Paint Spray Guns: Transfer Efficiency

SPRAY GUNS AND TRANSFER EFFICIENCY

There is a general misconception in industry that HVLP or electrostatic spray guns automatically yield high transfer efficiencies. Because of the wording that has been used in many air pollution regulations, there is a general belief that HVLP and electrostatic spray guns yield transfer efficiencies in excess of 65%. This is incorrect! It is true that HVLP and electrostatic spray guns, when properly used, are more efficient than conventional air atomizing spray guns, but their improved efficiency has no relation whatever to the 65% value that is often quoted in regulations and vendor literature.

For example, if a properly operating conventional air atomizing spray gun used to apply coating to a set of small parts yields a transfer efficiency of say 5%, then an HVLP or electrostatic spray gun might produce a slightly higher efficiency, say 7 - 10%, but not much higher than that.

On the other hand, if all three spray guns are properly used to apply coating to a large surface, then all three devices can be expected to yield relatively high transfer efficiencies, perhaps even greater than the mystical 65%. The HVLP and electrostatic guns will probably yield slightly higher results than those for the conventional air atomizing spray done.

By far the most important parameters for maximizing transfer efficiency are in the hands of painting operators. An operator using a conventional air atomizing spray gun who has been properly trained and uses efficient painting techniques will be far more effective in improving transfer efficiency than an operator who uses an HVLP or electrostatic spray gun, but has not been trained in proper painting practices.

Q: How can a company ensure/verify that they are meeting a permit condition that calls for a specified minimum transfer efficiency (TE)?

A: This is a very difficult, if not impossible task to perform with any reliability or repeatability. It is possible to perform TE tests on a production line, but the test protocol must be carefully spelled out. Even after following the protocol day-to-day variations will occur and the TE value will change, because there are too many variables that need to be controlled.

Perhaps and easier method that will achieve the same goal, namely to verify the efficiency of an operation, is to determine the monthly or annual “coating rate”. In essence, the company would keep records of the number of parts it paints over a period of weeks or months, and also keeps records of the amount of paint and solvent used to coat those parts. The “coating rate” could be based on the amount of paint and solvent used to coat say 100 or 1,000 parts. Better still, you could express this as lbs of VOC used for 100 or 1,000 parts.

When issuing a permit you would first make an initial determination and then compare the results after several weeks or months.

Q: I realize that will all of the variables, nothing is etched in stone, but could you give a general idea of the difference in transfer efficiency between airless, air assist, and electrostatic processes. This scenario would assume the same painter, the same product and the same target.

A: As you pointed out, there are lots of variables that determine transfer efficiency. One of the most important is the size and complexity of the object being coated. I mention this because each of the spray guns (compared with one another) mentioned in your question performs differently depending on the configuration of the object. For instance, airless spray guns are generally not suitable for applying coatings to small complex shapes. Electrostatic guns are generally not useful for painting objects that contain lots of crevices, tight angles and corners, etc. HVLP guns are not used on a beverage or food can coating production line because the high volume of air from the gun would blow the light weight cans off the conveyor line. Despite what I have just said above, specially designed airless guns are used to apply the internal epoxy coating into beverage cans, such as Coke, Pepsi, etc., with a transfer efficiency in excess of 90%!

This long preamble to my answer is given to illustrate that transfer efficiency for any spray gun is determined on a case-by-case basis.

To answer your question I will use only one scenario. If we were to apply a marginally conductive coating to a fairly large flat metal panel that is grounded, we could assume that the order of efficiency in descending order would be electrostatic, HVLP, air-assisted airless, and finally airless.

All other parameters being equal, this relative order will change as the part configuration changes.