 80% of painting is concerned with the maintenance of existing structures and plant. Although airless spraying can be more than 20 times faster than brush application and conventional spray some four times as fast, there are many occasions where access difficulties and safety considerations prevent their use. Rollers and brushes therefore continue to be widely used. 

Neither roller nor brush is capable of attaining the same film thickness as airless spray. A paint which consistently gives 100/im dry-film thickness when applied by the latter means is unlikely to produce more than 75 m d.f.t. comfortably by roller or brush the more complex the geometry of a structure the more unlikely it is that consistent results will be obtained. 

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. 

Air assisted airless spray This concept is a combination of air spray and airless methods. Paint can be atomised with full spray patterns at low pressures. Turbulence is reduced significantly and overspray is minimised. 

Airless spray Suitable for most components Superior penetration in awkward areas to normal spraying. 

Topcoats over primer are often applied by airless spray. Trends to higher standards of exterior durability have encouraged the use of methacrylated alkyds and two-component urethane finishes. 

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. 

AntifouJing composition Airless spray, brush or roller 1 50-80... 

Conventional non-bituminous system Tung oil/phenolic medium Airless spray. 2-3 150-200... 

Antifouling composition Airless spray brush or roller 1 80-100... 

High build chlorinated rubber Airless spray 2 175-225... 

Red lead primer in quick-drying Airless spray. 2 100-125... 

Gloss finish, alkyd medium Airless spray. 2 50-80... 

High build epoxy (2-pack) Airless spray 2 200-250... 

Gloss finish, chlorinated rubber Airless spray. 1 50... 

Zinc phosphate primer in quick- Airless spray. 2 80-100... 

Semi-gloss undercoat, alkyd Airless spray. 1 40-60... 

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