Differences Between Liquid Paint Guns and Bells
Mike Bonner of Saint Clair Systems put both gun and bell to the test at Carlisle Finishing Technologies’ lab in Toledo.
Q: What are the differences between guns and bells when it comes to applying liquid paint?
A. This question was put to the test in the Carlisle Finishing Technologies lab in Toledo, Ohio, 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.
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. Changing ambient temperature conditions were simulated using a Saint Clair Systems paint temperature control system implemented with a recorable 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.
Gun Testing: With parameters such as paint flow and shaping air held constant by the RCS system, temperature was varied in controlled increments from 65°F to 115°F (18°C-46°C) for the expressed purpose of varying clearcoat viscosity. At each step, the resulting Dv50 average particle size in the atomized cloud was measured using the Malvern analyzer.
With all other variables held constant, the average particle size for the gun applicator varied from 52.3 at 65°F (18°C) to just 38.6 at 115°F (46°C). 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 part surface, which will have a direct impact on the quality of the finish with regard to film build, gloss, orange peel and more.
Bell Testing: 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 to 115°F (18°C-46°C) for the expressed purpose of varying clearcoat viscosity and, at each step, the resulting Dv50 particle size in the atomized cloud was measured using the Malvern analyzer.
With all other variables held constant, the average particle size for the bell applicator is held constant at ~27, independent of the changes in temperature. 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, which 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 part surface—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 and more.
Q. What impact do ambient conditions have on particle temperature?
A. It is widely held that it is important to carefully control booth temperature because it directly affects the temperature of the paint as it is being applied. On first blush, this seems like a logical assumption. After all, the atomized droplets are extremely small and there are a huge number of them, which presents a large surface area to the ambient air when sprayed.
One of the things that has always bothered us about the concept of controlling paint temperature by controlling booth temperature is the degree of seasonal variation that coaters experience. If the ambient temperature is being held constant year-round, why is it still necessary to have a summer blend and a winter blend? Because seasonal variations are (by their very nature) temperature related, it seems there must be something else at work here.
It is fairly straightforward to calculate the change in temperature of the droplets. And, while it is virtually impossible to measure the temperature of individual droplets in the cloud, modern infrared technology allows us to measure the cloud temperature, in general.
According to Carlisle Fluid Technologies, bells create particles with speeds ranging from 150 to 300 mm/s, whereas guns create particles with speeds ranging from 300 to 600 mm/s (which is double that of the bells).
Assuming a common 10 inches (250mm) between the atomizer and the part, means that the average time the particles are in the air ranges from 0.42s to 1.69s. In spite of the large surface area presented to the ambient air, this is not a long time to effect a temperature change.
The impact is especially easy to understand when we consider the insulative properties of air, which has a U-value of just 0.2 BTU/ft² hr °F, and the fact that paint particles are mostly plastic. Again, this is not a good thermal conductor.
Consider a situation where the booth temperature is 77°F (25°C) and the paint temperature is 90°F (32°C) coming into the booth from a circulation system that is run in the truss level from the mix room on a summer day—a fairly common scenario for many painters. With the high particle velocities created by the gun and resulting shorter time in the air, the paint loses between 0.25°F and 0.75°F and reaches the part still above 89°F. Even with the relatively longer time in the air caused by the lower velocities of the bell, the paint only changes by 1.1°F-2.3°F. Again, in the worst case, the paint is still reaching the part at nearly 88°F. Even the particles in the cloud that have travelled past the part are still within 3.0°F of the temperature of the paint exiting the bell. This lends visual evidence to support the thermal model and proves, once again, that you just can’t argue with physics.
So, if you are assuming that your paint is being applied at 77°F and it is actually at (or above) 88°F, you may find it very difficult to make the right decisions to keep your finish quality in specification. This is why modern progressive coaters consider controlling paint temperature at the point of application to be more important for finish quality than controlling booth ambient temperature.
Mike Bonner is vice president of engineering and technology at Saint Clair Systems. Visit viscosity.com
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