Electrostatic spray painting: less spray mist, higher efficiency

A research project to avoid spray losses is investigating which physical processes interact in electrostatically assisted spray painting. The aim is to optimize processes for efficient painting.

High voltage helps to avoid spray losses. A computer model now shows for the first time the physical processes involved in electrostatically assisted spray painting. It can be used to optimize paints, painting systems and processes in paint stores. "You can only improve efficiency if you understand what you're doing," explains Dr. Oliver Tiedje. The physicist has been working at the IPA for years on the optimization of paints and painting processes. "In the past, electrostatically assisted spray painting had to rely on trial and error."

Electric paint application was developed in the 1940s to reduce spray losses. The principle is simple: a rotating atomizer, the so-called bell, which is under voltage, generates negatively charged paint droplets and accelerates them to several hundred kilometers per hour. The flight of the droplets is guided by a high-voltage field between the bell and the component to be coated. In this way, the spray mist, which normally leads to losses of over 50 percent, can be reduced to 20 percent.

"In order to further optimize the process, we need to know when, how and where the droplets pick up electrical charges - but that's exactly what no one has researched so far," explains Tiedje. In the MoELa research project, his team at the IPA, together with engineers from Esslingen University of Applied Sciences, has now closed this gap. In the first step, the researchers experimentally investigated how the conductivity of a specially produced model paint can be improved by adding additives. These modified paints were then sprayed onto components by a painting robot in a technical center. A high-speed camera documented the process. The images were used to determine the speed and size of the droplets. The quantity of negative charges on the coated component was also measured and evaluated.

"The result surprised us," recalls Tiedje: "We had expected the number of registered charges to increase when more paint was sprayed. But that wasn't the case: surprisingly, the amount of charges remained the same." The research team only found an explanation through the subsequent simulation. The experts spent two and a half years working on the computer model, which for the first time takes into account all phenomena of the electro-hydrodynamic painting process. Among other things, the viscosity and conductivity of the paint, the shape and rotational speed of the bell and the level of voltage applied were taken into account. The simulation of the previously conducted experiments showed that the charges emitted by the bell jar are located at the interface between the paint and the air. If the amount of paint is increased, the paint film on the bell jar becomes thicker, but the surface remains the same and therefore the amount of charges does not change.

In future, the simulations can help paint manufacturers to determine the optimum amount of additives for electrostatic painting processes. The digital models also support plant manufacturers in testing new bell designs or processes. Paint stores can use the simulations to virtually optimize the process parameters for different components - time-consuming and material-intensive test coatings can thus be reduced to an absolute minimum. (OM-5/24)