A Primer on Transfer Efficiency and Tip Sizes

Q. We manufacture many types of products that require finishing in the paint department. We are fairly new to the finishing process, and have heard about many different ways to apply the coating materials. Would it be possible to tell us the best type of spray gun and tip sizes to use?

A. This is a very good and commonly asked question. I will give you a general overview of the basics for determining the best spray gun technologies. You've probably heard salesmen telling you about all sorts of technologies, like HVLP (High Volume Low Pressure), airless, and air-assist airless—after your head quits spinning, how do you know which is best for your operation? For one thing, manufacturers who use spray finishing need to be cognizant of state and local EPA regulations, since many restrict certain types of older technologies such as cup guns. Without getting into the complex technical justifications, I will highlight what you need to know as far as application goes.

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Transfer efficiency—what is it, and why is it important? Regardless of the spray gun technology you select, each has its own transfer efficiency rating. In basic terms, transfer efficiency simply has to do with the amount of material that leaves the gun and how much of it lands on the surface you are coating (when applied properly). With other variables controlled, HVLP spray guns should provide 50- to 60-percent transfer efficiency, airless guns 40- to 50-percent and air-assist airless guns 60- to 80-percent. What does it all mean? If you were using the older, cup-type gun with a transfer rating of 30 percent, that would mean 70 percent of your material would literally be over-atomizing and heading straight up the exhaust stack. So, if you were paying $25 per gallon for finish material, about $17 to $18 dollars of it would be going, going, gone! The higher the transfer efficiency rating, the more money you keep in your wallet.

Material viscosity and tip size should also be considered when choosing spray gun technologies, since they will affect the quality of your finish. If you are using an HVLP gun, you will want to use the following guidelines:

• For low-viscosity material (e.g., dye stains), use a fluid nozzle with needle and air cap in the range of 0.6 to 1.2 microinches.
• For medium-viscosity material (e.g., sealers, light tint coats), use 1.4 to 1.8 microinches.
• For high-viscosity material (e.g., topcoats, primers, paints), use 2.0 microinches and above.

The same reasoning applies to air-assist airless spray guns with the following guidelines:

• For low-viscosity material, use a tip with needle and air cap in the range of 0.7 to 0.9 microinches.
• For medium-viscosity material, use 0.12 microinches.
• For high-viscosity material, use 0.15 to 0.27 microinches.

Regardless of the technology you ultimately select, the brief points I have highlighted should be part of your decision-making process.