The Expanded Reach of Laser Marking
The arguments in favor of laser marking are strong. It’s fast, flexible, reliable and permanent. It does not require consumables, such as ink.
It is inherently noncontact; the material being marked is not dimensionally altered or stressed by mechanical forces. And, lasers can process virtually any material with only parametric changes, instead of hardware or tooling changes.
All lasers work according to the same basic principle: A lasing medium, such as carbon dioxide, is excited by an energy source, or pump, and amplified between fully and partially reflecting mirrors. The resulting amplified light output is parallel and in phase. All lasers produce monochromatic light, but different lasing media produce different wavelengths.
Laser markers form characters by steering the laser beam with two computer-controlled scanning mirrors through a focusing optic that concentrates the laser energy into a small spot, typically 0.0025 to 0.005 inch in diameter. Graphics are generated by CAD-based software.
Laser marks are created by vaporizing, melting or annealing the material. Each has a specific effect for different marking applications. Vaporization produces a mark with depth in the material, like engraving. Melting creates a contrasting mark through a thermal-chemical reaction. This technique is often used with plastics. Annealing the surface of a material, such as steel or titanium, can produce a dark mark without noticeable surface penetration.
The most popular systems are lamp-pumped neodymium-doped, yttrium-aluminum garnet (Nd:YAG) lasers, which produce near infrared light at a wavelength of 1,064 nanometers, and CO2 lasers, which produce light at the 10,640 nanometer wavelength.
All laser marks are a function of wavelength, beam energy and time. Because of their wavelength, Nd:YAG lasers are more capable than CO2 lasers—but they are also substantially higher in cost. The 1,064 nanometer wavelength is more easily absorbed by a vast range of materials. CO2 lasers are an economical marking solution, as long as the wavelength and material match.
New ResearchRecent research has produced variations of basic laser technologies for specific marking applications. One example is the diode-pumped Nd:YAG laser. This laser offers high beam quality, excellent pulse-to-pulse stability, and long maintenance intervals—typically every 12,000 to 15,000 hours of use. Diode-pumped lasers also produce more reliable "green" light, which has a wavelength of 532 nanometers.
Diode technology lets laser producers use different types of crystals to create resonators with short pulse widths. In the past, lasers were rated purely by average power—the more the power, the faster or deeper the processing capabilities. In many cases, this power-to-process relationship remains true today. However, short-pulse-width lasers can be made smaller in size without sacrificing effectiveness. The rapid rise-time of the energy burst induces material changes without overprocessing. This is particularly useful for many polymers, where the goal is to create the highest possible contrast between the base material and the laser mark.
As little as 5 to 7 years ago, users had few choices other than the power of the laser. Today, a wide range of laser types and methodologies is available. Assemblers are well-advised to thoroughly test their application with different lasers. For example, low-power, diode-based lasers may not be able to keep pace with very high-speed applications. With advances in scanning technology, lasers can generate as many as 1,200 characters per second. Lasers used in these high-speed systems must have enough power to meet those rates.
Marking ApplicationsLasers are used in thousands of applications in a wide range of industries.
Metal parts. Lasers can handle all types of marking applications for metal parts, including 360 degrees around the product. Pressure transducers are a good example. These devices are typically cylindrical products made of metals such as 316 or 304 stainless steel, Hastaloy and Inconel. Lasers will mark clearly on the smallest circumference and can also add artwork, such as a company logo.
Direct marking obviates the need for nameplates or labels, which can be a source of contamination in the manufacturing process. Marks created with lasers do not produce crevices or uncleanable areas in the product. This makes laser marking a good choice for metal parts used in food, dairy and pharmaceutical industries.
Photographic imaging and invisible coding. Lasers can create high-resolution images that simulate black-and-white photography. It can also embed information into that photograph. This is an obvious benefit for manufacturing security cards and for making product tracking information invisible.
High-speed marking. Lasers have long been used in the packaging industry to mark production coding and expiration dates, but the technology is just as ideal for putting identifying nomenclature onto parts moving along a high-speed assembly line. Because of their speed, accuracy and flexibility, lasers are a cost-effective alternative to labels in high-volume, high-mix settings.
Data Matrix codes. Data Matrix is a 2D identification code that resembles a checkerboard. It offers tremendous data capacity and built-in error correction. Data Matrix has been embraced by the automotive industry for direct marking of parts, and it’s also in the ATA2000 specification for the aerospace industry.
Lasers are better at writing Data Matrix codes than any other programmable marking technology. For example, the size of the cells in the matrix can be equivalent to the focused spot diameter of the laser beam. As a result, up to 60 characters of information can be contained within a matrix code only 1 millimeter square. And, because Data Matrix codes do not require a 70 percent to 90 percent contrast ratio for decoding, lasers can be used in low-contrast marking applications, such as aluminum castings.
Plastic parts. Lasers once had limited applications for marking plastics because of low mark contrast. However, plastics companies have developed resins that change color when exposed to particular laser wavelengths. Today, virtually all plastic compounds can be effectively marked with lasers.
In fact, lasers are now the technology of choice for marking many plastics. Many inks will not adhere to some plastics, or will wear quickly. Many labels will not stick. Some engraving and etching processes create unclear imaging, and most require multiple process steps. Lasers generally do not require pre- or post-mark processes, so items can be marked and immediately packaged for shipment.
New short-pulse-width lasers (pulses of 10 to 15 nanoseconds) produce good thermal reactions in plastics without latent heat. This is particularly useful for achieving light marks on dark plastic items, such as switches and connectors. The automotive industry, for example, requires human or machine-readable traceability markings with the highest level of contrast. This can only be accomplished with lasers.
Electronics. For identifying integrated circuit (IC) packages, surface-mount devices and printed circuit boards, lasers produce permanent marks that are unaffected by the various stages of electronics assembly. Other marking methods may not withstand the harsh processes or may damage delicate substrates and components. Today, virtually every major producer of ICs in the world uses lasers to mark its products prior to shipment.
Circuit boards can be marked with text, bar codes or Data Matrix codes. Both Nd:YAG and CO2 lasers are used. CO2 lasers can produce marks directly on the solder mask without affecting the material’s properties. Nd:YAG lasers generally require a pad of either ink or copper, because of its poor interaction with solder mask materials. The CO2 marker is generally more cost-effective than the Nd:YAG laser. However, the Nd:YAG laser produces marks with somewhat better size and resolution.
Day and night marking. This concept was developed to make automotive controls easy to locate in the dark. A black topcoat, 1 to 2 mils thick, is applied to a translucent white substrate. The laser removes the coating from the substrate, producing clear, white characters that are crisply defined against a colored background. At night the characters are backlit for easy recognition.
The Nd:YAG laser is the marker of choice for this application. The coating can be removed at relatively low power at high scan speeds. To improve productivity, manufacturers often employ "stereo marking" for this application. The laser beam is divided equally by a 50 percent beam splitter and directed simultaneously through two marking heads. This doubles the productivity of the laser by allowing two parts to be identically processed simultaneously.
One-step label creation. This relatively new application has been widely accepted by automakers in Europe and the Pacific Rim. A typical vehicle can have more than 30 labels and nameplates. They come from a variety of suppliers and are applied at the factory. A laser marker can simplify this process.
Using acrylic label stock, the laser marks the information, then cuts the label to shape. This can save automakers money because they don’t have to stock a huge inventory of labels and they can change labels immediately without wasting inventory. And, because all the labels for a vehicle are produced in real time, vehicle labeling is more consistent. Payback on a laser-based labeling system is typically 6 to 9 months.
Laser LimitationsOther marking technologies are better suited than lasers for some applications. For plastic decoration, screen printing is better than lasers for marking very large areas. It’s also difficult to achieve jet-black marks on light or white plastics, other than polycarbonate resins. Lasers can provide dark gray marks, but not true, pure black markings.
Often, thermoplastics in their natural state—that is, unpigmented—will yield poor results with laser marking. Also, if a multicolor image is desired, laser marking is not the best solution. Even with today’s thermoplastics, direct laser marking still yields monochromatic results.
Creating conventional, high-contrast linear bar codes in cast aluminum is very difficult using laser marking technology. In fact, this tends to be true with any material in which lasers do not yield high-contrast marks. However, alternative symbologies, such as Bumpy Bar Code and Data Matrix, have enabled manufacturers to use lasers to produce machine-readable codes without high levels of contrast.
Lasers vs. Other TechnologiesThere are many methods of marking parts, including labels, ink systems, mechanical engraving and embossing, and chemical and dry etching. Each has it’s use, but lasers are growing more and more popular.
Laser marks do not degrade over time, as printing can. A label may live forever, but what is on it can disappear in hostile environments or just from continual wear. Ink can smear and many inks and labels will not adhere to many plastics. Often, heat stamping will react badly with some plastic surfaces.
For metals and other materials, engraving, embossing and other contact processes do not facilitate changeover. Tool wear is also an issue.
Dry etching requires special blast chambers to control debris. Chemical etching uses dangerous acids and requires costly waste disposal. Both etching processes require screens to be replaced each time a new mark is needed.
Unfortunately, the difference most often noted between laser marking and other marking technologies is usually the initial cost of a laser system. However, in numerous applications, the laser’s increased productivity, reduction in inventory costs, improvements in quality, and lower long-term cost of ownership more than offset the difference in initial capital investment.
Laser SafetyWhenever the subject of lasers arises, so does the issue of safety. This is a holdover from when the first laser-based systems came into use. The biggest fear seems to be "looking into the laser." This was addressed decades ago, and today all modern lasers provide safe windows for viewing the process and will not allow tampering while in operation.
Laser marking is an environmentally benign process. It requires no caustic chemicals or solvents. However, in some applications, the laser can generate debris that should be extracted from the laser chamber. Some plastics can generate a gas from the heat of laser processing, but this is easily removed with a special exhaust system.
For more information on laser marking, call Rofin-Baasel Inc. at 978-635-9100 or visit www.rofin.com.