MIL-HDBK-1110
7.5.4.3
Air-Assisted Airless Spray. Air-assisted airless spray
uses air to help atomize paint as compared with only fluid
pressure in the airless system. Thus, a lower hydraulic pressure
(typically 500 to 1000 psi) can be used. Air pressure is
typically 10 to 15 psi. Air-assisted airless spray provides a
finer spray than airless spray, and the lower hydraulic pressure
provides improved operator control. Consequently, finishes tend
to be smoother with fewer runs and sags. Transfer efficiency is
about the same as airless spray, but air-assisted airless spray
is more expensive to maintain.
7.5.4.4
High-Volume, Low-Pressure Spray. HVLP spray is an air
spray technique that uses low pressure and large volumes of air
to atomize the paint. It has much better transfer efficiency
that conventional air spray and some systems have been found to
meet the 65 percent transfer efficiency requirement of
California's South Coast Air Quality District. Because of the
lower air pressures, there is also less bounce back than with
conventional systems. Turbine air-supply systems, along with
large (1-inch diameter) hoses are commonly supplied with the
systems. Since the air supply is not turned off when the trigger
is released, air flows continuously through a bleeder valve in
the gun. An HVLP gun can be equipped with different fluid and
air tips depending upon several variables: the desired spray
pattern (wide fan to narrow jet), viscosity of the finish, and
output of the turbine. Although some special training of
painters may be required because of differences between
conventional air systems and HVLP, such as less recoil, higher
delivery volumes and continuous flow of air, an experienced
operator has good control.
Conversion kits for air compressor systems are
available which allow the use of them with HVLP systems. Spray
techniques may be slightly different depending upon the source of
pressurized air.
7.5.4.5
Multi-Component Spray. Multi-component (or plural-
component) spray equipment combines components of multi-component
paints in the nozzle. The equipment is more complicated than
other spray equipment, and its use is usually confined to large
or specialized coating applications. The components are metered
to the gun in the proper relative volumes, mixed and then
atomized by one of the previously described techniques. Thus,
pot life is not a factor in application of multi-component
coatings. However, it is essential that the metering be done in
accordance with the coating manufacturer's instructions. Volume
mixing ratios are usually from 1:1 to 1:4. Heating of the
components before mixing is also provided with some equipment.
By heating the components, both the viscosity during application
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