Laser Marking Steers a New Course in Manufacturing

As the technology of laser marking has advanced, new markets have evolved to take advantage of increasingly faster marking speeds as well as greater marking precision and imaging capabilities. Continuing developments in laser-cavity design, beam-steering and focusing optics, and computer hardware and software are expanding the role of the systems.

Steering the beamOf the available marking technologies, beam-steered laser marking systems provide users with the greatest amount of image flexibility in a fast, permanent, noncontact marking process. As manufacturing processes become more automated and after-sale tracking more prevalent, laser markers are frequently the only method available to produce individually unique, permanent images at high speed.

Beam-steered laser marking systems usually incorporate either a CO2 or Nd:YAG laser. The CO2 laser emits a continuous-wave output in the far-infrared (10.6-um wavelength) while the Nd:YAG laser emits in the near-infrared (1.06 um) in either a CW or pulsed mode (1 to 50 kHz). The Nd:YAG laser is also unique in its ability to produce very short, high-peak-power pulses when operated in the pulsed mode. For example, a typical 60-W-average-power Nd:YAG laser can produce peak powers on the order of 90 kW at 1-kHz pulse rate.

The delivery optics consist of either a simple focusing lens assembly or a combination fixed upcollimator and flat-field lens assembly. In either instance, the laser beam is directed across the work surface by mirrors mounted on two high-speed, computer-controlled galvanometers.

The simple focusing assembly offers the advantages of low cost and fewer optical components and is routinely used with CO2 lasers. The flat field lens design, though more expensive, maintains the focal point of the marking beam on a flat plane for more consistent image characteristics throughout the marking field. The flat-field lens also produces higher power density on the work surface than the simple focusing assembly due to the shorter effective focal length. The flat-field lens design is always preferred for high-accuracy and high-image-quality applications and is usually incorporated with Nd:YAG lasers.

Both designs provide the user with a selection of lenses that establish both the diameter of the marking field and the marking-line width. Longer-focal-length lenses provide larger working areas, but the line width is also enlarged, thus reducing the power density on the work surface. The user must compensate by either increasing the laser output power and/or decreasing the marking speed which usually consists of two lenses and may be placed anywhere in the beam path before the focusing lens. A beam expander often is used instead of extending the beam path approximately 10 more feet, in which the beam expands through its inherent tendency to diverge as it exits the resonator cavity. A spatial filter inserted within the beam expander produces the best mode quality in close-coupled systems, by passing the beam through a small aperture.

The last optical element that a laser beam encounters is the focusing lens. With CO2 lasers, this lens is usually made from one of several materials: Zinc selenide (ZnSe), gallium arsenide (GaAs) or germanium (Ge). ZnSe, a dense, yellow material that is transparent to visible wavelengths, is by far the most common of these materials, and it allows a low-power, HeNe laser beam through for alignment purposes. This is a great advantage over GaAs or Ge which are opaque to light from the visible portion of the spectrum.

Nd:YAG lasers almost always employ beam expansion, usually in the 2x to 5x range, because of their initially small beam diameters. Spatial filters for CO2 lasers must be external, but those for Nd:YAG lasers can be located inside the laser cavity itself, and many different sizes are available for mode selection.

Nd:YAG lasers employ optical glasses such as BK-7 or fused silica for lenses. The 1.06-um wavelength of these lasers is close enough to the visible spectrum to permit adaptation of standard optical devices with the correct AR coating to direct the laser light. For example, microscope objectives can deliver Nd:YAG laser light to the surface of VLSI circuitry for micromachining of conductor paths. As discussed earlier, delivering a Nd:YAG laser beam with fiber optics offers incredible advantages over fixed-optic delivery. The fiber advantage is unique to Nd:YAG lasers and has created an enormous growth in their use for industrial materials processing.

Fiber optic delivery for Nd:YAGThe use of fiber delivery with YAG lasers is so extensive in the industry that it should be discussed in more detail. Approximately 90 percent of new Nd:YAG welding installations involve fiber optic delivery. Because the 1.06-um wavelength is transmitted by glass optics, it can be used in standard fiber optics. Conventional beam delivery is extremely cumbersome, prone to misalignment and contamination to the optics, and can be very expensive due to custom layouts. Fiber provides a real answer to all of these problems. The benefits are:

• Fibers deliver laser energy over distances which, in practice, would be impossible to achieve using conventional optics. Distances of up to 50 meters are achieved quite routinely.
• Stability and accuracy are improved since only the final focus optics need to be held in an accurate relationship to the workpiece.
• Most applications can be handled with standard delivery hardware (avoiding custom design).
• Fibers are flexible and, within the limitations of minimum bend radius, can follow any desired route to the workpiece.
• The workpiece may be held stationary while the fiber and output optics move during processing making them the ideal delivery system for use with robotic manipulation.
• Fibers make the design of time and energy sharing beam distribution systems a practical possibility. The use of such systems significantly increases the flexibility and versatility of individual lasers by allowing them to address multiple workstations or produce multiple simultaneous outputs.
• Access to the laser head for routine maintenance is improved since the positioning of the head is not dictated by the beam delivery system. The low-cost fiber can be delivered to areas that are dangerous because of explosives or radiation while the laser head is located in a non-hazardous area.
• Spot size at focus does not alter with changes of average power.

The optics of fiber delivery are simple and straightforward. Fiber optics used for laser delivery are typically step-index fibers. This type of fiber consists of an optically uniform core between 200 and 1500 um in diameter, surrounded by a thin cladding which has slightly different optical properties.

There are several options to fiber optic beam delivery. The first is single-fiber delivery from a single laser. This type of delivery is generally used for a dedicated production process or in development labs where moving the beam delivery to other workstations is infrequent. The choice of a single-fiber delivery is easily justified by its ease of use, ease of integration to workstations, and the capability for upgrading the system with other options in the future. Other reasons for single-fiber delivery are for robotic delivery of the laser beam and other multiaxis systems where conventional delivery would be a nightmare. With fibers, the output housing is mounted on the final-motion component so integration is incredibly economical and simple.

Another fiber delivery option is time sharing, whereby all of the laser output can be directed into any one of the several fibers on demand. A single laser with this system can provide laser energy to several different workstations switching among them at up to 40 Hz. These systems are typically used for laser welding at many different workstations, or to deliver the laser beam to separate areas of one large assembly station.

The last option is termed energy sharing. These systems divide the laser output and send the energy into several fibers at the same time. Mirrors skim portions of the beam from the laser and divert them into the input housings for each of the fibers.

The relative extent that each skimming mirror is moved into the beam path determines the sharing ratio. Typical energy-share systems can split the beam into as many as four fibers. These systems are used to weld many parts simultaneously, in order to increase throughput, or to eliminate the part distortions that often result from sequential welding of a single assembly.

The system computer creates marking images by sending beam-motion signals to the galvanometer drivers while simultaneously blanking the laser beam between marking strokes. The motion of the galvanometer-mounted mirrors directs the marking beam across the target surface much like a pencil on paper to draw alphanumeric and graphic images.

Laser selectionLaser marking uses the high power density of the focused laser beam to generate heat on the work surface and induce a thermal reaction. A readable, contrasting line is produced by increasing the target surface to annealing temperatures, the melting point or to vaporization temperatures. Annealing and melting are employed to induce a contrasting color change on a wide variety of metallic’s as well as plastics, ceramics and other nonmetallic’s. The fastest marking speeds are obtained by increasing the temperature to the vaporization point to engrave metallic’s and many nonmetallic’s.

The near-infrared wavelength of the Nd:YAG laser is well suited to most metallic’s and many plastics. The Nd:YAG can anneal or melt in both the CW and pulsed mode and can provide the necessary peak pulsed power to engrave. With many materials, the Nd:YAG can simultaneously engrave the surface and induce a contrasting color change in the engraved trough.

The far-infrared wavelength of the CO2 laser is compatible with plastics, ceramics and organic materials. However, without the high-peak-power capability required to achieve vaporization temperatures, the CO2 laser is limited to annealing or melting the surface.

AdvantagesBeam-steered laser marking offers several advantages over other marking methods. Most apparent is the unique combination of speed, permanence and the flexibility of computer control. Although other technologies can provide one or two of these attributes, no other method offers all three to the same degree.

Many users also benefit from the noncontact nature of laser marking. The only force applied to the part during the marking cycle is the very localized thermal effect of the laser beam. No additional physical force is applied, with the exception of any appropriate part-handling motion designed into the system. Silicon wafers, silicon disk drive read/write heads and many medical devices are examples of components that are too fragile for any type of mechanical marking. In addition, laser marking provides the permanence necessary to satisfy image-lifetime requirements, while printed marking does not.

Laser-marking systems also excel at creating intricate graphic images. Nd:YAG lasers can produce marking-line widths on the order of 0.001 inch or less, which, when combined with marking resolution of 0.0002 inch/step, can produce images with much more detail than mechanical contact or stencil systems.

Regardless of the specific process justifications for incorporating laser marking, the application of the technology can result in significant cost savings. With operating costs for the Nd:YAG system, users have reported cost savings of greater than 90 percent and associated reductions in quality control and inventory expenses.

As manufacturing industries continue to automate their manufacturing processes, incorporate aftershipment traceability, reduce manufacturing cycle times, apply more sophisticated graphics and develop products requiring new marking techniques, the laser-marking manufacturers will continue to improve the power, speed, image-generation capabilities and user-friendliness of their products.