Editor’s Note: Our VP of Engineering & Technology Mike Bonner recently spoke at the 2018 Waterborne Symposium held in New Orleans, LA. His presentation went in-depth on the differences in atomization between spray guns and bell atomizers, shedding light on how paint finishers can further improve their applications. For those not in attendance, he’s also prepared his insights here for our blog as a series of posts. To view the rest of the posts in the series, visit the Guns vs. Bells blog series page. Also, watch for the video of this presentation to be loaded soon on our YouTube page.
In our last post, we discussed the differences in atomization between spray guns and bells. How do these differences affect our day-to-day coating operation?
This question was put to the test at Carlisle Finishing Technologies’ lab in Toledo, OH, where we used their Malvern Particle Size Analyzer to measure the distribution of particle sizes in the atomized cloud for a typical gun and bell as shown in Figure 1, “Cloud Measurement Setups.”
To maintain consistency, both gun and bell tests were performed using an HCNTX 2K clearcoat. Ratio, fluid flow, and atomizing and shaping air were all held constant with a Ransberg RCS system. Ambient conditions were simulated with a Saint Clair Systems paint temperature control system implemented with a re-corable coax hose as the heat exchanger.
This configuration allowed accurate control of temperature all the way to the point of dispense in controlled, repeatable steps. This system is shown in Figure 2, “Test Control System.”
Though sequence was not important due to the accuracy of control with this test system, the first tests were performed with the gun setup.
With all other parameters held constant by the RCS system, temperature was varied in controlled increments from 65°F - 115°F (18°C - 46°C) for the expressed purpose of varying clearcoat viscosity. At each step, the resulting Dv(50) particle size in the atomized cloud was measured using the Malvern.
The results are summarized and shown in Figure 3, “Gun Cloud Particle Size.”
Here we can see that, with all other variables held constant, the average particle size for the gun applicator varied from 52.3m at 65°F (18°C) to just 38.6m at 115°F (46°C).
Based on the discussion of gun atomization in our last post, it is reasonable to conclude that the change in atomization is directly related to the change in clearcoat viscosity resulting from the change in temperature.
In addition to variations in particle size, the change in viscosity will affect particle recombination and flow out on the surface of the part. This will have a direct impact on the quality of the finish with regard to film build, gloss, orange peel, etc.
Next, the gun was replaced with a bell in the setup shown in Figure 1, “Cloud Measurement Setups,” and Figure 2, “Test Control System.”
The cup speed was set at 32,000 RPM, and, as with the gun, all other parameters were held constant by the RCS system. Temperature was again varied in controlled increments from 65°F - 115°F (18°C - 46°C) for the expressed purpose of varying clearcoat viscosity and, at each step, the resulting Dv(50) particle size in the atomized cloud was measured using the Malvern.
The results are summarized and shown in Figure 4, “Bell Cloud Particle Size.”
Here we can see that, with all other variables held constant, the average particle size for the bell applicator is held constant at ~27m independent of the changes in temperature.
Based on the discussion of atomization from our last post, it is also reasonable to conclude that bell atomization is not affected by the change in clearcoat viscosity resulting from the change in temperature.
This theory was confirmed by increasing the cup speed from 32,000 RPM to 60,000 RPM at the median temperature of 85°F. This shifted the average particle size from ~27m to ~16m.
Because these are both plotted on a 20m particle size scale and a 65°F to 115°F temperature scale, they can be combined on the same graph, as shown in Figure 5, “Cloud Particle Size Comparison.”
This allows us to readily compare the atomization performance as a function of temperature (viscosity) for the two applicator types.
Though there is no change in particle size as a function of temperature with the bell applicator, the change in viscosity will still affect particle recombination and flow out on the surface of the part – just as with the gun applicator – and this will still have a direct impact on the quality of the finish with regard to film build, gloss, orange peel, etc.
In our next post, we’ll discuss how ambient conditions affect particle size and whether it’s important to control booth temperature.
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