Spraying is by far the most frequently used application when it comes to Industrial painting. Spray-painting equipment can be classified by atomization method: air, hydraulic or centrifugal. These classifications can general be broken down further into conventional air atomize, airless, air-assisted airless, air electrostatic, airless electrostatic air-assisted airless electrostatic; high-volume low-pressure (HVLP) and rotating electrostatic discs and bells. The most common of these being the air atomize, HVLP, Airless, Air Assisted Airless and electrostatic spray gun.
Conventional Air Atomizing
Air atomizing guns used to be the most popular for applying high quality paint finishes. Because they are notorious for yielding lower transfer efficiencies than HVLP, many states have passed air pollution regulations that outlaw them or discourage their use. These guns rely on paint pumped under pressure to conventional spray guns, so that it mixes with a stream of compressed air either internally or externally. The compressed air breaks up the liquid stream or atomizes it, causing it to break up into droplets that form a spray. Most internal-mix guns have controls to regulate fluid flow, atomizing air and spray patterns. Since these adjustments allow the guns to meet the finishing requirements of a variety of sizes and shapes, conventional spray guns are used for coating many high-quality items. They can apply catalyzed, high-solids and waterborne coatings as well as more traditional finishes.
The greatest asset of conventional air-spray equipment is its versatility. In the hands of a skillful sprayer, a spray gun can coat practically any object. It can apply various coatings whose viscosity, flow and drying rate are controlled by solvent blending and adjustment of the application environment. A problem is that organic solvents used for thinning are costly. And in the application process they evaporate into the atmosphere, lowering transfer efficiency. The solvents also contribute to formation of photochemical smog.
In addition to their relatively low transfer efficiency, conventional spray guns used with organic-solvent-thinned paints produce overspray and solvent evaporation requiring large volumes of make-up air and high exhaust rates to protect workers. Despite all these considerations, however, the versatility of conventional spray is difficult to match. Thus it is still widely used.
HVLP guns come in a variety of designs concepts. Some are based on turbine technology, some on ventura and others on inductor technology. Regardless, the regulatory definition of HVLP mandates that the atomizing pressure must not exceed 10 psig. Consequently, transfer efficiency (or transfer rate) usually improves in comparison to conventional air atomizing guns. These guns rely on turbines to supply high volumes of low-pressure, heated air to HVLP spray guns. The use of heated air lessens the cooling associated with air and airless atomization. This not only reduces the tendency to condense atmospheric moisture but also stabilizes the evaporation of solvent from the coating droplets.
Airless Spray Guns
Use very high fluid (coating) pressures (1,000–3,000 psig) to achieve atomization. These guns are used for high volume coating flow rates rather than for high quality finishes. Airless spray guns are used to paint buildings, houses, ships, bridges, storage tanks, and more. Because it is generally difficult to control film thickness to 1.0-1.5 mils (as is common for industrial finishes), airless guns are often banned under air pollution regulations for industrial applications. These guns force paint under pressure through a small orifice in the gun, atomizing it in the same manner as a nozzle attached to a garden hose atomizes water. Upon emerging from the orifice, the tremendous internal pressure causes the paint stream to blow apart into atomized droplets. The viscosity of the paint, the size of the orifice and hydraulic pressure determine the speed at which the fluid emerges from the gun. Unlike air-atomized spraying, where compressed air imparts a greater velocity to coating droplets, causing overspray and blow-back, airless spraying lessens overspray and blow-back. Only their own momentum (not compressed air) propels the droplets. This accounts in part for the higher transfer efficiency. Heating the coatings applied by airless spray, using in-line heaters, has the same effect as adding solvents—it lowers viscosity. Thus hot airless spraying can apply paint at lower fluid pressure. The coating viscosity, amount of solvent required and overspray are commensurately reduced. Higher-viscosity paints, using less solvent, can be sprayed. Consequently, finishing with hot airless spray equipment reduces air pollution and decreases the amount of coating residue requiring disposal. It also compensates for seasonal changes in ambient temperatures.
Air-Assisted Airless Spray Guns
Air-assisted airless spraying units add compressed air to airless spray. The increased use of high-viscosity, high-solids coatings requires painters to use higher temperatures and higher fluid pressures to atomize paint. In air-assisted airless spraying, compressed air provides additional atomization and pattern control for application of viscous, high-solids coatings, allowing use of lower fluid pressures and paint temperatures.
Electrostatic Spray Guns
Electrostatic Spray Guns have several advantages. An electrostatic gun has a very high transfer efficiency (40-98%) which results in a lower paint cost, less VOCs, less cleanup and faster production. Electrostatic spraying equipment charges coating droplets as they pass or contact an electrode. It relies upon the attraction of opposite electrical charges. Charged paint particles are attracted electrostatically to the surfaces of the products to be finished, which are usually at ground potential. Not only are the charged coating droplets attracted electrostatically to the front surfaces of these products, they also wrap around and, in some cases, completely coat back surfaces. Because of this “wraparound” effect, electrostatic applicators are especially suitable for coating tubular products. Electrostatic spray guns can be conventional, HVLP, Airless or Air-Assisted Airless.
Airless electrostatic spraying uses high-pressure hydraulic spray guns with a power pack that charges the atomized droplets. The low velocity imparted to these droplets reduces blow back and overspray. Airless electrostatic spray guns are used manually or in automatic modes. They may be installed either as fixed units or in conjunction with reciprocators, to coat various products on a conveyor line.
Air-assisted airless electrostatic spraying is really a modification of airless electrostatic spray. The increased use of high-viscosity and high-solids coatings presents problems for airless electrostatic. When heating and higher fluid pressures were used to aid in the atomization of more viscous materials, other problems arose. With the introduction of air-assisted airless spraying equipment, which uses compressed air to provide additional atomization and pattern control, many of these application problems were solved.
Air-atomize electrostatic spraying improves transfer efficiency by use of power packs that electrostatically charge air-atomized droplets. These units may be used manually or automatically, in fixed or reciprocating mountings. Since they impart a higher velocity to coating droplets, their transfer efficiency is lower than that of the other electrostatic units.
Rotating electrostatic discs and bells take advantage of centrifugal and electrostatic forces to atomize paint. Liquid coating material is pumped into the center orifice of a rotating disc or bell. Centrifugal force propels the coating to the edge of the rapidly spinning disc or bell and into the atmosphere. High voltage concentrated on the machined, razor-edged outer rim charges the coating droplets as they spin off the edge. Paint is attracted to oppositely charged surfaces of parts.
High-viscosity, high-solids coatings (65% volume solids and higher) can be atomized and electrostatically applied using high-rotational-speed discs and bells.
Electrostatic rotating discs are used mostly on automatic lines. A conveyor loaded with parts to be painted loops around the disc in a horseshoe shape.
Rotating electrostatic bells apply coatings in either fixed or reciprocating modes, and also can be used manually or attached to robot arms.
Multi-Component Spray Equipment
Multi-component spraying equipment meters, mixes and sprays multi-component coating materials in one operation. Spraying can be by hydraulic or air-atomizing, internal or external mixing. Aside from having multiple supply and metering pumps feeding a common applicator, these units have the same components as other spray equipment.