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Beam-steered Laser Marking of Plastics

Beam-steered Nd:YAG (Neodymium:Yttrium Aluminum Garnet) laser marking provides a unique combination of speed, permanence, and imaging versatility in a noncontact marking process. Laser marking can generate considerable savings in reduced manufacturing and tooling costs; elimination of secondary processes and consumable disposal; and reduced inventory expense, quality-control costs, and maintenance downtime. Laser marking frequently improves the aesthetic appearance of the marking image, thereby increasing the product's perceived value.

Of all materials, plastics are the most challenging in terms of the laser's interaction with the material and the required image quality. The wide variety of material chemistries and colors and the aesthetic requirements of most plastics applications require special consideration in both material chemistry and imaging techniques. The successful implementation of laser marking technology requires a working knowledge of the laser marker's function and capabilities and a committed, team approach by the user.

Marking Fundamentals
Laser marking is a thermal process that employs a high-intensity beam of focused laser light to create a contrasting mark on the material surface. As the target material absorbs the laser light, the surface temperature increases to induce a color change in the material and/or vaporization of material to engrave the surface.

Beam-steered laser marking employs mirrors mounted on high-speed, computer-controlled galvanometers to direct the laser beam across the target surface. Each galvanometer provides one axis of beam motion in the marking field. A multi-element, flat-field lens assembly subsequently focuses the laser light to achieve high power density on the work surface while maintaining the focused-spot travel on a flat plane. The laser output is gated to blank the beam between marking strokes.

Marking can be accomplished at speeds of up to 5000 mm/sec with positioning speeds between marking strokes of 50,000 mm/sec. Because the process relies on heat conduction into the plastic, marking speeds are usually slower than the system's maximum capability to allow sufficient conduction to achieve the desired results.

The beam-steered marker can duplicate virtually any black-and-white image, including variable line widths and images as small as 0.0001 inch. Present computer-imaging technology produces highly intricate graphics with line widths, resolution, and accuracy well below 0.001 inch. Because the image is created by "drawing" with the laser beam, the marking time is dependent on the amount and complexity of the text and graphics. With computer-generated imaging, any graphic element or the entire marking program can be instantly changed before a new part is positioned for marking.

Nd:YAG lasers amplify light of 1.06mm wavelength in the near-infrared. They are unique among the different types of lasers in that they operate much like an "optical capacitor." In pulsed operation, the Nd:YAG laser stores energy between pulses, resulting in peak powers of kilowatts of light energy. A Nd:YAG laser emitting 75 watts of continuous light, pulsed at 1 kHz, emits a train of pulses with peak powers of 110,000 watts. The "optical capacitor" effect provides the peak power necessary to vaporize material. For plastics applications, the laser must also be run in a "top hat" mode, where the power distribution is fairly even across the cross section of the laser beam in order to eliminate "hot spots" in the marking path.

The beam-steered Nd:YAG marker frequently replaces acid and electro-etch systems, stamping and punching systems, and those other marking systems that permanently mark products by imprinting or engraving. It also replaces other, less permanent printing systems, including ink jet.

Uncoated Plastics
Most uncoated plastics must be doped with a material reflective to the laser wavelength to prevent over-absorption of the laser light, which results in loss of control of the temperature rise and excessive melting on the surface. Light-colored plastics are doped with mica, titanium dioxide or carbon-containing materials. The heat generated by absorption of the laser light causes the carbon to migrate to the surface, producing a contrasting dark mark against the unaltered background plastic.

Plastics are semitransparent to the near-infrared wavelength of the Nd:YAG laser. Depending on the degree of transparency and the laser output power, the laser beam can alter the material surface to depths of more than 0.025 mm without achieving vaporization temperature on the surface. If material vaporization occurs, the layer of carbon is thinned and the marking image will appear washed out.

There has been considerable success in altering the depth of carbon migration to create gray-scale graphics on light plastics. Adjusting the power and/or pulse rate of the laser controls the depth of penetration and therebv the darkness of the mark. Increasing the laser power will increase the overall depth of penetration and thickness of the carbon layer. Increasing the pulse rate will result in a longer pulse width and lower peak power. The longer exposure also increases the depth of penetration and associated carbon layer.

Dark plastic is doped with a material that produces a lighter color as the material expands and the density decreases. As the temperature of the plastic increases, the plastic expands to form a "blister" on the surface and a lighter-colored mark. As with light plastics, the temperature must be tightly controlled to avoid over absorption. If the temperature rises too high and the blister bursts, material is lost and the mark will lose contrast.

Not all plastics require dopant to achieve a contrasting mark. Several plastics do yield excellent results without additives; for example, most black polycarbonates produce a snow-white mark without altering the chemistry.

Coated Plastics
Coated plastics consist of a solid, translucent, or transparent plastic with one or more coats of ink or paint. The marking image is created by achieving vaporization temperature on the surface to remove the top coat and expose the underlying plastic or second coat.

Coated plastics allow a great deal of control over color selection and marking contrast. Transparent plastics allow the designer to use an underlying part to establish the background color (marking image) while the top coat determines the foreground color. Solid plastics establish their own background with the color of the plastic. Translucent plastics are frequently used for back-lit applications. The plastic is initially coated with a white paint and overlaid with a dark top coat. The laser removes the top coat, exposing the white paint for daytime visibility. When the part is back-lit at night, the lighting illuminates the translucent plastic from behind and the marking image appears in the color of the plastic.

The paint or ink used must be conducive to laser processing. Standard paints and inks are neither predictable nor controllable when exposed to the laser output. The inks burn easily and can mix with the underlying plastic while in the molten liquid state. Laser-compatible inks are mixed with a silicone-based material reflective to the laser output, thereby reducing the ink's light absorption and rate of thermal reaction. Paints must be suitable for high-temperature processing and be free of any contaminants that may absorb the laser wavelength and speed up the thermal rise.

To achieve a quality image, the top coat must be completely removed with minimal impact on the underlying plastic or secondary coat. To maximize the ratio of light absorption between the two layers, the top coat must always be a dark color and the contrasting underlying layer must be a light color. The dark color will absorb a comparatively higher percentage of the laser light, resulting in a higher surface temperature, while the light color reflects a higher percentage and minimizes the temperature rise. The underlying plastic, paint, or ink should also be thick enough to tolerate a minor amount of material removal during marking.

Marking coated plastics is a multi-step process in which the first marking pass removes the majority of the top coating. The remaining residue is removed with a second, lower-power pass to minimize the effect to the underlying material. For precise edge definition, the outline of the image is marked prior to filling in the image. The outline is marked with a heavy edge pass (i.e., 50 kHz, 250 mm/sec, 2.5 watts) followed by a lower-power cleanup pass (50 kHz, 250 mm/sec,1.75 watts). The image is then filled, if desired, with a heavy fill pass (50 kHz, 650 mm/sec, 6 watts) and subsequent cleanup pass (50 kHz, 6.50 mm/sec, 4.5 watts). Care in determining the process parameters for each pass and the edge and fill beam paths will result in a crisp, high-contrast, high-quality marking image.

Preparation and Installation
Perhaps the most critical element in the successful application of laser marking is the composition of the part programs. When replacing an existing marking technology, one must allow up to six months for conversion of existing art work to part-marking computer programs. Even if the present artwork resides in AutoCAD files, time must be allotted to convert the files to optimized marking programs.

Many users start with thousands of sheets of Mylar artwork. (Mylar is a DuPont trade name.) Each Mylar film is scanned to create a bitmap image. The scanned bitmap could be directly converted to the laser marker format with good image quality, but the cycle time would be unnecessarily long, with excessive marking line overlap.

For best results, import the scanned bitmap into AutoCAD as a positional template. Create a separate marking "logo" for each alphanumeric character and graphic image, and, in AutoCAD, place each logo in position on a separate layer, using the bitmap template as a positioning guide. A library of optimized logos facilitates the creation of programs from the scanned artwork, allows nonstandard text kerning and line leading, and ensures low cycle time and high image quality. After all the logos are in place, the template layer is removed, and the final CAD file is converted to the laser marker program format.

If the art work already exists in a CAD file format, the image elements could be optimized without using a separate library of logos. Every element including repetitive elements shared between drawings must be individually optimized. It will take considerably longer to convert large quantities of files, and there is no guarantee that every clement is optimized correctly. It is far more efficient to use the original AutoCAD file as the placement template for optimized logos.

Implementation of beam-steered laser marking requires a team effort. With cooperative implementation. manufacturing can ensure product flow and integration with existing controls, the materials department ensures that plastics and coatings are appropriate for laser marking, and engineering will produce part-marking programs with low cycle times and high-quality images. Careful team planning, preparation, and execution will result in a smooth application of laser marking technology and the associated benefits in manufacturing efficiencies, quality, and product value.

About the Author

Richard Stevenson is the Sales Director for Control Micro Systems, Inc. a manufacturer of beam-steered laser marking systems. He has published and presented numerous technical papers and articles on laser marking in trade publications. For information on Industrial Laser Systems, Welding, Engraving, Cutting, Etching or Marking call 407-679-9716 or email sales@cmslaser.com