The painting of automobile consists of the plural layers due to be required diverse function such as rust prevention, tolerance for climate and the appearance quality. Especially primer, base and clear coat called topcoat is required the high quality such as gross and the high designs such as color design. Due to achieve both high design and productivity, the spray painting with atomized paint is used by automobile painting. This atomization is mainly used by air, but this air decreases transfer efficiency due to scatter the paint by bounce of this in the vicinity of object such as automobile. Further, the paint scattered pollutes the equipment and makes worse paint quality. So the improvement of transfer efficiency is a universal problem.
It is development paint atomization without using the air and apply to the automobile painting. And there was little scattering of the paint (Figure 1) and improved transfer efficiency more than 30% conventionally.
Figure 1 Comparison with the conventional painting
2. Technical contents
This technology is applied electrostatic atomization without air to automobile painting.
The electrostatic atomization principles are as follows. Paint is enlarged in pillar shape to the grounded side such as automobile by electrostatic gravitation when it supplies an electric charge to the paint which exited from the nozzle. Electrostatic force exceeds surface tension and viscosity when it supplies an electric charge more, and columnar paint becomes division. Because this paint is charged with electricity, this flies towards to automobile along an electric field and attach.
Unfortunately, this atomization ability is only 0.3cc/min, so it was not used automobile painting that requires more than 200cc/min.
In our technology, the 600 special grooves were arranged on the circumference of the head on painting system, as seen in the 600 paint nozzles. And by rotating the head at a low speed, the paint material is uniformly and stably supplied to the groove on the circumference of the head (Figure 2).
Figure 2 Painting machine tip and electrostatic atomization
The stable supply of charge to the paint is also an important factor to achieve high transfer efficiency. If the charge supply is not stabilized, the particle size is not uniform at the time of atomization, and the appearance quality is negatively affected. Conventional high-voltage controllers adopt a “constant-voltage control system” in which the voltage at the top of the atomizer is controlled to be constant, and the current fluctuates depending on the distance from the object to be painted, and the electric charge is not stabilized. It is difficult to stabilize the distance between the top of the atomizer and the object which has complicated surface such as an automobile body.
So, this time, a “constant-current control system” for controlling the current on the top of atomizer was developed, and the current could be controlled to be constant by varying the voltage in accordance with the distance to the object to be painted(Figure 3). As a result, even in automobile body painting, the electric charge supplied to the paint can be kept constant, the painting quality can be stabilized.
Figure 3 Constant current control
Improved transfer efficiency can reduce the loss of sprayed paint and the space required to recover the unpainted paint, thereby realizing super compact painting booth (Figure 4). The objectives of the super compact painting booth are to achieve reduced CO2 emissions of 80%, compared with our conventional booth.
Figure 4 airless painting booth section
Tanaka Kazuki *1
Tani Shinji *2
Numasato Akira *2
Yasunari Fujisawa *1
*1 Member, Toyota Motor Corporation(〒471-8573 1, Motomachi, Toyota-shi)
*2 Toyota Motor Corporation (〒471-8573 1, Motomachi, Toyota-shi)