Paint Spray Guns

Non-Electrostatic

GUN-TARGET DISTANCE (GTD)

Get Those Spray Guns Closer to the Part

Regular readers of this column will vouch that I have written many column inches on the advantages of bringing the spray gun closer to the part.  During the past month I visited a large paint shop in which two automatic, stationary HVLP spray guns were used to apply coatings to small parts that required an automotive quality finish. The reason for my visit was that the company was experiencing an unacceptably high reject rate due to dust, solvent popping and other surface defects.

After having gone through similar scenarios in the past few years, it immediately became evident that the guns were too far from the parts.  How far is “too far”?  In this case both guns were 8 -10 inches away; the one pointing down onto the part, the other pointing up and aimed at the lower portion.

I asked the operator if we could perform some experiments by bringing the guns 4 - 6 inches from the surfaces.  He agreed, and we spent several minutes reducing the fluid and air atomizing pressures to prevent too much paint from being applied. At the same time we narrowed the fan, which even before any changes had been made was far too wide for the part. As a result of the new set-up it, was also necessary to change the position of the guns relative to each other, by bringing them closer together.  Admittedly, it took several minutes, and the testing of several parts to confirm that the coating film thickness was consistent across the entire 3-dimensional face of the part.

How did these changes improve the finish, and what other benefits were enjoyed?

• With the low atomizing pressures all of the overspray was captured within the paint booth.  Before the adjustments, much of the overspray had entered the factory area maliciously waiting to contaminate the freshly painted parts that were on their way to the oven.

• The lower air pressure reduced the tendency for the solvents to flash-off so violently, and allowed for a slightly wetter film on the parts.  This reduced the solvent popping that had been experienced before.

• Transfer efficiency increased immediately, although we did not have the time to measure the increase.  We expect that spray booth filter replacement will be less frequent and overall production costs will nosedive. (At least I hope so!)  

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