Technical data

4.2.3 Airless Spray. The term airless comes from the
fact that no air pressure is used with this paint application
method. Instead, hydraulic pressure is used to deliver the
coating material, heated or unheated, to the gun head and
atomize it by ejecting it from special spray nozzles. These
nozzles increase the pressure by a factor of approximately
100. Atomization of the coating material and formation of the
spray pattern are created by the gun nozzle. The droplets
move toward the work surface by their momentum and are
appreciably slowed down by air resistance. There is less
bounce of the coating material on arrival at the work surface,
and, therefore, less overspray. The cooling effect of expand-
ing air associated with conventional spray is not present, so
the only heat loss in the cold airless method is through solvent
evaporation. In the hot airless method the material arrives at
the work surface warmer than with other methods of spray-
ing, usually at or above ambient air temperatures.
4.2.4 Air-Assisted Airless Spray. In this method the
coating material is atomized by hydraulic pressure the same
as airless spray but at a much lower pressure. Low pressure
air is added at the gun head and directed at the paint mist to
control and form the spray pattern. While the coating can be
atomized at lower hydraulic pressure through the spray
nozzle, proper spray pattern formation requires the assistance
of the low pressure air through jets at the nozzle. This allows
the operator control of the atomized coating pattern that
cannot be done with standard airless. It offers almost equiva-
lent advantages in spraying as the airless spray method, while
being safer and requiring lower maintenance on pumps.
These advantages are due to the lower hydraulic pressures
used. In addition, the appearance of coatings applied by this
method is better as the tendency to orange peel is lessened.
4.2.5 Electrostatic Spray.
Electrostatic spray painting of JP-8 fueled aircraft
constitutes a significant hazard when the on-
board fuel temperature exceeds 100° F.
This method is a variation of the spray methods previously
described which adds the feature of electrostatic charging
(60,000 volts at about 200 microamps) of the paint material
which is then attracted to the grounded workpiece. Charging
of paint material can occur either inside the gun or at a fine
metal probe at the gun nozzle exit (the most common
method). Typically, this requires specially designed paint
guns as most HVLP, airless, or air assisted airless guns cannot
be modified to add this feature. This method is more effective
with airless or air-assisted airless as the combination of
low-particle velocity of the airless spray and the electrostatic
attraction to the workpiece produces an excellent transfer
efficiency rate. Electrostatic spray painting equipment can be
powered by an external electrical source or a self-generating
electrical source contained within the spray gun. Overspray is
greatly reduced and hard-to-coat areas such as edges or
geometric shapes are more effectively painted. The workpiece
(aircraft, etc.) is not charged electrically, but is grounded as in
normal painting practices. This method has limited effectivity
in coating interior corners, crevices, and cavities due to the
Faraday effect, that causes charged paint particles to be
repelled from the deepest points, and on some aircraft
exterior surfaces, due to aluminum components being insu-
lated by anodize, or due to composite materials that cannot be
grounded. The safety precautions, operational parameters,
and equipment maintenance for this method in Paragraph
5.5.3 must be strictly followed.
4.3 SPRAY PAINTING EQUIPMENT, GENERAL.
4.3.1 HVLP Spraying Systems. All HVLP spray sys-
tems have certain basic components necessary for their
efficient operation. There must be an adequate source of
compressed air, a supply of the finishing material from a
reservoir or feed tank, a spray gun, and a device for
controlling the combination of air and finishing material.
Other refinements, such as an air-pressure transformer (regu-
lator), air filter, water drain, hose cleaner, etc., are incorpo-
rated in the system to provide more efficient and satisfactory
results. Figure 4-2, Figure 4-3, and Figure 4-4 are diagrams of
complete spray systems.
4.3.2 Spray Gun, General. Spray guns are mechanical
devices for atomizing or breaking-up coating materials into a
spray and applying it under control, to a surface to form a
continuous film. Figure 4-5 illustrates in sectional view of a
typical spray gun. It is a precision instrument and must be
treated as such. Its daily care and maintenance determine the
effectiveness of spray painting. It should not be used by
untrained personnel.
4.3.3 Classes of Spray Guns. HVLP spray guns are
classed in three general types: suction feed, gravity feed and
pressure feed. Each type is further subdivided by having
either external or internal mix air caps. For the most part, the
Air Force uses the external mix type.
4.3.3.1 The suction feed (or siphon) cup gun is usually
fitted with a fluid cup. Its nozzle assembly is designed to feed
paint into the air-stream by the vacuum created from the air
flowing past the fluid tip which protrudes into the air stream
beyond the air cap. The amount of spraying at one time is
limited to the contents of the cup. This gun is most commonly
used in painting smaller areas, usually within the confines of
a spray booth. (Figure 4-2, Detail B, illustrates suction feed
hookup.)
4.3.3.2 The pressure feed gun is designed to fluid feed to
the gun under pressure from an external tank through a hose.
The air cap and fluid tip are flush with each other, and no
siphoning effect is necessary. It is suitable for high volume
painting. (Figure 4-2, Detail A, illustrates pressure feed
hook-up.)
4.3.3.3 The gravity feed gun is designed with the cup
located on the top of the spray gun. This allows paint to
TO 1-1-8
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