Spray Painting of Automobiles - Problems and Solutions

Paint is applied to car parts and bodies on assembly lines in several coatings, for protection against rust and to provide the required color finish.  Only a few minutes is available to complete each paint process.  Robots with multiple nozzles apply the initial coats, followed by hand spraying.  With increased on-demand manufacture, frequent color changes are required as parts pass the robots on the assembly line.  The geometry of body parts poses special problems for the spraying process.  As much as forty percent of the spray paint misses its target and the pollution generated by the toxic off-spray can exceed the pollution generated by the engine emissions during the lifetime of the vehicle.

During the painting process, orange peel, blistering, running, and other imperfections in the coating can be cause for rejection and may result in scrapping the body part.  Metal flakes in the paint must be oriented parallel to the body surface to increase reflectivity.  The consequences of paint surfaces which do not meet the required minimum standards are very costly.  The overall quality of the paint on the vehicle can play a major role in selection of a car for purchase.

In order to process the very large volumes of paint that miss the target (up to 40%), large wind tunnels surround the painting operation.  Wind is blown over the vehicle body during painting at a velocity high enough to drag the paint droplets into a tunnel.  Since paint has toxins and metal flakes, it needs to be processed and treated as toxic waste.  As a consequence, the painting section is the most expensive component of the assembly line.

The atomizers generate small droplets of paint which pass through an electric field, which induces an electrostatic charge on each droplet.  The initial trajectory of each droplet is governed by the diameter, velocity and angle of flight as it leaves the atomizer.  The subsequent trajectory is governed by interaction with air streams and other droplets.  Final impact on the target surface is assisted (by about 10%), by the electrostatic attraction between the body surface and the charged droplet.

The overall costs associated with painting are very large.  Failure to achieve the required quality and luminosity in the paint surface adds greatly to the overall cost.  Robots that pan across the body surface cannot provide adequate coverage to components with complex geometries.

The automobile industry needs a solution to the spray problem. They are spending a great deal of money on painting, much of which is wasted. The industry must increase the percentage of paint that is deposited on the target and correspondingly, reduce the percentage of offspray. If the automotive industry used instrumentation and computation for controlling drop size, trajectories and deposition, they would see large scale increases in efficiency, reduced failure to meet painting requirements and reduced generation of toxic pollutants. Use of this advanced spray technology would provide very large financial benefits to the automobile industry.