| Beam-steered Nd:YAG (Neodymium:Yttrium Aluminum | | | | yield excellent results without additives; for example, |
| Garnet) laser marking provides a unique combination | | | | most black polycarbonates produce a snow-white |
| of speed, permanence, and imaging versatility in a | | | | mark without altering the chemistry.Coated |
| noncontact marking process. Laser marking can | | | | PlasticsCoated plastics consist of a solid, translucent, |
| generate considerable savings in reduced | | | | or transparent plastic with one or more coats of ink |
| manufacturing and tooling costs; elimination of | | | | or paint. The marking image is created by achieving |
| secondary processes and consumable disposal; and | | | | vaporization temperature on the surface to remove |
| reduced inventory expense, quality-control costs, and | | | | the top coat and expose the underlying plastic or |
| maintenance downtime. Laser marking frequently | | | | second coat.Coated plastics allow a great deal of |
| improves the aesthetic appearance of the marking | | | | control over color selection and marking contrast. |
| image, thereby increasing the product's perceived | | | | Transparent plastics allow the designer to use an |
| value.Of all materials, plastics are the most challenging | | | | underlying part to establish the background color |
| in terms of the laser's interaction with the material | | | | (marking image) while the top coat determines the |
| and the required image quality. The wide variety of | | | | foreground color. Solid plastics establish their own |
| material chemistries and colors and the aesthetic | | | | background with the color of the plastic. Translucent |
| requirements of most plastics applications require | | | | plastics are frequently used for back-lit applications. |
| special consideration in both material chemistry and | | | | The plastic is initially coated with a white paint and |
| imaging techniques. The successful implementation of | | | | overlaid with a dark top coat. The laser removes the |
| laser marking technology requires a working | | | | top coat, exposing the white paint for daytime |
| knowledge of the laser marker's function and | | | | visibility. When the part is back-lit at night, the lighting |
| capabilities and a committed, team approach by the | | | | illuminates the translucent plastic from behind and the |
| user.Marking FundamentalsLaser marking is a thermal | | | | marking image appears in the color of the plastic.The |
| process that employs a high-intensity beam of | | | | paint or ink used must be conducive to laser |
| focused laser light to create a contrasting mark on | | | | processing. Standard paints and inks are neither |
| the material surface. As the target material absorbs | | | | predictable nor controllable when exposed to the |
| the laser light, the surface temperature increases to | | | | laser output. The inks burn easily and can mix with |
| induce a color change in the material and/or | | | | the underlying plastic while in the molten liquid state. |
| vaporization of material to engrave the | | | | Laser-compatible inks are mixed with a silicone-based |
| surface.Beam-steered laser marking employs mirrors | | | | material reflective to the laser output, thereby |
| mounted on high-speed, computer-controlled | | | | reducing the ink's light absorption and rate of thermal |
| galvanometers to direct the laser beam across the | | | | reaction. Paints must be suitable for high-temperature |
| target surface. Each galvanometer provides one axis | | | | processing and be free of any contaminants that |
| of beam motion in the marking field. A multi-element, | | | | may absorb the laser wavelength and speed up the |
| flat-field lens assembly subsequently focuses the | | | | thermal rise.To achieve a quality image, the top coat |
| laser light to achieve high power density on the work | | | | must be completely removed with minimal impact on |
| surface while maintaining the focused-spot travel on | | | | the underlying plastic or secondary coat. To maximize |
| a flat plane. The laser output is gated to blank the | | | | the ratio of light absorption between the two layers, |
| beam between marking strokes.Marking can be | | | | the top coat must always be a dark color and the |
| accomplished at speeds of up to 5000 mm/sec with | | | | contrasting underlying layer must be a light color. The |
| positioning speeds between marking strokes of | | | | dark color will absorb a comparatively higher |
| 50,000 mm/sec. Because the process relies on heat | | | | percentage of the laser light, resulting in a higher |
| conduction into the plastic, marking speeds are usually | | | | surface temperature, while the light color reflects a |
| slower than the system's maximum capability to allow | | | | higher percentage and minimizes the temperature rise. |
| sufficient conduction to achieve the desired | | | | The underlying plastic, paint, or ink should also be |
| results.The beam-steered marker can duplicate | | | | thick enough to tolerate a minor amount of material |
| virtually any black-and-white image, including variable | | | | removal during marking.Marking coated plastics is a |
| line widths and images as small as 0.0001 inch. Present | | | | multi-step process in which the first marking pass |
| computer-imaging technology produces highly intricate | | | | removes the majority of the top coating. The |
| graphics with line widths, resolution, and accuracy well | | | | remaining residue is removed with a second, |
| below 0.001 inch. Because the image is created by | | | | lower-power pass to minimize the effect to the |
| "drawing" with the laser beam, the marking time is | | | | underlying material. For precise edge definition, the |
| dependent on the amount and complexity of the | | | | outline of the image is marked prior to filling in the |
| text and graphics. With computer-generated imaging, | | | | image. The outline is marked with a heavy edge pass |
| any graphic element or the entire marking program | | | | (i.e., 50 kHz, 250 mm/sec, 2.5 watts) followed by a |
| can be instantly changed before a new part is | | | | lower-power cleanup pass (50 kHz, 250 mm/sec,1.75 |
| positioned for marking.Nd:YAG lasers amplify light of | | | | watts). The image is then filled, if desired, with a |
| 1.06mm wavelength in the near-infrared. They are | | | | heavy fill pass (50 kHz, 650 mm/sec, 6 watts) and |
| unique among the different types of lasers in that | | | | subsequent cleanup pass (50 kHz, 6.50 mm/sec, 4.5 |
| they operate much like an "optical capacitor." In | | | | watts). Care in determining the process parameters |
| pulsed operation, the Nd:YAG laser stores energy | | | | for each pass and the edge and fill beam paths will |
| between pulses, resulting in peak powers of kilowatts | | | | result in a crisp, high-contrast, high-quality marking |
| of light energy. A Nd:YAG laser emitting 75 watts of | | | | image.Preparation and InstallationPerhaps the most |
| continuous light, pulsed at 1 kHz, emits a train of | | | | critical element in the successful application of laser |
| pulses with peak powers of 110,000 watts. The | | | | marking is the composition of the part programs. |
| "optical capacitor" effect provides the peak power | | | | When replacing an existing marking technology, one |
| necessary to vaporize material. For plastics | | | | must allow up to six months for conversion of |
| applications, the laser must also be run in a "top hat" | | | | existing art work to part-marking computer |
| mode, where the power distribution is fairly even | | | | programs. Even if the present artwork resides in |
| across the cross section of the laser beam in order | | | | AutoCAD files, time must be allotted to convert the |
| to eliminate "hot spots" in the marking path.The | | | | files to optimized marking programs.Many users start |
| beam-steered Nd:YAG marker frequently replaces | | | | with thousands of sheets of Mylar artwork. (Mylar is |
| acid and electro-etch systems, stamping and punching | | | | a DuPont trade name.) Each Mylar film is scanned to |
| systems, and those other marking systems that | | | | create a bitmap image. The scanned bitmap could be |
| permanently mark products by imprinting or | | | | directly converted to the laser marker format with |
| engraving. It also replaces other, less permanent | | | | good image quality, but the cycle time would be |
| printing systems, including ink jet.Uncoated | | | | unnecessarily long, with excessive marking line |
| PlasticsMost uncoated plastics must be doped with a | | | | overlap.For best results, import the scanned bitmap |
| material reflective to the laser wavelength to prevent | | | | into AutoCAD as a positional template. Create a |
| over-absorption of the laser light, which results in loss | | | | separate marking "logo" for each alphanumeric |
| of control of the temperature rise and excessive | | | | character and graphic image, and, in AutoCAD, place |
| melting on the surface. Light-colored plastics are | | | | each logo in position on a separate layer, using the |
| doped with mica, titanium dioxide or carbon-containing | | | | bitmap template as a positioning guide. A library of |
| materials. The heat generated by absorption of the | | | | optimized logos facilitates the creation of programs |
| laser light causes the carbon to migrate to the | | | | from the scanned artwork, allows nonstandard text |
| surface, producing a contrasting dark mark against | | | | kerning and line leading, and ensures low cycle time |
| the unaltered background plastic.Plastics are | | | | and high image quality. After all the logos are in place, |
| semitransparent to the near-infrared wavelength of | | | | the template layer is removed, and the final CAD file |
| the Nd:YAG laser. Depending on the degree of | | | | is converted to the laser marker program format.If |
| transparency and the laser output power, the laser | | | | the art work already exists in a CAD file format, the |
| beam can alter the material surface to depths of | | | | image elements could be optimized without using a |
| more than 0.025 mm without achieving vaporization | | | | separate library of logos. Every element including |
| temperature on the surface. If material vaporization | | | | repetitive elements shared between drawings must |
| occurs, the layer of carbon is thinned and the | | | | be individually optimized. It will take considerably |
| marking image will appear washed out.There has | | | | longer to convert large quantities of files, and there is |
| been considerable success in altering the depth of | | | | no guarantee that every clement is optimized |
| carbon migration to create gray-scale graphics on light | | | | correctly. It is far more efficient to use the original |
| plastics. Adjusting the power and/or pulse rate of the | | | | AutoCAD file as the placement template for |
| laser controls the depth of penetration and therebv | | | | optimized logos.Implementation of beam-steered laser |
| the darkness of the mark. Increasing the laser power | | | | marking requires a team effort. With cooperative |
| will increase the overall depth of penetration and | | | | implementation. manufacturing can ensure product |
| thickness of the carbon layer. Increasing the pulse | | | | flow and integration with existing controls, the |
| rate will result in a longer pulse width and lower peak | | | | materials department ensures that plastics and |
| power. The longer exposure also increases the depth | | | | coatings are appropriate for laser marking, and |
| of penetration and associated carbon layer.Dark | | | | engineering will produce part-marking programs with |
| plastic is doped with a material that produces a lighter | | | | low cycle times and high-quality images. Careful team |
| color as the material expands and the density | | | | planning, preparation, and execution will result in a |
| decreases. As the temperature of the plastic | | | | smooth application of laser marking technology and |
| increases, the plastic expands to form a "blister" on | | | | the associated benefits in manufacturing efficiencies, |
| the surface and a lighter-colored mark. As with light | | | | quality, and product value.Richard Stevenson is the |
| plastics, the temperature must be tightly controlled to | | | | Sales Director for Control Micro Systems, Inc. a |
| avoid over absorption. If the temperature rises too | | | | manufacturer of beam-steered laser marking |
| high and the blister bursts, material is lost and the | | | | systems. He has published and presented numerous |
| mark will lose contrast.Not all plastics require dopant | | | | technical papers and articles on laser marking in trade |
| to achieve a contrasting mark. Several plastics do | | | | publications. |