Modern automotive paints, with their metallic formulations that appear to change color at different viewing angles, present a unique color quality control challenge. The automotive manufacturers have established stringent color matching standards for virtually every colored component in the vehicle, from painted surfaces to molded plastic components to interior fabrics. Making certain that these new paint formulations will meet the automotive manufacturers' color standards requires advanced quality control techniques.
The traditional methods of visual color evaluation, even using established color matching procedures, are no longer acceptable. That is why PPG Industries, Inc., Pittsburgh, Pennsylvania, has adopted a color quality control system based on spectrophotometers and associated computer software as its standard for controlling color quality at its paint and resin plant in Oak Creek, Wisconsin.
World's second largest paint plant. The 400,000-sq-ft Oak Creek facility is the second largest paint plant in the world, the largest in the U.S. Housing its coatings and resins group in a three-building campus, PPG produces resins in addition to coatings for both automotive and industrial customers.
The facility also includes a laboratory complex dedicated to assuring consistent high quality and color integrity in its products. Formulations for these product batches are drawn from a file of more than 3,000 individual recipes.
Color measurement and control. What is seen as an object's color is actually the individual's subjective response to a given combination of light waves that the object transmits and/or reflects. Therefore, visual color matching has been based essentially on human perception, despite a number of efforts over the years to assign numerical values to various color properties in an attempt to lift color matching above a purely subjective technique.
According to Ray Gravelle, manager of Automotive Quality Services at the Oak Creek facility, visual inspection, while still used, does not completely meet the quality control standards of today's automotive industry. "The auto manufacturers are demanding exact, documented color matching from bumper to bumper," said Mr. Gravelle. Automotive engineers want to move away from human subjectivity for color identification and toward a system that measures light waves in terms of specific numerical values. "This type of color evaluation enables us, as a paint manufacturer, to measure and compare color accuracy every step along the way," stated Mr. Gravelle.
To meet the color quality control requirements of its automotive and other industrial customers, PPG has been working cooperatively with X-Rite, Inc., Grandville, Michigan, since 1988 to develop more precise color measuring instruments and procedures. "The biggest improvements in spectrophotometer technology," explained Mr. Gravelle, "are instruments that measure color at several viewing angles. This makes it possible to evaluate more accurately modern automotive finishes that appear to change color when viewed from different angles."
Understanding color measurement. Different type instruments are designed for making different types of color measurement. "If the wrong instrument is selected, the measurement may not be reliable, even though the instrument itself may be well designed and properly calibrated," said Mr. Gravelle.
Although bench-top instruments are available for use in laboratories, fully portable hand-held units are often more convenient and practical because they allow color measurements to be taken in the plant where color is actually applied.
A spectrophotometer is the most accurate type of instrument used for the kind of color quality control evaluation performed by PPG. It measures light at many points along the visual spectrum, and these multiple measurements result in a mathematically defined curve that describes the combination of individual light wave components making up the complete color.
Different types of spectrophotometers are designed for measuring color on different surfaces and finishes. Thus, a truly accurate measurement accomplished by a spectrophotometer requires an understanding of not only what the instrument was designed for but also the type of surface or finish on which the color is measured.
The single-angle instrument only looks at color from one viewing angle. "We have used these for many years, and we still use them for some types of finishes," Mr. Gravelle said. "But with the advent of mica flakes and aluminum flakes in automotive paint, the color can look different from one spot to another. The single-angle instrument is not accurate enough for these new finishes."
The multi-angle spectrophotometer that PPG uses to measure color for automotive finishes was designed specifically to evaluate paints containing metallic or mica particles. These particles create shifts in hue at different viewing angles. This makes accurately comparing the color against an established standard more challenging.
The multi-angle spectrophotometer developed by X-Rite uses a directional light source at 45 degrees, and measures reflected light at four discrete angles: 15, 25, 45 and 110 degrees. The different angles of measurement provide the instrument with a combination of data, which interprets a color change as the viewing angle changes. The instrument integrates all these readings to provide objective, numerical information about the color.
A Windows-based computer software program, working in conjunction with the spectrophotometer, helps to establish an acceptable numerical tolerance for the instrument's readings. The software package creates a user-defined program that is downloaded to the portable spectrophotometer. The program prompts the operator for a measurement and compares the sample to a previously specified standard.
The software program can also associate any stored notes and tags, such as formula, process and performance information as well as sample identification and other data. All this data is retained for upload back to the system. The program can import and export stored data for access in a spread sheet program, manipulation in other formats and sharing databases via local area networks.
"The software works with computers in our plant-wide area network," Mr. Gravelle explained. "We can share numerical color data along the entire corporate network as well, allowing us to look at each other's color results. This is a feature we will be using more extensively in the future."
Controlling color quality. "Spectrophotometers and their associated computer software are used for everything that requires precise color control," Mr. Gravelle continued. "This includes exterior topcoats, accent colors and colors for a range of automotive interior products such as dash board components, steering wheel coatings and some types of molding. Today's topcoats are 90 pct metallic. Since these are the formulations that include mica and aluminum particles, the spectrophotometer that evaluates color at different angles has been invaluable to us."
Most automobile manufacturers are now working toward a ten-year finish. The manufacturers want the vehicle to look almost new when a used car customer goes to buy the car five to ten years later. PPG has an ongoing program with automotive design engineers to develop finishes for the long term. To preserve the finishes, it designs resin systems that help protect the paint from exterior and UV exposure.
"We have an extensive pigment development evaluation team, which is to go through all our automotive products to screen new pigments to make sure they are not only light stable, but accurately colored too. And again that requires color measurement instrumentation and evaluation with related software," stated Mr. Gravelle.
PPG color data is also made available to auto body repair facilities where damaged components are repaired or replaced. The vehicle is often spot painted to match the original finish. Color matching, even when a vehicle is several years old, can be critical to the customer's satisfaction.
In the early 1990's, the PPG Automotive Refinish Group introduced the Prophet version of the supplier's MA-58 spectrophotometer. The color analyzing system reads the color data from the car's surface. This information is then transmitted through a custom-designed computer system to the collision center, providing them with the correct color formulation for a precise color match.
The advent of quality programs such as ISO 9000 and the automotive industry's QS 9000 standards have caused manufacturers to impose stricter measurements on products not required a few years ago. The introduction of these higher quality standards, along with new paint formulation and finishing technologies, have made measuring and duplicating exact color even more critical.
Mr. Gravelle pointed out that the physical act of mixing a paint formulation is a fairly standard process. But what goes into that formulation can change radically, affecting not only the color, but its performance and longevity too.
Numeric color measurement is enabling PPG to more accurately track the development of an automotive color from the original formulation in the laboratory, check pigments and other constituents, check the formulated product and check the performance of the applied finish against an established standard.
The electronic communication of color information between departments is contributing valuable data to statistical process control systems as well as tightening quality control from raw materials to finished product. The spectrophotometer is the result of a cooperative effort to develop and implement a color management system that is a vital component in PPG's overall on-spec and on-time delivery initiatives.