When we shop for spray equipment (including a spray gun air cap set), we’re flooded with all sorts of technical information that, at times, makes the waters a bit muddy in trying to make a final purchase.
Do we want a turbine unit? Do we want a compressed air gun? Is my compressor the right size for the gun I am looking at? What type of coatings will I be spraying?
Yes, it can get complicated – especially when we need to decide on the right fluid assembly parts, i.e., the spray gun air cap and the needle/nozzle size to ensure we obtain the right quality of final finish.
Many of my seasoned clients are still heavily invested in guns that are set up to spray low solids, high solvent content finishes such as nitrocellulose lacquer and conversion varnishes.
The guns they purchased 30-40 years ago were designed and set up to spray these “hot solvent” finishes with small needle/nozzle sets. This is due to the low viscosity of these types of coatings.
Also, certain types of high-pressure spray guns have air cap designs that can handle the low solids being atomized – configurations sometimes referred to as “lacquer guns.”
As we know, lacquer has changed over the past 30 years and therefore so has the type and engineering quality of the spray equipment being used to deliver the newer waterborne coatings that are now prevalent in the market.
Most of the good spray gun manufactures offer a chart that helps you determine how your spray gun air cap assembly should be configured based on the type and viscosity of the coating be applied.
Very often these charts are good starting-points, but they certainly leave room for adjusting and fine-tuning your finishing performance.
Many of these charts corollate the viscosity of the coating based on a unit of measurement called Zahns or Ford viscosities.
These viscosities are measured in specially designed cups and are timed is seconds. To complicate these charts a bit more, these types of viscosity cups come in different sizes – and they all vary based on the quality of the cup being used.
As you continue to read the chart, you’ll see specific air cap and needle/nozzle sets suggested for each viscosity measurement. This is where I find most of my “specifications oriented” customers fall down the precision rabbit hole. Many take these charts to be the final word, and very often the charts are misleading.
Do these charts get you into the right neighborhood? Yes, but they should not be taken as gospel – like I said above, these things get complicated and you need to be ready to make adjustments on your own accord or through the recommendations of the coating supplier.
So, let’s circle back to the air cap and nozzle/needle set size requirements…
We know that caps, tips, and nozzles range in size from less than 1 millimeter (1.0mm) to 2 millimeters (2.0mm) is diameter.
The purpose of the starting-point viscosity chart is to help you select the right fluid nozzle/needle set. These sets are engineered with great precision to allow the coatings to flow through the nozzle based on viscosity and specific inbound fluid pressure.
The air cap, which is what you see on the outside of the gun head is matched to the size of the fluid/needle set that is set inside the gun. The fluid nozzle and needle help to get the fluid to the air cap and the air cap helps to break-up and atomize the coating as it exits the gun.
Nine times out of ten, the air cap is well matched with its mated nozzle and fluid needle, however, there are occasions when you may want to try a larger or smaller air cap to see if it helps in the final atomization process.
Swapping out an air cap with one that’s one or two sets larger can really help you dial-in your final finish – especially when using a turbine unit that has limited CFM adjustments.
We’ll drill deeper into the nuances of setting up your spray gun in future blog posts.
For now, tell me how you set up YOUR gun and how you dial-in your atomization quality.
Please share your thoughts or read what others are saying below in the comments section.