Low-Power Lasers Make Their Mark
Bill Stull, Synrad Inc
Although sealed CO2 lasers have been used to mark plastics, paper and other organic materials for years, two recent areas of growth include the marking of metals and glass — applications once considered the domain of other, often higher-cost technologies. The increased use of CO2 lasers for these applications is attributable to improved process developments, as well as to the emergence of new laser designs that better address the demands of working with these materials.
Metal marking is usually associated with Nd:YAG lasers. However, low-power CO2 lasers can mark a number of metals, including mild and stainless steels, inconel and titanium. These systems are considerably less expensive and have fewer maintenance and safety demands than Nd:YAGs, and, with their good beam quality and fast rise time, they are increasingly replacing YAG lasers for the marking of text, bar codes and graphics on metal parts. These applications generally require 50 to 200 W of CO2 laser power and, as new, smaller, air-coolable CO2 lasers in these power levels enter the market, their appeal continues to grow.
Because CO2 lasers are controlled through pulse-width modulation, their output power can be controlled by varying the duty cycle of the radio-frequency amplifiers. These lasers create a contrasting, rather than an engraved, mark on bare metal. Because the mark has minimal part penetration, there is no degradation of the part’s mechanical properties. Metal marking is accomplished in one of two ways — with or without surface melting (Figure 1).
Figure 1. Engineers used 90 W of CO2 laser power to mark this stainless steel with (left) and without (right) melting.
In the latter case, discoloration results from surface oxidation when the material is heated by the focused laser beam. In the former, additional energy is applied to melt the surface of the material, and the molten metal resolidifies to form the mark. Up close, this type of mark resembles tiny weld beads. The energy input, the atmosphere in which the mark is made and the material being marked all help to determine the type of mark produced.
For years, glass manufacturers and fabricators have used sandblasting and chemical etch methods to mark automotive glass, medical devices, windows and electronic parts. Until now, CO2 lasers have not been an option because of the highly fractured marks that they typically produce. The only way they could be used for glass marking was to coat surfaces, which produced readable and durable marks but which had the obvious drawbacks of application, curing and post-mark cleaning. New techniques, however, have made it possible to produce high-quality marks directly on glass with CO2 lasers.
Sandblasting and chemical etch methods have distinct disadvantages compared with laser marking. Both require a mask, which restricts their flexibility, and both are relatively dirty processes, problematic because of airborne dust particles and corrosive chemicals, respectively. Laser marking, in contrast, is a clean, noncontact process that is highly flexible and that requires no hazardous chemicals. Moreover, marking speeds of up to 200 characters per second are easily attained with galvo-based systems.
The key to quality glass marking lies in careful control of power, speed, resolution and laser spot size, along with the use of multiple laser passes. It is important to control the laser’s heating effect during processing because overheating of the material can produce a highly fractured mark.
To begin with, engineers must choose an energy level appropriate for the glass. Laser marking usually requires making several passes over the material while gradually increasing the power, until an optimum level — one that will just break the surface of the glass, but not overheat it — is reached. The optimum energy input varies from one type of glass to the next, and it depends on a number of factors, including the toughness of the material and its thermal expansion coefficient. This multipass method may be used for marking serial numbers, batch codes, logos, data matrix codes and bar codes in a variety of glass types (Figure 2).
Figure 2. Most glass marking applications, such as applying this data matrix code, are easily accomplished with a 25-W CO2 laser.
With improvements in both processing technique and laser design, CO2 laser technology should continue to infiltrate diverse industrial marking applications.
Meet the author
Bill Stull is applications manager at Synrad Inc. in Mukilteo, Wash.; e-mail: email@example.com.
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