Happiness Is a Warm Pneumatic Spray Gun
Every spray gun setup has three essential components: a gun
, which sprays the finish; a cup or pot, which holds the finish; and a power system, which moves the finish through the gun and atomizes it at the air cap. Although there are many types of guns, the operation is basically the same. They contain inlets for air and fluid, a fluid needle, tip, and an air cap.
The airline from the compressor enters the gun handle. Fluid enters the gun near the front, either a cup mounted directly to the gun or from a remote pressure pot. A screw on the back of the gun adjusts the amount of fluid passing through the tip. Another screw adjusts the air passing through the air cap, controlling the fan pattern.
A spray gun must have a container for holding the spray material. These containers range in size from an 8-oz. cup to a 10-gallon pressure pot, and can be located below, above, or away from the gun. The location of the container will determine how the material is fed into the gun.
Pneumatic spray guns utilize an air compressor to move and atomize the material. Depending on the spray gun, this compressor should deliver between 3 and 12 SCFM at 40 PSI (most 5 to 6 HP units are adequate). Most spray guns operate between 30 and 80 PSI. It's important to be aware of the maximum air pressure, and failure to regulate the air pressure under the maximum rating can damage the gun.
What are the different types of guns?
General purpose spray guns are adaptable to a wide range of applications. Most can be converted from siphon-feed to pressure-feed to allow a wide range of material from stains to latex to be sprayed. They can also be converted from non-bleeder to bleeder style. Non-bleed guns-the best and most common practice-shut off the airflow when the trigger is released. Bleeder guns allow air to continually pass through the gun regardless of whether the trigger is pulled or not. This set-up is typically used on tankless air compressors that run continuously.
If painting a larger job, most of these guns can also be used with a paint tank or remote canister. In order to work properly, the gun must be set in siphon-feed mode to prevent pressure from entering the fluid hose from the wrong direction.
Because these guns are designed to work in a variety of set-ups, they spray well in each function, but not excellent. If a high-quality furniture or automotive finish is desired, it's best to stick with a gun specifically designed for siphon, gravity or pressure-feed.
In a pressure-feed system, air is used to pressurize the material container, forcing fluid to the tip of the gun, where it's atomized by air passing through the air cap. Pressure-feed guns can be set up with an attached 1 qt. canister or by using a remote canister. Remote canisters can range from 2 quarts to over 10 gallons. With a remote canister, or pressure pot, the container is pressurized and material flows through a hose to the fluid inlet on the gun. A separate hose provides the necessary air for atomization.
Pressure-feed guns offer several advantages over other designs:
Material pressure can be regulated to move fluid to the gun. This allows materials to be sprayed that would otherwise be too thick or heavy to be siphoned from a container.
Air pressure at the tip can be adjusted independently of fluid pressure. As a result, material can be applied at a more rapid rate, saving time.
With a remote canister, guns are much lighter and easier to maneuver in tight spaces.
The gun can be sprayed in any position including upside down.
A much larger material container can be used, reducing the need to interrupt production.
The main disadvantage of a pressure-feed system is the increase in clean-up time of the pot and fluid hose. A secondary disadvantage is the high equipment cost.
On siphon-feed guns the material container is located below the gun. Compressed air enters the inlet and moves through the gun to create a vacuum at the air cap. The lid has a vent hole to allow air to enter the cup. The difference in pressure between the air leaving the gun at the tip and the air entering the vent hole creates a siphon that draws material up the fluid tube. The material is then drawn through the fluid tip, where it mixes with air coming from the cap and is atomized.
Because some of the air entering the gun is needed to draw material to the fluid tip, and the siphon effect is not very powerful, these guns are not as efficient with heavier materials as other designs. Siphon-feed guns should be used with low- to medium-viscosity materials. This includes dyes, stains, lacquers, acrylics and enamels. Siphon-feed guns generally won't work with heavy material like latex. Another disadvantage is that the gun must be held fairly upright and is less than ideal in tight quarters due to the cup.
Gravity-Feed Spray Guns operate similar to a siphon-feed gun except the material container is located above the gun. This allows gravity to assist the fluid when entering the gun, creating a positive fluid pressure. With a positive fluid pressure, air entering the gun can be better utilized for atomization rather than for siphoning paint from a container. This is particularly important when using reduced gun pressures such as those in HVLP equipment. Furthermore, the positive fluid pressure allows slightly heavier materials to be sprayed through gravity-feed guns.
Besides better performance, gravity-feed offers huge material cost savings over siphon-feed and pressure-feed guns. Gravity-feed guns allow almost all of the paint material to be used. Siphon and pressure-feed designs will always leave several ounces of paint at the bottom of their containers.
Finally, when paint runs out in gravity-feed guns, there is no "sputtering" which often leads to refinish issues. For these reasons, gravity-feed guns have become the products of choice in the automotive refinishing industry.
Happiness Is a Warm Pneumatic Spray Gun
By: Sean Murphy
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