Airless paint spraying

FIGURE 1.6 Airless or high pressure paint spraying. (From Wang, L.K. et al. Case Studies of Cleaner Production and Site Remediation, Training Manual DTT-5-4-95, United Nations Industrial Development Organization, Industrial Sectors and Environment Division, Vienna, Austria, April 1995.)... 

Spray painting is a widely used application technique for most industrial maintenance and commercial architectural jobs. Spray painting is much faster than using brushes, pads, or rollers. A large variety of spray equipment is available, including air, airless, plural spray, and electrostatic. 

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. 

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. 

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 conventional bituminous or oleoresinous paints previously described are still used on the bottoms of smaller ships, the chief difference being that they are applied mainly by airless spraying. The formulations may be adjusted to permit application of thicker coats than by brush or roller, although the coats must not be too thick because oleoresinous paints require... 

Airless Spraying the process of atomisation of paint by forcing it through an orifice at high pressure. This effect is often aided by the vaporisation of the solvents especially if the paint has been previously heated. The term is not generally applied to those electrostatic spraying processes which do not use air for atomisation. 

Note 5 In calculating paint requirements up to 50% more than the theoretical dry film weight requirement should be allowed to cover thick coatings, wastage, repairs, losses (typically 30% for air spray 15% airless spray 10% electrostatic spray 5% roller or brush) etc. Manufacturers should be asked to quote percentage volume solids in their paint to facilitate calculations. 

Note 6 Labour costs for the application of paint increase in the order airless spray normal air spray roller brush... 

Figure 17 Preparation steps for the coated polycarbonate cylinders (top) (1) blank (2) tape applied (3) painted by airless spray (4) tape removed. Rotating equipment (bottom)...

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. 

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. 

Spray coating is used before and after a product is assembled particularly if already assembled and has complex shaped and curved surfaces. Many different types of spray equipment are in use to handle the different forms of paints used. They are classified by their method of atomization (airless, air, rotary, electrostatic, etc.) and by their deposition assist (electrostatic or nonelectrostatic, flame spray, etc.). Spraying techniques may fall into several of these categories. They range from simple systems with one manual applicator to highly complex, computer-controlled, automatic systems. They can incorporate hundreds of spray units. Automatic systems may have their applicators mounted on fixed stands, on reciprocating or rotating machines, on robots, and so on. 

The most widely used method of application in heavy industrial projects is airless spraying. This utilises high pressure, the liquid paint being ejected through a fine specially designed nozzle which causes the paint stream to break up into fine droplets in the form of a fan. This rapid method of paint deposition also allows application of high dry-film thickness with each coat, 150 /an being not unusual in this respect. A coat of "decorative paint applied by brush would probably yield a thickness of 30-40 fim. 

As part of the NAFTA Technical Working Group on Pesticides, priority areas for development of biocide applicator data have been agreed upon. These include, in order of priority, high-pressure spray, low-pressure spray, painting (roller/brush), wipe/mop, place solids, aerosol spray, painting (airless), pour solid, pour liquid and pump liquid. 

The most common methods of paint applications are brush and roller, air or airless spray, roll coating, electrostatic spraying, electro deposition, and dip coating. Many factors affect the choice of method to be used for a particular application. These include film thickness, appearance requirements, and operating cost. 

Ethyl silicate zinc dust paints are applied by air or airless spraying, often directly after jet cleaning of the substrate. They are seldom applied with a brush or roller. Layer thicknesses range from 15 to 20 pm (shop primers) through 50 to 70 pm (primers) to >100 pm (single thick-layer application). 

In conventional spraying, atomization is the result of external mechanical forces, i.e., the exchange of momentum between two free jets (air and paint). Atomization may be classified as compressed air atomization (air 0.02-0.7 MPa, paint 0.02-0.3 MPa), airless atomization (paint 8 40 MPa), air-assisted airless atomization, also termed airmix process (air 0.02 0.25 MPa, paint 2-8 MPa), and special technologies (Table 8.3). 

In airless hydraulic) atomization the paint is forced through a slit nozzle of hard metal under high pressure (8-40 MPa). On account of the high degree of turbulence, the paint stream disintegrates immediately after leaving the fluid tip. A similar atomization process occurs in spray cans where the paint pressure is produced by the propellant gas. 

The combined airmix process operates at a lower paint pressure (2-8 MPa). Additional low-pressure air jets (0.02-0.25 MPa) from the air-cap bores impinge on the spray jet to mix and homogenize it. In addition to the atomizer and a compressed air generator (airless pump), the airmix unit therefore also requires compressed air for postatomization. Advantages over the airless method are the less sharply defined spray jet and the smaller droplet size. Compared with compressed air atomization, a low-mist coating is possible. 

Heavy-duty coatings are often still applied manually with brushes or rollers that completely wet the metal surface holes and pores are filled with paint. This is especially important when old, partially rusted constructions are repainted after sanding. Brushing and rolling, however, only allow a slow working speed. Larger surface areas must be painted with airless spraying equipment. 

Application. The standard application method for most paints used on railroad vehicles is airless spraying. A combination of airless and compressed air spraying has been recently introduced, it is mainly used for applying waterborne topcoats. 

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. 

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