Understand the advantages and differences between air spray, air assist, electrostatic and rotary bell applicator technology.
Choosing the Right Liquid Spray Equipment
There is a wide range of applicator technology to choose from. Depending on the desired finish quality, transfer efficiency, speed of application, type of paint used, type of object, shape of object and coating technology that is chosen, it helps to understand the advantages and differences between each technology.
Air spray uses a low-pressure fluid stream that is mixed with compressed air at the aircap to atomize material in a controlled manner. It’s used for the application of low to medium viscosity fluids for products requiring a high quality, Class A or decorative finish.
Due to changing requirements for environmental regulations, different versions of air spray technologies have been developed:
- Conventional is the traditional form of air spray technology and delivers the highest finish quality and production speeds. To achieve these advantages, a lot of air is utilized, resulting in low transfer efficiency.
- High Volume, Low Pressure (HVLP) was developed for areas regulated by the U.S. Environmental Protection Agency. To meet EPA regulations, the amount of air used is limited to 10 psi at the aircap. The result is a low velocity pattern with good finish quality and higher transfer efficiency than conventional.
- Compliant technology, commonly referred to as Low Volume, Medium Pressure (LVMP), was developed to meet European standards. The requirement is the air pressure cannot exceed 29 psi at the air inlet. This allows for an aircap design that delivers a high finish quality while achieving transfer efficiency equal to or better than HVLP.
Airless spraying uses a high-pressure fluid supply for atomization without the use of compressed air, only fluid pressure. It’s used for medium to high viscosity fluids, delivers a lower finish quality and is ideal for speed and transfer efficiency.
Airless spray atomization is created by hydraulic force pushing material through an orifice. As the fluid exits the orifice, friction between the fluid stream and atmosphere disrupts the stream into small particles. The tip size and pressure is what determines the material flow rate. High pressure is used to create a complete pattern. Therefore, the higher the material viscosity, the more pressure required.
Air-assisted airless uses a medium- to high-pressure fluid supply for atomization and compressed air at the cap for pattern control. It’s used to spray medium to high viscosity fluids and delivers a finish quality that is better than airless, but not as high quality as air spray.
Air-assisted airless spray atomization is created by hydraulic force, which is less than airless so it atomizes the center of the pattern. A fluid tip is used to determine the pattern size and fluid flow rate. Then air is applied to assist in filling out the pattern. An aircap is used to complete the spray pattern and eliminate the pattern defects also known as tails.
Air-assisted airless solves many problems that arise with the use of high-viscosity and high-solids coatings, and issues associated with heating and using higher fluid pressures to aid in the atomization of more viscous materials. Many waterborne materials require the higher fluid pressure of air-assisted airless. The lower pressure and the air-assist create finer atomization for a finer finish than airless.
Electrostatic applicators charge material particles as they pass or contact an electrode to achieve higher transfer efficiency. They rely upon the attraction of opposite electrical charges.
- The material is electrostatically charged as it passes through an electrostatic field produced between the electrode on the front of the gun and a grounded object.
- The charged particles of the material are attracted to the grounded (neutral) object and form an even coating.
- The charged material will then wrap itself around the object, which will increase surface space that is coated.
Because of this wraparound effect, electrostatic applicators are especially suitable for coating tubular products.
This video compares the transfer efficiency of an electrostatic air spray gun to that of a conventional spray gun:
Types of Electrostatic Spraying
Air spray electrostatic spraying uses a low-pressure fluid stream that is mixed with compressed air at the aircap to atomize material in a controlled manner. It is used for the application of low to medium viscosity fluids for products requiring a high quality, Class A or decorative finish.
Air-assisted airless electrostatic spraying uses a high pressure fluid supply for atomization and compressed air at the cap for pattern control. Air-assisted airless electrostatic spraying solves many problems that arise with the use of high-viscosity and high-solids coatings, and other issues associated with heating and using higher fluid pressures to aid in the atomization of more viscous materials.
Rotary atomization electrostatic spraying is another form of air spray. It uses an electrostatic charged bell cup which is spinning at high rotation speed, so the paint is subjected to centrifugal force. The paint flows along the surface of the cup and when it reaches the edge, the high centrifugal forces break it apart into fine cloud of fluid particles. The paint droplets carry the electrostatic charge from the bell cup and are directed or shaped by the shaping air coming out of the air cap. The charged droplet size is finer and more consistent than other atomization methods resulting in high transfer efficiency and finish quality.
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