The paint comes out of the narrow nozzle of the air sprayer, mixes with the compressed air, and is sprayed through the surface in tiny droplets in the conventional spray application method.
The paint sprayed into the sprayer in one of three ways. For each method, there must be a unique sprayer construction design.
Pressurized (Feed) Coated Sprayer: The paint in a container is delivered to the sprayer by passing through the hose with the pushing force created by applying 0.1–0.5 atm pressure.
(See Figure.1). It is used for large-scale applications. Better atomization is provided, and fewer paint particles are dispersed throughout the surroundings.
Top chamber sprayer: Paint is fed into the sprayer by gravity from a hopper mounted to the sprayer
(Figure.2). It is very suitable for small and frequent color changes.
Bottom chamber sprayer: Paint is fed into the sprayer by suction from a chamber attached to the bottom of the sprayer
(Figure.3). Well suited for small-scale operations with frequent color changes, such as overhead chamber sprayers
The paint fed into the air sprayer passes through the channels in the sprayer and reaches the paint nozzle mixing with air during exit via the help of a compressor with 2.5 - 5.5 atm. Mixing of paint and air can occur in two ways, depending on the air cap of the sprayer. The paint can be atomized both within and outside the sprayer (internal mix) (external mix).
Inner Mixture: As illustrated in Figure.4, the mixing of air and paint occurs right before the sprayer's nozzle. It allows for the use of less and lower pressure air in the internal mix. It produces a wider beam, maximum film thickness, and reduced overspray formation. After each application, the needle and nozzle must be disassembled and cleaned.
External Mixture: In such applications, the air is sprayed from the air ducts in the sprayer's head and collides with the paint coming out of the sprayer's nozzle, resulting in a mixture (Figure.5). It is a popular method. External mixing can lead to better atomization. Because of its high pressure, the beam can be better controlled and used in bottom chamber sprayers. It can be preferred in applications that require fast drying and high surface quality. It is less difficult to clean.
When air is pressurized, the moisture in it condenses, and the water droplets that occurred can reach the paint surface and form a surface defect known as a bubble. Furthermore, because compressor oil might mix with air, "water and oil filters" are inserted between the compressor and the sprayer.
There are two regulating valve heads at the back of the sprayer and one at the bottom. One located at the back is used to adjust the size of the paint jet, and the other one is used to adjust the flow rate of the paint to be sprayed from the nozzle. The adjustment valve at the bottom is used to adjust the amount of air fed to the air ducts (Figure.6).
In conventional spray systems there is no change in pressure while adjusting the paint flow and air flow with the valves. However, after the paint spray emerges from the orifice of the air spray gun, it encounters the air flow of the outside environment. For this reason, the spray pattern, which is expected to be circular, turns into an elliptical cross-sectional pattern that provides better efficiency (Figure-7).
The breakdown of the paint into very small droplets, also called atomization, is determined by the following variables.
- Application viscosity of paint (paint droplets become larger as paint viscosity increases),
- Diameter of the nozzle orifice (the smaller the diameter of the nozzle orifice, the smaller the droplets),
- The ratio of fluid pressure to air pressure (the smaller this ratio, the smaller the droplets),
- Surface tension of the paint (droplets get smaller as the surface tension decreases),
- Paint pressure that drives paint into the nozzle orifice (droplets get smaller as paint (fluid) pressure increases, provided the paint pressure/air pressure ratio remains constant),
A good atomization means that the paint droplets are as small as possible. In conventional air spray applications, droplet diameters between 20 µm and 50 µm can be achieved. These paint diameters allow a very good paint levelling on the surface.
At this point, wrinkled paint application can be mentioned as an exceptional application method. In wrinkled paint applications, it is aimed to obtain an wavy surface that does not spread well as a result of spraying the paint in large droplets by reducing the air pressure and choosing large diameter spray nozzles. It is especially preferred that some industrial machines and vehicle trunk interiors are painted to have this appearance.
In air spray applications, 600 volumes of air are used for 1 volume of paint. Therefore, the sprayed material is a paint-air mixture containing plenty of air. Having so much air in the mixture leads to three important consequences.
1) In air spray, the paint droplets go towards the target surface as a cloud with unclear boundaries. Although the application is targeted directly to the surface, some of the paint overflows the surface.
2) Paint droplets, carried with very high-pressure air stream, hit the surface with great speed and a significant part of the paint droplets bounce back and spread to the environment (Figure-8).
3) Air with high pressure such as 2.5 – 4 bar and paint with less pressure than air stream reaches the surface to be applied at very high speeds. Due to the bounce back of the air stream when hit to the surface, small air cloud accumulates on the surface that creates the effect called bouncing effect (Figure-9). Some of the paint that emerges from the gun has a tendency to bounce back by hitting this air cushion before it reaches the surface. Scattered paint droplets (also known as overspray dusts) spread to the environment as waste. Bouncing effect decreases the transfer efficiency since some of the sprayed coating is not transferred to the application surface.
For these two reasons, paint losses are high and transfer efficiency (percent of coating solids leaving the gun that is actually deposited on the coated object) is low in air spray applications. Depending on the shape and size of the object to be painted, the transfer efficiency can drop to 25%, and on large surfaces it can go up to 50%. Due to the low transfer efficiency, the amount of paint required to cover the surface and the amount of solvent emission into the environment also increase. The high amount of paint dust and solvent (VOC value) emission to the environment creates unhealthy conditions and does not allow applications to be made in open areas. Therefore, applications should be made in paint booths. Along with the paint booths, facilities where paint dust, paint sludge and dry paint can be filtered should also be established. In addition, the person who will do the application should wear necessary personal protective equipment.
- It can be applied on all kinds of surfaces.
- It provides good atomization.
- Its application area is wide. It can be used in various areas where tons of paint are used by selecting the appropriate gun.
- Provides good levelling and surface quality.
- It can be used with bottom reservoir (fed by suction), top reservoir (fed by gravity) and pressure vessel (fed by pressure). Systems that allow the flow of paint with the help of pressure vessels are generally preferred for large surfaces.
- It provides the opportunity to change the pressure and flow rate of the air and paint and the mixing ratio at the the nozzle.
- Özellikle renk hassasiyetinin önemli olduğu efektli renk uygulamalarında iyi sonuç vermektedir.
- It gives good results especially in color effect applications where color sensitivity is important. It is the most ideal application method for effect paints.
- Equipment cost is relatively lower.
- Too much paint dust and solvent are scattered around during the application. Therefore, it requires the installation of special application cabinets. Cabin filters should be changed frequently.
- The cleaning of the spray gun is also important. Cleaning processes require precision and time.
- Transfer efficiency of the application is low.