Beam-Steered Laser Marking of Plastics

Beam-steered Nd:YAG (Neodymium:Yttrium Aluminumyield excellent results without additives; for example,
Garnet) laser marking provides a unique combinationmost black polycarbonates produce a snow-white
of speed, permanence, and imaging versatility in amark without altering the chemistry.Coated
noncontact marking process. Laser marking canPlasticsCoated plastics consist of a solid, translucent,
generate considerable savings in reducedor transparent plastic with one or more coats of ink
manufacturing and tooling costs; elimination ofor paint. The marking image is created by achieving
secondary processes and consumable disposal; andvaporization temperature on the surface to remove
reduced inventory expense, quality-control costs, andthe top coat and expose the underlying plastic or
maintenance downtime. Laser marking frequentlysecond coat.Coated plastics allow a great deal of
improves the aesthetic appearance of the markingcontrol over color selection and marking contrast.
image, thereby increasing the product's perceivedTransparent plastics allow the designer to use an
value.Of all materials, plastics are the most challengingunderlying 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 offoreground color. Solid plastics establish their own
material chemistries and colors and the aestheticbackground with the color of the plastic. Translucent
requirements of most plastics applications requireplastics are frequently used for back-lit applications.
special consideration in both material chemistry andThe plastic is initially coated with a white paint and
imaging techniques. The successful implementation ofoverlaid with a dark top coat. The laser removes the
laser marking technology requires a workingtop coat, exposing the white paint for daytime
knowledge of the laser marker's function andvisibility. When the part is back-lit at night, the lighting
capabilities and a committed, team approach by theilluminates the translucent plastic from behind and the
user.Marking FundamentalsLaser marking is a thermalmarking image appears in the color of the plastic.The
process that employs a high-intensity beam ofpaint or ink used must be conducive to laser
focused laser light to create a contrasting mark onprocessing. Standard paints and inks are neither
the material surface. As the target material absorbspredictable nor controllable when exposed to the
the laser light, the surface temperature increases tolaser output. The inks burn easily and can mix with
induce a color change in the material and/orthe underlying plastic while in the molten liquid state.
vaporization of material to engrave theLaser-compatible inks are mixed with a silicone-based
surface.Beam-steered laser marking employs mirrorsmaterial reflective to the laser output, thereby
mounted on high-speed, computer-controlledreducing the ink's light absorption and rate of thermal
galvanometers to direct the laser beam across thereaction. Paints must be suitable for high-temperature
target surface. Each galvanometer provides one axisprocessing 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 thethermal rise.To achieve a quality image, the top coat
laser light to achieve high power density on the workmust be completely removed with minimal impact on
surface while maintaining the focused-spot travel onthe underlying plastic or secondary coat. To maximize
a flat plane. The laser output is gated to blank thethe ratio of light absorption between the two layers,
beam between marking strokes.Marking can bethe top coat must always be a dark color and the
accomplished at speeds of up to 5000 mm/sec withcontrasting underlying layer must be a light color. The
positioning speeds between marking strokes ofdark color will absorb a comparatively higher
50,000 mm/sec. Because the process relies on heatpercentage of the laser light, resulting in a higher
conduction into the plastic, marking speeds are usuallysurface temperature, while the light color reflects a
slower than the system's maximum capability to allowhigher percentage and minimizes the temperature rise.
sufficient conduction to achieve the desiredThe underlying plastic, paint, or ink should also be
results.The beam-steered marker can duplicatethick enough to tolerate a minor amount of material
virtually any black-and-white image, including variableremoval during marking.Marking coated plastics is a
line widths and images as small as 0.0001 inch. Presentmulti-step process in which the first marking pass
computer-imaging technology produces highly intricateremoves the majority of the top coating. The
graphics with line widths, resolution, and accuracy wellremaining residue is removed with a second,
below 0.001 inch. Because the image is created bylower-power pass to minimize the effect to the
"drawing" with the laser beam, the marking time isunderlying material. For precise edge definition, the
dependent on the amount and complexity of theoutline 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 islower-power cleanup pass (50 kHz, 250 mm/sec,1.75
positioned for marking.Nd:YAG lasers amplify light ofwatts). The image is then filled, if desired, with a
1.06mm wavelength in the near-infrared. They areheavy fill pass (50 kHz, 650 mm/sec, 6 watts) and
unique among the different types of lasers in thatsubsequent cleanup pass (50 kHz, 6.50 mm/sec, 4.5
they operate much like an "optical capacitor." Inwatts). Care in determining the process parameters
pulsed operation, the Nd:YAG laser stores energyfor each pass and the edge and fill beam paths will
between pulses, resulting in peak powers of kilowattsresult in a crisp, high-contrast, high-quality marking
of light energy. A Nd:YAG laser emitting 75 watts ofimage.Preparation and InstallationPerhaps the most
continuous light, pulsed at 1 kHz, emits a train ofcritical element in the successful application of laser
pulses with peak powers of 110,000 watts. Themarking is the composition of the part programs.
"optical capacitor" effect provides the peak powerWhen replacing an existing marking technology, one
necessary to vaporize material. For plasticsmust 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 evenprograms. Even if the present artwork resides in
across the cross section of the laser beam in orderAutoCAD files, time must be allotted to convert the
to eliminate "hot spots" in the marking path.Thefiles to optimized marking programs.Many users start
beam-steered Nd:YAG marker frequently replaceswith thousands of sheets of Mylar artwork. (Mylar is
acid and electro-etch systems, stamping and punchinga DuPont trade name.) Each Mylar film is scanned to
systems, and those other marking systems thatcreate a bitmap image. The scanned bitmap could be
permanently mark products by imprinting ordirectly converted to the laser marker format with
engraving. It also replaces other, less permanentgood image quality, but the cycle time would be
printing systems, including ink jet.Uncoatedunnecessarily long, with excessive marking line
PlasticsMost uncoated plastics must be doped with aoverlap.For best results, import the scanned bitmap
material reflective to the laser wavelength to preventinto AutoCAD as a positional template. Create a
over-absorption of the laser light, which results in lossseparate marking "logo" for each alphanumeric
of control of the temperature rise and excessivecharacter and graphic image, and, in AutoCAD, place
melting on the surface. Light-colored plastics areeach logo in position on a separate layer, using the
doped with mica, titanium dioxide or carbon-containingbitmap template as a positioning guide. A library of
materials. The heat generated by absorption of theoptimized logos facilitates the creation of programs
laser light causes the carbon to migrate to thefrom the scanned artwork, allows nonstandard text
surface, producing a contrasting dark mark againstkerning and line leading, and ensures low cycle time
the unaltered background plastic.Plastics areand high image quality. After all the logos are in place,
semitransparent to the near-infrared wavelength ofthe template layer is removed, and the final CAD file
the Nd:YAG laser. Depending on the degree ofis converted to the laser marker program format.If
transparency and the laser output power, the laserthe art work already exists in a CAD file format, the
beam can alter the material surface to depths ofimage elements could be optimized without using a
more than 0.025 mm without achieving vaporizationseparate library of logos. Every element including
temperature on the surface. If material vaporizationrepetitive elements shared between drawings must
occurs, the layer of carbon is thinned and thebe individually optimized. It will take considerably
marking image will appear washed out.There haslonger to convert large quantities of files, and there is
been considerable success in altering the depth ofno guarantee that every clement is optimized
carbon migration to create gray-scale graphics on lightcorrectly. It is far more efficient to use the original
plastics. Adjusting the power and/or pulse rate of theAutoCAD file as the placement template for
laser controls the depth of penetration and therebvoptimized logos.Implementation of beam-steered laser
the darkness of the mark. Increasing the laser powermarking requires a team effort. With cooperative
will increase the overall depth of penetration andimplementation. manufacturing can ensure product
thickness of the carbon layer. Increasing the pulseflow and integration with existing controls, the
rate will result in a longer pulse width and lower peakmaterials department ensures that plastics and
power. The longer exposure also increases the depthcoatings are appropriate for laser marking, and
of penetration and associated carbon layer.Darkengineering will produce part-marking programs with
plastic is doped with a material that produces a lighterlow cycle times and high-quality images. Careful team
color as the material expands and the densityplanning, preparation, and execution will result in a
decreases. As the temperature of the plasticsmooth application of laser marking technology and
increases, the plastic expands to form a "blister" onthe associated benefits in manufacturing efficiencies,
the surface and a lighter-colored mark. As with lightquality, and product value.Richard Stevenson is the
plastics, the temperature must be tightly controlled toSales Director for Control Micro Systems, Inc. a
avoid over absorption. If the temperature rises toomanufacturer of beam-steered laser marking
high and the blister bursts, material is lost and thesystems. He has published and presented numerous
mark will lose contrast.Not all plastics require dopanttechnical papers and articles on laser marking in trade
to achieve a contrasting mark. Several plastics dopublications.