Controlling Orange Peel's Impact on Today's Brilliant Auto Finishes
Any way you look at it, orange peel is bad news. When a car rolls off the assembly line with this wavy, light-and-dark pattern hiding somewhere in the paint, the only good news is that it was caught. Otherwise it would have arrived at the dealership or even the consumer and caused a lot of dissatisfaction. And that would be even more expensive than sending it to the rework department in the first place. Add to those problems the potential losses in scrapped products, and the possibility that paint is being wasted because of overkill in the spray booth.
According to Bob Hertel, transportation sales manager with BYK-Gardner, the orange peel phenomenon is by no means restricted to passenger cars. It is a concern for manufacturers of SUVs, motor homes and luxury buses, and is gaining attention in the pleasure boat and aircraft markets. In short, more people are requiring a great paint job.
"There was a time when a truly high-quality car finish meant 12 coats of hand-rubbed lacquer," Hertel explains. "Today's car buyer expects a flawless finish right off the production line, which requires sophisticated painting systems and precision quality controls."
The "wetness" of today's shiny paint jobs is not the result of paint alone. A high-quality finish depends on what coatings experts call "distinctness of image." So, while paint makers have succeeded in developing very high gloss liquid coatings, there are still underlying problems in controlling waviness (orange peel) in the final appearance of finishes.
"Each layer of paint - E-coat, primer, topcoat and clearcoat - must have a smooth, lustrous appearance as well as the requisite physical and chemical properties," Hertel says. "This is not easily achieved, since vehicle parts have varying contours and the sheet metal from which they're made has an uneven texture from the stamping process."
Also, the application process must accommodate varying contours and orientations, including straight, curved, horizontal and vertical.
Add to that the fact that automakers are shifting to powder coatings in order to comply with the EPA's year-2003 ban on the VOCs found in liquid coatings. Powder coatings differ in rheology (deformation and flow) from today's liquid coatings. And most automakers don't yet have a lot of experience in applying powder coatings to car finishes. Chrysler, GM and Ford are working together in the Low Emissions Paint Consortium (LEPC) to develop application techniques and finish standards for powder coatings.
"There's a lot more to this than just paint," says Brian Prylon, GM staff engineer with the LEPC at the Wixom, MI, Ford assembly plant. "Converting to powder paints also involves new painting tools, filtration processes, the entire painting system." All of which makes powder coating with little or no orange peel - a most noticed appearance factor - a rather daunting challenge.
One tricky aspect of detecting and measuring orange peel is that it is a visual, not tactile, phenomenon. In an effort to better define this, BYK-Gardner conducted studies on the affects of substrate roughness and paint rheology on appearance using its Wave-Scan Plus, a hand-held meter designed to evaluate orange peel by taking laser-optical measurements on the finished surface. (Orange peel was traditionally evaluated using samples with varying degrees of orange peel, or by profilometry, a laboratory process that measures the texture of the painted surface.)
Using three different paint systems and substrates with varying textures, test panels were subjected to different paint processes and baking positions. Then the Wave-Scan Plus laser light took a measurement every 0.08 mm for scan lengths of 10 cm on the painted surfaces, resulting in 1,250 measuring points per scan. Measurements of waviness were automatically correlated in mathematical terms to other industry-known, visual measurement scales including tension value and orange peel standard ranking. This also provided an indirect measure of orange peel-influencing factors such as substrate roughness, flow/leveling properties of paints and process parameters.
The test concluded that changes in short-term waviness - the condition that causes the most noticeable orange peel - are caused by sheet metal roughness and the use of a clear (top) coat. The less noticeable long-term waviness is affected mainly by baking position. The test results also indicated that there is a need to measure for orange peel on a continuous basis and take corrective actions as necessary.
The importance of the Wave-Scan Plus measuring long- and short-term waviness is that it simulates the resolution of the human eye, or "how you see." The profilometer, on the other hand, measures microscopic topography, or "what you feel." The other quality control measure was to place cameras on the assembly plant finishing line to record and analyze the gloss. These visual assessments were quite subjective, based on comparing the final paint job with orange peel samples of varying degrees. The Wave-Scan provides a more objective eye for the assessment of appearance.
Instrument use helps to avoid orange peel problems in two ways. First, it detects and measures excessive orange peel in final vehicle finishes. But equally important, the instrument collects data that can be transferred to a PC to perform statistical and analytical evaluations. This enables the optimizing of quality-control systems for the prevention of losses through avoidable rework, scrapped parts, and wasted paint. The software program "gardner-soft" provides a Windows/PC link for the data collected by the Wave-Scan Plus so that the measurements can be processed through statistical process control to expedite paint system adjustments.
The Wave-Scan Plus is battery powered, so the device can be used at any location without the interference of cables. It is easy to operate; user-definable displays permit guided operation according to your measurement sequence. The instrument is reliable and self-calibrating; you receive reproducible results whether on test panels, curved surfaces, or metallic or non-metallic paints.
The Wave-Scan Plus is in wide use among automakers today, including manufacturers of European prestige models like Mercedes, BMW, Jaguar and Rolls Royce. According to Bob Hertel, Toyota uses the device in the production of its Lexus and Camry models, which "are setting the benchmarks for paint quality in the automotive industry."
Hertel says the instrument is becoming popular in the luxury bus industry, too, and is used by Boeing and Cessna for evaluating aircraft finishes.
For more information on color measurement instruments, write 9104 Guilford Road, Columbia, MD 21046; phone 800/343.7721; visit www.byk-gardner.com; or Circle Number 66.