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