Airless Spray

MIL-HDBK-1110

7.5.4.2

Airless Spray

a) Equipment. Airless spray relies on hydraulic

pressure alone. Atomization of paint is accomplished by forcing

the material through a specially shaped orifice at pressures

between 1000 and 3000 psi. Because of the high pressures,

extreme care must be taken to prevent worker injury. The spray

manufacturer's instructions must be followed carefully. The

basic parts of airless spray equipment are a high-pressure paint

pump, a fluid hose, and an airless spray gun. The high-pressure

pump must deliver sufficient pressure and material flow to

produce a continuous spray of paint. The fluid hose must be able

to withstand the very high pressures required to deliver the

paint to the gun and atomize it. A filter screens out particles

that might clog the tip. Since atomization is controlled by the

size and shape of the orifice of the tip, a different tip is used

to obtain different patterns and atomization rates. The tip

angle controls the fan width. Tips having the same orifice size

but different angles deliver the same amount of paint, but the

area covered with one pass is different. Viscous materials

require a larger tip than less viscous materials. Coating

manufacturers recommend tip sizes on their data sheets. The

larger the orifice, the greater the production rate. But, if too

large an orifice is used for a thin coating, the rate of delivery

may be such that the operator cannot keep up with the flow. This

will result in sagging and running of wet paint. Airless spray

is available with heaters to reduce paint viscosity, permitting

spraying of coatings having higher ambient viscosities at a

faster production rate.

b) Setting-Up, Adjusting Equipment, and Shutting-Down

Procedures. The manufacturer's instructions should be followed

for setting up the spray equipment. To minimize tip clogging

problems, airless spray equipment must be scrupulously clean

before setting-up for a spray application and the coating must be

free of lumps. The manufacturer's recommendations should be

followed rigorously for the setting-up, using, and shutting-down

procedures. Since the pressures used are high, two safety

features are required for guns: a tip guard and a trigger lock.

The tip guard prevents the operator from placing a finger close

to the tip and injecting paint into the skin. The trigger lock

prevents the trigger from accidentally being depressed. Other

safety measures include never pointing the gun at any part of the

body; not making adjustments without first shutting off the pump

and releasing the pressure; making sure the fluid hose is in good

condition, free of kinks, and bent into a tight radius; and using

only high-pressure hose fittings. Also, never clean systems

containing aluminum with chlorinated solvents. Explosions may

occur. Causes of and remedies for faulty patterns are described

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