Laser Engraving Makes Robust Bar Codes on Automotive Parts
Automotive manufacturers cannot afford even short delays on the production line, and each part must be labeled clearly to avoid mistakes. Anytime the line stops, it costs money. However, traditional methods of labeling automotive parts, such as affixing bar codes on sticky labels, are prone to damage in the dirty, busy environment of a production line.
Using a laser to engrave, rather than mark, automotive parts yields robust, scanner-readable bar codes in just a few seconds.
Lasers can be used to mark bar codes on parts made of aluminum, galvanized steel and stainless steel. Aluminum is finding more use in automotive manufacture, but marking on uncoated aluminum with lasers is challenging. The metal has a high reflectivity at infrared wavelengths and, because aluminum oxide is white, laser marks created by oxidation usually have low contrast. This means that conventional laser-marked bar codes on aluminum are not scanner-readable because there is insufficient contrast between the marked and unmarked regions.
Many industrial processes for laser marking on aluminum require an intermediate coating step such as anodizing, which is unacceptable in the automotive industry because coating large, complex parts would increase manufacturing cost and time. Powerlase Ltd. in Crawley, UK, has developed a technique for laser engraving scanner-readable bar codes directly onto metals. The codes hold all the information needed for subassembly traceability on the production line and also can be used in security applications for high-value components and vehicle authentication.
The process is different from standard methods because it involves milling, or engraving, the surface instead of just marking it. It uses high-average-power Q-switched diode-pumped solid-state lasers to locally change the surface roughness of an aluminum substrate. These lasers offer high-intensity nanosecond pulses at kilohertz repetition rates, allowing high-quality, precision material removal through fine melting and vaporization, but with sufficient repetition rate and average power to achieve high removal rates and productivity. A typical bar code takes just 6 s to engrave.
The company has experience in the flat panel display industry, where lasers are used to selectively ablate and pattern transparent thin films. “We have found that the same laser technology can be applied to metals used in the automotive industry,” said Matt Henry, applications manager at the company.
In testing the technique, Henry and his colleagues used a Starlase AO2 Nd:YAG Q-switched diode-pumped solid-state laser at the fundamental wavelength of 1064 nm to engrave aluminum, galvanized steel and stainless steel. This device offers average powers greater than 200 W at a range of repetition rates and pulse durations between 3 and 50 kHz and 30 and 200 ns, respectively. The output beam power was varied using a Powerlase attenuator unit, collimated with a Galilean telescope and directed into a galvanometric scanner.
The scanner was fitted with an 80-mm-focal-length f-Theta telecentric objective lens with a working target area of 25 × 25 mm, which produced at best focus a 160-μm-diameter focal spot. All of the processing work was performed in air at standard atmospheric conditions, and no gas assist was used.
Powerlase worked with industrial scanner manufacturer SICK GmbH of Düsseldorf, Germany, to develop the process. The scanner for this application uses a visible red laser diode to scan the laser-engraved sample. “Because the laser-engraved regions scatter more light back to the scanner than the bulk aluminum substrate, we engraved a negative, or reverse, image of a bar code onto the aluminum substrate,” Henry said. This means that the laser-engraved regions read as white and that the bulk substrate reads as black, in reverse logic to that of conventional laser marking.
When working with aluminum, a negative image bar code read at a 30° angle yields the best results, but Henry and his colleagues found that, for galvanized and stainless steel, the best results were obtained using conventional bar-code images read at an angle of around 5°. These are read at a lower angle because bar-code readability in this case is less a function of scatter and more one of specular reflection.
The fact that this technique works best when the bar code is read at an angle is an unforeseen advantage. “It is preferable for industrial scanners to be positioned at an angle to the bar code on production lines because it makes industrial setup more tolerant of variation,” Henry said. The process also requires no postprocessing.
“This represents a highly flexible solution, as the laser parameters required for each different material can be controlled by software, and so a single laser-processing station could bar code a wide variety of components made of dissimilar metals,” Henry said.
The process is not yet in production but is being tested by several major car manufacturers. The company hopes to see implementation in volume production in 2007.
Contact: Matt Henry, Powerlase Ltd., Crawley, UK; e-mail: email@example.com.
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