Manufacturing: Surface Finishing

Industrial Spray Painting Application Processes

Air Atomized Systems

Spray Painting & Curing Booth

In general, traditional systems combine compressed air and paint, producing small atomized particles. This approach can be applied at a rapid rate without sacrificing finish quality and it can cover complex shapes with recessed areas. Bounce back has been a problem in the past that has lowered the transfer efficiency. However, newer gun designs allow for greater control of the direction of emerging air jets and heating of the paint allows the viscosity to be reduced and thus the atomization can take place at lower air pressures, thus less bounce back (see spray application processes).

High Volume Low Pressure Guns

HVLP is similar to the traditional method, except that the air and fluid nozzle are designed to reduce the spray velocity. Higher air volumes are used at lower pressures (8-10 psi). The disadvantage of this approach is that it sometimes has difficulty in reaching high quality atomization at higher rates of application (above 200 cc/min.) which effects the quality of the finish. It also can have clogging problems with higher solids paints. Non-turbine HVLP guns do not offer metallic control of conventional style guns, making the spraying of metallic topcoats difficult to master.

Airless Spray Guns

Paint is forced through a very small orifice using high hydraulic pressure. The internal passage of the atomizing nozzle, however, is shaped so that paint passing through it is mechanically forced into a highly unstable form--usually a fan shaped film. As with air guns, heat may be used to lower paint viscosity. It also produces a soft spray. One of the biggest problems with this system is that the nozzles tend to wear out faster, creating tails in the pattern; and the very high pressure (1000 psi) in the hydraulic hoses restricts the ease of movement of the hand gun.

Air-assisted Airless Guns

Compressed air is introduced through orifices in the airless spray tip holder where the hydraulically formed paint film is atomized. This allows hydraulic pressure to be reduced which helps lessen nozzle wear and the formation of tails in the spray pattern.

Electrostatic Spray Guns

Liquid electrostatic spray uses one of two methods to charge the paint: bombardment charging or contact charging.

Bombardment charging happens to atomized paint from a spray gun as it enters an ionizing zone. The particles are charged by air molecules in the intense electrical field created by a thin grid wire network. The charged atomized paint particles are then attracted to the parts which are grounded by a conveyor. The main advantage of this approach is the relatively high transfer efficiency and ease of color change. A comparative disadvantage is that this system requires a much higher voltage source than direct charging. Contamination of the charging system can occur if the parts to be paointed are not properly grounded.

Contact charging uses rotary disks (sometimes called bells) to form a thin film of paint by centrifugal action. The paint film is charged directly by high voltage contact on the rotator assembly.

Both bombardment and contact methods have high transfer efficiencies. One disadvantage of the liquid electrostatic method is the Faraday Cage Effect which does not allow uniform coverage on narrow recesses and sharp interior corners.