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Advantages And Disadvantages Of Etching With Beam-steered Laser
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.
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