This year, over one-third of all material processing lasers will be installed for product or package marking applications. Since their introduction in the early-1970's, laser markers have evolved as an effective tool for manufacturers who require a combination of speed, permanence, and image flexibility not available from more traditional marking technologies.

Two marking system designs have emerged with notably different strengths and weaknesses. Careful consideration of these laser and imaging optics combinations can provide the optimum tool for a wide range of marking requirements. Process Fundamentals

Laser marking is a thermal process that employs a high-intensity beam of focused laser light to create a contrasting mark. The laser beam increases the surface temperature to induce either a color change in the material and/or displace material by vaporization to engrave the surface. Both marking system configurations utilize this principle of surface modification but differ in the method used to project the laser beam and create the marking image.

The beam-steered laser marker provides the greatest degree of image manipulation. To create the marking image, two beam-steering mirrors mounted on high-speed, computer-controlled galvanometers direct the laser beam across the target surface. Each galvanometer provides one axis of beam motion in the marking field. The beam projects through a multi-element, flat-field lens assembly after reflecting off the final steering mirror. The lens assembly focuses the laser light to achieve the highest power density possible on the work surface while maintaining the focused spot travel on a flat plane. The laser output is gated between marking strokes. This design offers the user the advantages of a computer generated marking image and utilization of the entire laser output for the highest marking power possible.

The mask or "stencil" marking system sacrifices image quality and versatility for significantly increased marking speed. The marking image is created by enlarging the laser beam, projecting it through a copper stencil of the desired image, and refocusing the beam on the target surface to "burn" the image into the material. A single pulse of the laser creates the entire image. If the alphanumeric characters must be altered part-to-part, (i.e., serialization, etc.), computer-controlled rotary stencil wheels index the characters. This technique is aesthetically limiting in that images exhibit a "stencil" appearance with breaks in the marking lines. Since the mask blocks a high percentage of the laser beam, marking power and resultant surface penetration is limited. Laser and Imaging Combinations

Beam-steered Nd:YAG

The combination of the Nd:YAG (Neodymium:Yttrium Aluminum Garnet) laser and the beam-steered delivery optics marks the widest range of materials and provides the versatility of computer controlled image generation.

Nd:YAG lasers amplify light in the near-infrared at 1.06 mm. Metallic materials absorb a comparatively high percentage of the light in this region of the spectrum. In the pulsed mode, the Nd:YAG laser produces peak powers considerably higher than the normal continuous-wave output. A 90 watt CW Nd:YAG laser, pulsed at 1 kHz, will emit a train of pulses with peak powers of 110,000 watts. The Nd:YAG lasers ability to emulate an "optical capacitor" provides the power necessary to vaporize metallics and other materials. The high peak power will vaporize material up to 0.005 inches deep in a single pass or greater with multiple passes. The non-metallic materials normally associated with the far-infrared wavelength of the CO2 laser are usually highly reflective to the Nd:YAG. However, the high peak power of the Nd:YAG can often overcome the higher reflectivity. Some overlap does occur among many plastics that absorb both wavelengths equally well.

The beam-steered marker can duplicate virtually any vector graphic image including variable line widths and images as small as 0.010 inch or less. In addition, the computer can instantly change any graphic element or the entire marking program before a new part is positioned for marking. The Nd:YAG laser offers a greater range of adjustable process variables to achieve a specific material modification but at a correspondingly higher purchase price than the CO2 laser.

Beam-steered CO2

The continuous-wave CO2 laser can also be combined with the beam-steered delivery system. CO2 lasers emit a narrow bandwidth of light in the far infrared at 10.6 mm. This wavelength is most suitable for organic materials such as paper and other wood products, many plastics, removing thin layers of ink or paint from a substrate, and for marking ceramics. It does not produce high peak powers when pulsed.

Typically utilizing laser powers up to 50 watts, these systems combine the far infrared wavelength with the image control and flexibility of beam-steered image generation. Typical uses include serialization of ceramic and plastic products that require high-quality graphics such as company logos and/or significant amounts of additional alphanumeric text. The lower power CO2 marker does not provide the power to "engrave" substrates but, due to the comparative simplicity of design, can be purchased at a lower cost than the beam-steered Nd:YAG marker.

Mask CO2

Applications that require high speed but not high power and do not vary the marking image except for alphanumeric text (i.e., serialization, date code, etc.) utilize the mask CO2 marker. The CO2 laser is pulsed at rates of up to 1,200 pulses per minute. The high repetition rate provides marking of parts "on-the-fly" at high part-transfer speeds. Computer controlled masks can alter up to three lines of text at speeds of up to 720 parts per minute if the alphanumeric code must be changed.

Advantages and Disadvantages

Beam-steered Nd:YAG

The beam-steered Nd:YAG provides more marking power and far superior imaging than any other laser marker configuration. The available high peak power can mark or engrave a wide variety of materials including hardened metallics. Present computer technology produces highly intricate graphics with linewidths and accuracy's of less than 0.001 inch. Because “drawing” with the laser beam creates the image, the marking time is dependent on the amount of text and the complexity of any graphics. The Nd:YAG laser marker is the most costly of the three system configurations. The beam-steered Nd:YAG marker frequently replaces acid and electro-etch systems, stamping and punching systems, and those other marking systems which permanently mark products by imprinting or engraving. It also replaces ink jet and other color printing systems. Typical applications include marking pistons, bearings, valves, gears, and a multitude of other components in the automotive industry; heart pacemakers, replacement hip joints, and surgical tools in the medical industry; computer chassis, disk drives, and integrated circuits in the electronics industry; tool holders, drill bits, and cutting tools in the tool industry; and writing pens, nameplates, and golf club grips.

Beam-steered CO2

The acquisition and operating costs of the beam-steered CO2 marker are lower than the Nd:YAG marker due to the relative simplicity of the laser. Image generation is equal to that of the other beam-steered system while speed and depth of penetration are considerable lower due to the lower power of the CO2 laser. Although not as popular as the beam-steered Nd:YAG and mask CO2 markers, the beam-steered CO2 system is frequently used for marking general plastics and plastic and ceramic connectors and packages within the electronics industry.

Mask CO2

Although the mask CO2 does not offer the imaging capabilities of the beam-steered design, it is far superior in speed. Because a single pulse of the laser creates the entire image, throughput is typically limited only by the pulse rate of the laser and the transfer speed of the parts handling system. While the part must be stationary while marking with the beam-steered design, parts are marked in motion with mask systems. Depth of penetration is less than the beam-steered CO2 marker since the laser output is spread over a large area with correspondingly low power density.

Masked CO2 markers most frequently compete with ink-jet marking. The mask CO2 laser is often the marker of choice for sequenced coding, batch coding, open or closed date coding, and real-time coding of paper or cardboard, ink or paint coatings, glass, plastics, coated metals, and ceramics.

While the beam-steered design provides superior imaging and material penetration and the mask design provides superior speed, either system provides a better combination of speed, permanence, and imaging flexibility than other marking techniques. Many users also benefit from the non-contact nature of laser marking and the elimination of additive materials such as inks or paints.

The development of a successful marking application requires careful consideration of the laser output characteristics, the design of the optical beam delivery and image generation system, the properties of the target material, and the aesthetic and physical properties of the desired mark. Industrial laser marking systems provide prospective users with several system designs from which to choose to match the optimum marking performance with the users unique requirements.