As challenging paint materials are becoming increasingly common, automotive manufacturers are becoming more aware of the role contamination control plays in the ultimate goal—the emotional showroom experience
By Ernest M. Otani
The automotive paint shop is responsible for providing a quality paint job to every vehicle while simultaneously managing the cost of the specialized paint materials as well as an array of environmental regulations for air quality.
To meet with success, the manufacturer must focus on every detail of the process. For just as a quality finish is a major advantage for the manufacturer, paint defects are obvious to any observer. They taint not only the vehicle, but the reputation of a whole product line.
A successful painting operation is vigilant against contamination—which comes from personnel, equipment or even the painting process itself—and has a very consistent and effective means for delivering paint to the car body.
In a typical paint shop, personnel are required to wear basic cleanroom apparel—hairnets, shoe covers, eye protection and paint suits. Despite these safeguards, people are probably still the main source of contaminants in the paint environment. By judicious use of quality automation—robots and other painting machines—operators can stay out of the paint booths during production with only occasional intrusions for service or to clean equipment.
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Even painting equipment can be a contamination trouble spot if proper care is not taken. A painting robot should be easy to clean, with no features that would tend to trap paint or dirt. Materials exposed to the paint environment should not deteriorate in the presence of harsh paint solvents.
The paint itself may prove to be a contaminant if it builds up on the surface of painting equipment to the point where it might drip onto a job. Many plants use robot coverings or “pajamas” as both a means to manage contamination and as an aid to shorten maintenance time. It is typically much faster and more economical to swap a disposable cover than it is to adequately clean equipment with solvent sprayers and rags.
A consistent process begins with the paint. Typically, paint is delivered to the plant in 550-gallon containers, with each batch of paint essentially identical to all others of the same color. Nevertheless, a careful paint shop logs in each batch by manufacture date and container number, and takes steps to assure that viscosity and agitation are maintained at manufacturer specifications.
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The “mix kitchen,” where paint enters the delivery or paint circulation system, is often temperature-controlled. All these steps are meant to eliminate any source of variation.
The spray booth itself can affect paint quality by the painting environment it presents.
Extreme temperatures and changes in humidity affect the rate at which solvents evaporate from the paint, both while airborne and on the painted job. Humidity also affects the efficiency of the electrostatic charging of the paint, particularly on those waterborne systems that don't charge the paint directly but use high-potential cathodes that emit ions into the air. Although not all automotive plants control temperature and humidity as closely as other factors, those with less control are faced with the task of fine-tuning their process as these environmental factors shift from day to day and from season to season.
Atomization parameters
The most important factors are those related to the specific processing of the job—atomization parameters, and the painting path executed by the robot carrying the atomizer.
In principle, the job is quite simple: push a uniform cloud of droplets toward the car body in a predictable manner in the allotted cycle time. Need more paint? Increase the paint flow rate. In practice, the quality measures that result in the emotional “showroom” experience make the task anything but simple.
Today, atomizers fall into two styles. The spray gun has an atomizing nozzle and fan of air to disperse the droplets in a predictable pattern. The bell has a spinning bell-shaped cup that creates droplets by centrifugal force, and an air nozzle that shapes the droplets into a circular pattern directed at the vehicle.
For bell atomizers, the key parameters are paint flow rate, bell cup rotation speed, and the velocity and shape of the shaping air. For a given flow rate, higher cup rotation or higher shape air velocity lead to smaller particles. And up to a point, smaller particles tend to lead to better quality numbers.
But the specifics of how the two factors interact with paint density and viscosity to produce a uniform, brilliant paint job are different for different styles of applicators. And the physics of how the film of paint on the bell cup becomes a uniform cloud of droplets is still an active research area, both in industry and academia.
A final aspect when considering job processing is the inclusion of electrostatic effects. For optimal painting efficiency, the paint in this cloud of droplets is charged with high potential (60 kV and higher) ions. The droplets are then drawn to the grounded vehicle body by electrostatic force. This attraction leads to high transfer efficiency—it is common for 90 percent or more of the paint atomized to actually arrive on the target.
Paint can be charged directly by placing it in contact with a high-voltage source; for example, charging the bell cup, or a fluid passage. Paint can also be charged indirectly, a strategy usually used for conductive materials, such as waterborne paint. Applicators emit streams of ions into the surrounding air and have them impinge on the droplets once airborne.
Indirectly charged paint, however, has some disadvantages, since the cathode “antennae” used to emit the ions can be cumbersome and might make processing more difficult for some jobs. The many slender rods also present a large surface area, which, if they collect paint, can be an additional source of contamination.
An atomization strategy, electrostatics, and the equipment to produce the desired behavior with consistency and control all come together in the spray applicator design.
Continuing challenges in painting
The objectives of the automotive paint shop are the same as any manufacturing endeavor: quality and efficiency. But competition makes the standards for these objectives very demanding.
All glamour and emotion aside, automotive paint quality is measured quantitatively with several specialized metrics. Color is compared to a standard paint sample using an electronic colorimeter. Measurements are taken from several viewing angles because the metal or mica flakes within the paint subtly shade the color as it is viewed from different angles.
Film build is often controlled to tolerances in the range of 0.01 mil. There are also several appearance measures for the surface of the finish, including such things as “orange peel” and Distinctness of Image (DOI), which is essentially a measure of the finish's glossy, mirror-like quality. The metrics offer a clear definition of the quality objective, but meeting that objective with a new or troublesome paint can require extensive experimentation. Once a desired result is achieved, maintaining it requires a good process control plan.
The other key objective is efficiency—reduce wasted paint, maximize uptime, and keep maintenance and service overhead to a bare minimum. Here, the right choice in automation can make a substantial difference. Automation, which is dedicated to the application, often comprehends practical matters like cable and hose management much more effectively than a general-purpose robot converted to serve the painting application.
The future
Challenging paint materials are becoming increasingly common. Two-component cyanoacrylate paints have very desirable anti-chip properties but must be mixed “on-the-fly,” just upstream of the applicator.
Waterborne paints have very low levels of environmentally-unfriendly volatile solvents. Because the paint is conductive, however, it presents some additional challenges in isolating the paint to be sprayed from the rest of the paint circulation system.
In the coming years, a number of trends will drive the next generation of painting equipment and processes:
- Emergence of “special handling” paints as mainstream;
- Further advances in productivity for paint atomizers and robot arms;
- Bell-style applicators with traditional atomizing spray guns as the equipment-of-choice in the automotive industry;
- Improved understanding of atomization physics will lead to even higher quality measures with less setup and experimentation;
- Environmental concerns will lead to a shift towards water-based paints and more demands on air-handling systems in the paint booth.
Ernest M. Otani is a staff engineer in the Product Development Department of FANUC Robotics America. He is the lead mechanical engineer supporting current and new paint atomizer products for FANUC. He can be reached at [email protected]