Spray Application 101
When it's time to upgrade from older cup guns, what spray gun technologies should a shop be considering?
Q. We apply several types of coatings to many types of metal substrates, and it has become apparent to us that we are not being efficient in the finish department. We are not getting the look we are wanting with our older cup guns. What spray gun technologies should we be looking at to make the proper decision on newer equipment? –L.L.
A. This is probably one of the most asked questions I receive on an ongoing basis. Whether it is manual or automated spraying, the application process is much the same. As you evaluate equipment you will quickly learn that there are many options to sift through before a final decision can be made. So, you are asking yourself, “What is the best technology for me?” The short answer is whatever technology best fits all your needs, as we will discuss. You didn’t mention if you are doing manual or automated spraying, but we can assume that manual, hand spraying will typically be used for your area of industry. That said, let’s look at a list of typical criteria that should prove helpful in your decision-making process.
For coating materials, proper spray gun technology is generally dictated by paint body or viscosity. So it is important to look at some basic objectives as to what the proper technology needs to be. When you mentioned that you felt you were not getting the results you would like, where you referring to the overall aesthetics, material usage, etc.? Without really knowing for sure what you were describing in this statement, let’s look at some basics.
As a general rule, the lower the viscosity, the less energy is needed to effectively atomize your coating material, and the opposite is true of higher-viscosity materials.
HVLP (high volume low pressure) spray gun technology is typically recommended for the application of lower-viscosity material. HVLP gun technology uses large volumes of air at low pressure to atomize the coating material. The velocity of the coating material particles (in relation to viscosity being delivered to the part surface) is slower when compared to higher-pressure atomization technologies with higher viscosities. Because of this, HVLP technology applies the coating material more effectively into complex recesses, corners, etc. whereas higher particle velocities tend to blow the coating through such areas without proper application of material, which results in uneven thicknesses and rough paint surfaces. HVLP guns also require fairly large volumes of compressed air to operate properly, so adequate air delivery is also important when reviewing this as an option. Tip sizes for HVLP guns should range from 1.6 to 3.0 or higher depending on the material viscosity. The higher the viscosity, the larger the tip size needs to be, and vice versa. As a general rule, 1.8 to 2.4 is a good tip size range for many coatings for metal, using an air pressure of 35 to 45 psi.
If you are applying higher-viscosity materials, air-assisted airless (AAA) application technology should be considered as an option for sure. AAA technology uses hydraulic fluid pressure (rather than compressed air) with low pressures of 400 to 900 psi. If you have a 20-to-1 pump, you are going to get 20 psi of hydraulic pressure for every pound of air to the pump. So, a small amount of compressed air in relation to the proper tip size would be used to deliver a very soft spray pattern. AAA technology is very efficient for breaking up higher-viscosity coatings then its HVLP or siphon cup counterparts. This is why AAA is normally the first choice when a spray operator wants minimal overspray, high finish quality and excellent transfer efficiencies.
As for tip sizes, low-viscosity materials would normally require a tip size of around 9 to 11 mils, whereas the higher-viscosity materials would require 13 to 30 mils. The higher the solids, the larger the tip size should be.
The hydraulic pressure for lower-viscosity material (for example, on a 20-to-1 pump) should be around 400 to 500 psi in relation to the tip size. Higher-viscosity material would require 500 to 600 psi with the proper tip size. The atomization air trim pressure should be in a range of 10 to 20 psi in relation to the viscosity of the material with a fan width of around 8" to 12".
It is also important to highlight the fact that the spray operator, regardless of the spray gun technology you settle on, will need to pay close attention to the speed at which the gun moves, as well as the proper gun-to-part distance. Spray technique plays a major role in successful material delivery.
In conclusion, when you are making a decision on which spray gun technologies to use, it is also important to understand that spray operator training, following the standards or guidelines of HESHAP (National Emissions Standards for Hazardous Air pollutants) will also make a huge impact on a successful spray operation. All too often, individuals buy the right spray equipment without really considering the operator’s ability and knowledge. When proper training is paired with proper equipment, you will see major impacts on your bottom-line profitability.
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