Air Atomizing Spray Guns
The sectioned view of the spray gun above is a good example of how an air atomizing spray gun works. The blue color represents the path of the air. The red color is the finish material. While different manufacturers may have the location or operation of their controls a little different from one another, the basic concept of atomization will be the same for all manufacturers, as well as, for gravity feed, siphon feed, or pressure feed guns. Air Assisted Airless guns operate a little differently and we will discuss those separately.
As stated in an earlier article, a spray gun works by taking liquid from a cup or tank, allowing a stream to flow between controlled jets of compressed air. As the material flows between the jets they break apart the stream of liquid into little droplets. Other jets create a rush of air that carries the droplets away and onto the target that is being finished. The factors that determine how well a spray gun does this job are:
- How big in diameter is the stream of liquid
- How fast is it flowing past the jets of air
- How much air is breaking the stream into droplets
- How much air is carrying these droplets away
All of these factors are directly influenced by the viscosity of the material you are spraying.
Regardless of any of the other manufacturer features, the gun components that control these factors are the fluid nozzle, fluid needle, and air cap.
The process starts with your fluid tip or nozzle. The tip is chosen to match the viscosity of the material you are spraying. High viscosity material requires a larger diameter fluid nozzle. Thinner material uses a smaller diameter fluid nozzle. The concept is to allow a certain flow rate, ounces per minute, of material to pass thru the fluid nozzle and into the air jets. The fluid needle seats itself in the opening of the nozzle. The needle is attached to the trigger and it moves in and out of the nozzle which starts and stops the flow of liquid. The fluid control knob limits the travel of the needle, which in turn, limits the flow of material out of the nozzle. You can adjust the material flow from zero up to the total Oz/min capacity of the nozzle. Fluid nozzles and needle valves should be matched sets to ensure a perfect fit which will prevent leakage at the tip. This is especially true for pressure feed guns as the material is presented to the nozzle at elevated pressures.
So now we have material flowing out of our nozzle at a prescribed rate that is optimum for the viscosity of the liquid we want to atomize. The air cap is the next link in the atomizing chain. Air cap design will vary by gun type, as in conventional versus HVLP.
HVLP air caps have larger air holes than conventional guns. Air cap design will also vary by how the material is presented to them, as in pressure feed versus siphon-feed. Pressure feed air caps are usually shorter and have larger air holes in the horns which enable it to atomize the larger flow rate per minute of the pressure feed gun. So when you are setting your gun up, make sure that you have the correct air cap for the type of gun that it is on. The second thing that you want to check is that you have the proper air cap for the fluid nozzle that you are using. Remember that your material is coming out at a certain flow rate and you will need the correct amount of air to atomize it. Too much air and you have excessive overspray, too little air and you end up sanding a lot of orange peel. An air cap will usually work for a range of fluid nozzle sizes. Check your manufacturer’s part list or website for air cap and nozzle combinations and details.
You will notice that the air cap has holes on the face and holes on the ears (or horn of the cap). The center hole on the face is where the fluid nozzle comes out. The holes on either side of this center hole blow air into the material stream as it leaves the gun. They atomize the stream as well as propel it toward the target. By adjusting the air pressure coming into the gun you control the flow of air through the air cap. This in turn determines the atomization of the stream of material. Again, too much air and you have excessive overspray; too little air and atomization is not complete. The holes in the horns not only aid in atomization, but they help form the spray pattern. The fan control knob on the gun diverts air into the horns. With little air coming to the horns, the spray pattern is a round spot. As the air to the horn increases it changes the spot to a fan shape. As you change from a spot to a fan, the pattern shape does not change the flow rate of the material. This is important to note especially when you reduce the size of the fan to do a little spot touch-up. Remember to turn down the material flow or runs will surely follow.
As you are starting to see, there is a balancing act going on. You balance the material flow with the atomizing air pressure and fan size. You set the major parameters with the selection of the fluid nozzle/needle and air cap, followed by “fine-tuning” the system by adjusting your line of air pressure, then the fan and fluid controls on the gun.
In a siphon feed gun, the velocity of the air running through the air cap also establishes the suction that siphons the material from the cup. Siphon feed HVLP guns are a little touchy because the air at the tip of an HVLP gun can not exceed 10 PSI. This reduced pressure creates a slower exit velocity and less of a vacuum in which to suck up the material out of the cup. You may find that you have to over-reduce high solids materials in order for them to spray properly.
When you adjust the controls on the gun, you may ask what are you adjusting them for?
The answer is to compensate for:
- The viscosity of the material you are spraying
- The pressure at which the material is presented to the gun
- The size of the part you are spraying
- The distance you hold the gun from the part
- The speed at which you move the gun across the part as you spray
This whole process can be thought of as a large mathematical equation. If you change any one of the parameters, you can still get the same answer by changing one or more of the remaining parameters. If you have too much material coming out of the gun, move the gun faster across the work, hold the gun farther away, or turn the fluid control down. You can even do a little of each. When you have this many variables, you have a lot of control.
You can be comforted by the fact that the viscosity of the material and the pressure that it is delivered to the gun should hopefully not vary during a given setup. It boils down to the fact that you are adjusting for things that will change gun speed and distance, as well as the size of the parts you are spraying. While we often unconsciously compensate for the changes that mood, fatigue, and coffee intake have on our physical control of the speed and distance of the gun – it is a fact that the more consistent you are with maintaining your target distance with what the gun was set for will result in a more uniform final film thickness and less material usage.