Advancements in direct laser marking of plastics yield unprecedented marking
quality, contrast, and speed. This article presents the newest generation of
laser material science and laser equipment systems. With proper application,
laser marking can provide manufacturing advantages and bring value to a
product’s appearance and function.
Basic Principles of Laser Marking
Beam-steered Nd:YAG lasers (“YAG”) at 1064nm wavelength (near infrared
spectrum) are popular in the laser marking industry due to their emission
wavelength, power performance and versatility. This results in faster marking
speeds, higher quality, and greater production. As reference, the continuous
wave (CW) CO2 lasers operate at a wavelength of 10.6 µm (far infrared spectrum).
CW CO2 lasers generate comparatively much lower peak power and normally cannot
produce high contrast markings on many plastics.
The mechanism of laser marking is to irradiate the polymer with a localized
high-energy radiation source (laser). The radiant energy is then absorbed by the
material and converted to thermal energy. The thermal energy induces reactions
to occur in the material. Beam-steered YAG laser markers (arc lamp & diode light
pumping sources) utilize mirrors that are mounted on high speed computer
controlled galvanometers to direct the laser beam across the surface to be
marked. Each galvanometer, one on the Y-axis and one on the X-axis, provides the
beam motion within the marking field. A flat-field lens assembly focuses the
laser light to achieve high power density on the substrate surface. A basic YAG
laser marking configuration is shown below in Figure 1.
Laser Material Science
The material science chemistry for achieving high contrast laser marking is
both art and science. Since many polymers do not possess absorption properties
at 1064 nm, experts utilize additives and colorants (pigments & dyes) that
enhance the absorption of laser energy yielding contrasting color changes.
Contrary to popular belief, a single laser additive that solves all marking
problems does not exist. Vastly different chemistries and laser parameters are
used depending upon the desired marking contrast. Figure 2 shows “dark-on-light”
computer keycap (left), “light-on-dark” interior automotive lever (center), and
gold “color” advertising specialty product (right).
Three unique surface reactions are demonstrated in Figure 2. First, the
charring process occurs when the energy absorbed raises the local temperature of
the material surrounding the absorption site high enough to cause thermal
degradation of the polymer. While this can result in burning of the polymer in
the presence of oxygen, the limited supply of oxygen in the interior of the
substrate results in charring of the polymer to form a black or dark-on-light
Second, the foaming process occurs when the local polymer temperature
surrounding the absorption site is sufficiently high that the polymer generates
gases via burning or evaporation. The hot gases are themselves surrounded by
molten polymer and expand to form bubbles. If the energy of the laser is
controlled, foaming can result in bubbles that scatter light in a way that
results in white or light-on-dark marking contrast.
Third, laser energy is used to heat/degrade one colorant in a colorant
mixture resulting in a color change. An example is a mixture of carbon black and
a stable inorganic colorant. When heated, the carbon black is removed leaving
behind the inorganic colorant. These mixed colorant systems are dependent on
specific colorant stabilities and not all color changes are possible. Laser
formulations cannot be toxic or adversely affect the product’s appearance,
physical properties, or functional properties.
A recent advancement is Mark-it™ Laser Marking Pigment by BASF Corporation
(formally Engelhard Corporation). This additive product is an antimony-doped tin
oxide pigment that is easily dispersed in polymers. Mark-it™ pigment is the
first to receive U.S. Food and Drug Administration (FDA) approval for use in YAG
laser marking processes to generate dark markings (light markings also are
achievable by incorporating additional additives). The product has FDA approval
for use at loadings up to 0.5 percent in polyolefins that contact food under
conditions A-H of 21 CFR 178.3297 Colorants for Polymers.
Laser Marking Equipment Systems
All beam-steered YAG lasers are not created equal. The hardware and software
components a laser manufacturer incorporates into its systems makes significant
differences in marking quality, speed and versatility. When procuring laser
systems, it is important to remember there is not a single universal solution.
Each application is unique relative to the plastic substrate composition and
color, marking quality, speed, laser efficiency, contrast (dark-on-light,
light-on-dark, or color), and total system costs.
Beam quality output mode refers to the energy distribution within the laser
beam and is critical to marking performance. Lasers can be supplied by
manufacturers as multimode (MM), TEM00 (Transverse Electromagnetic Mode) or
anything in between including Low-Order Mode (LOM). These output modes relate to
factors including the beam divergence and power distribution across the diameter
of the laser beam. A TEM00 laser beam can be focused to the smallest size spot
that the focusing optics permit and the energy distribution in a TEM00 laser
beam is most intense at its center and tapers off uniformly from its center to
its edges. TEM00 laser output provides the highest beam quality. Multi-Mode (MM)
laser output provides the poorest beam quality. Reference Figure 3. Low-order
and TEM00 mode lasers are particularly well suited for high speed vector marking
of single-stroke alphanumerics, filled true-type fonts, and complex graphics
because of their ability to achieve a small focused spot with very high power
densities, resulting in a very narrow line with well defined edges that can be
drawn quickly. Most plastics applications are optimal using near TEM00 or TEM00
Power density is a function of focused laser spot size (laser power per unit
area, watts/cm2). This is different than the raw output power of the laser.
Focused laser spot size for any given focal length lens and laser wavelength is
a function of laser beam divergence which is controlled by laser configuration,
mode selecting aperture size and upcollimator (beam expander) magnification.
Pulse repetition rate (via acousto-optic Q-switch) and peak power density are
critical parameters in forming the mark and achieving the optimal contrast and
speed. High peak power at low frequency increases the surface temperature
rapidly, vaporizing the material while conducting minimal heat into the
substrate. As the pulse repetition increases, a lower peak power produces
minimal vaporization but creates more heat. Beam velocity (speed of the laser
beam across the work surface) is also a critical factor.
Two types of solid-state beam-steered YAG lasers are traditionally used for
marking plastics – lamp and diode-pumped systems (referred to as “lamp/YAG” and
“diode/YAG”). Major differences exist between these two laser types. Both lamp/YAG
and diode/YAG can potentially yield acceptable marking results relative to
marking contrast and speed. Table 1 provides comparative data (page 38) for
lamp/YAG and diode/YAG lasers ranging from low to high power and configured for
multimode (MM) to TEM00 beam quality modes.
For a direct comparison of diode/YAG versus lamp/YAG, it is important to
evaluate near equivalent lasers for a specific application, e.g., 100-Watt.
Using a 100-Watt laser as a basis, diode/YAG lasers are inherently more
efficient than lamp/YAG in terms of output beam power as a fraction of input
electrical power. Diode/YAG lasers rely on a bank of laser diodes as the optical
“pump” source for the YAG laser rod, rather than a krypton arc lamp. Laser
diodes are more sensitive than arc lamps to electrical noise so greater circuit
protection is required. Contrary to common perception, both high-powered diode
and lamp pumped lasers require a cooling system, although diode systems can use
smaller cooling units but require greater temperature control. Low power diode/YAG
systems are sometimes air-cooled. Diode/YAG lasers can produce TEM00 beam output
quality, resulting in higher peak power and subsequent fast marking. Both lamp/YAG
and diode/YAG systems can produce TEM00 beam output quality, or near TEM00
outputs, with proper apertures and collimation to produce similar spot sizes.
Lifetimes of laser-diode bank versus arc lamps are an important
consideration. Most commonly advertised lifetime of laser diodes operating in
Q-switch mode are in the range of 10,000 hours, although the actual lifetime is
dependent upon a variety of factors and can vary significantly. When replacing a
bank of diodes, the laser head is returned to the factory and the replacement
cost can be in the range of $12,000 to $15,000. In contrast, arc lamps have an
operating range of 400-600 hours, based upon average usage conditions, and can
be easily replaced by a technician for about $100 or less. Advantage goes to
diode/YAG lasers relative to beam power stability since arc lamps age over time.
At present, lamp/YAG lasers are significantly less expensive to procure. Lamp/YAG
is a much more mature technology and has been in use for decades (1960s). Diode/YAG
lasers (1980s) are the newer technology and they have longer mean time between
maintenance intervals and lower electrical consumption and heating requirements.
Lamp/YAG lasers often times can be more versatile when the marking of various
substrates is required. The diode/YAG is a more specialized laser machine.
Comparison of Lamp Pump YAG versus Diode Pump YAG
(Basis: 100-Watt laser, 220V, water-cooled)
||Lamp Pump YAG
||Diode Pump YAG
||10 - 100 Watt MM
||10-100 Watt MM
|Power with Best Quality Output
||1 - 22 Watt near TEM00
||1 - 22 Watt TEM00
|Beam quality mode
||MM to near TEM00
||MM to TEM00
|Beam power density
||Low to High
||Low to Very High
||Focus lens, Upcollimator and Mode
||Focus lens, Upcollimator and Mode
|Laser Pump Source Life
||400 - 600 hours
||8,000 - 10,000 hours
|Pump Source Replacement Cost
||$50 - $100
||$12,000 - $15,000
|Laser System Size
||Large to Small
|Wall Plug Efficiency
|Operating cost annual (B)
|Initial laser cost
||$48,000 – 60,000
||$60,000 – $75,000
Laser control software is as important as any hardware component in the
marking system. Advanced software algorithms enable unprecedented speed.
Beam-steered laser markers are sometimes wrongly conceptualized as (desktop)
printers. In fact, they are plotters. Rather than placing individual pixels to
create alphanumeric letters or graphics, the laser draws lines much like writing
with pencil and paper. Regardless of the input file format originally used to
create the laser marking objects, all marking is eventually reduced to its most
simple form, a list of vector lines to be drawn by the scan head and marked by
the laser. Complex input file formats often used by design engineers may not
necessarily yield the best (or fastest) vector laser marking. Laser marking
equipment systems must be safe and conform to ANSI Z136 standards.
1. Bruce Mulholland (Ticona, formally Hoechst Technical Polymers) and Scott
Sabreen (The Sabreen Group, Inc.), “Enlightened Laser Marking”, Lasers&Optronics,
2. BASF Corporation (formally Engelhard Corporation) Mark-it™ Laser Marking
Pigment Technical Bulletin 2002, with technical content contributions from The
Sabreen Group, Inc.
Scott R. Sabreen is founder and president of The Sabreen Group Inc. (TSG).
TSG is an engineering company specializing in secondary plastics manufacturing –
laser marking, surface pretreatments, bonding, decorating/finishing and product
security. For more information, call toll-free at 888-SABREEN or visit