Lasers in Manufacturing Conference 2017

Galvanometer Scanning Technology and 9.3µm CO2 Lasers for
On-The-Fly Converting Applications
M. Hemmericha,*, M. Darvisha, J. Conroyb, L. Knollmeyerb, J. Wayb,
X. Luoc, Y. J. Hsuc, J. Lic
Novanta Europe GmbH, Münchner Strasse 2a, 82152 Planegg, Germanyb
Synrad Inc., 4600 Campus Place, Mukilteo, WA, USA 98275
cCambridge Technology, 125 Middlesex Turnpike, Bedford, MA, USA 01730
aAbstract

Digital converting processes are used to transform a roll of material into a different form or shape andprovide the flexibility to deliver unique designs or changes on-the-fly, unlike traditional mechanicalprocesses. Tremendous progress has been made in the field of digital printing; its increased adoption
requires that converting processes also be more flexible and cost-effective while delivering high cut quality.
Due to the high cost of storage and maintenance of a plurality of conventional dies and long set up time,using CO2 lasers in combination with fast and precise laser scanning has proven to have great potential inpaper and cardboard processing, flexible packaging and label cutting. At the same time, the capability tocontrol the laser beam power density delivered on the material processed is critical to achieve high quality
finish goods.
In this paper, we are showing the capabilities of an all-digital galvanometer scanner in combination with ahighly frequency stable CO2 laser that provides stable laser power density by modulating the laser power incoordination with beam scanning speed. Our system also demonstrates high scanning speed of more than
10 m/s and a focal spot size of less than 150µm.
Keywords: 9.3µm CO2-Laser; Uniform Laser Density; Digital Galvanometer Scanning Technology; Digital Converting; Cutting; Drilling;
Micro-Cutting;* Corresponding author: Tel.: +49-89-31707-441; fax: +49-89-31707-222
E-mail address: Malte.Hemmerich@synrad.com

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1. Introduction
The rapidly growing market of roll-to-roll converting systems for thin materials, where flexibility andprecise repeatability is key to success, is ideally suited for CO2 lasers paired with high-speed galvanometerscan heads. The materials involved are often thin plastic films, paper, cardboard, fabric and textiles. Thesematerials fall into a wide range of end uses including flexible packaging, product labels, greeting cards /envelopes, high performance optical films, and athletic apparel fabrics.
The digital printing revolution has changed the landscape of these roll-to-roll converting machines
significantly, especially for label and customized packaging. Traditionally, cutting has been done withmechanical die-presses where the user was locked into one design for a production shift and the cost andhassle of storing multiple dies made it unfeasible to have many cut shapes available. With the newly added
flexibility of digitally printed designs, the need to easily change the cut or perforation shape has become
more important. Replacing die presses with laser cutting in combination with high-speed scan heads haschanged this to a dynamic and easily flexible process, where cut design changes can be done on-the-fly via
software without stopping the production line.
Additionally, there are some special features which have been difficult or impossible to create until now.
A non-contact process can enable the processing of finer features in more sensitive materials, and the digitalnature also allows short-production runs or one-off prototypes that would not have been time- or costeffective in the traditional mechanical system. Further unique examples can be found in the apperal market.
Innovation is a key driver in this market, especially in the booming segment of active wear, where
performance and unique designs increase sales opportunities. Materials like polyester and spandex(commonly used for their wicking or shape-retention qualities) do not breathe, so manufacturers look to
perforation to increase breathability. While this can be done mechanically or by altering the weave itself, thefabric’s durability or opacity can be negatively impacted. Using lasers, holes can be strategically placed in the
fabric for ventilation or aesthetic embellishments. Lasers can also seal the fabric, preventing fraying causedby mechanical processing and preserving the fabric’s integrity for demanding use. For on-the-fly processes,
these design changes can easily be made using scanning software and controllers.
Successful laser processing of thin materials for converting applications requires careful controlling of thelaser power in synchronization with the actual scanning speed. The conventional method to coordinate thegalvanometer movement and laser control can lead to a non-uniform laser energy input on the material
during the acceleration and deceleration at ends of patterns or contours than may result in failed cuts or
perforated base material. In the case of the athletic apparel processing, if too much laser energy is delivered
to the fabric, the edge becomes melted and rough which can scratch the skin. In the case of label kiss-cuts,
cutting requires cleanly penetrating through the top coatings, laminate, and label base, while maintaining
minimal damage to the liner underneath; otherwise the liner may rip during the separation process. Thus, acareful balance between the laser energy and the real-time cut velocity of the scan head must be used. One
strategy to solve the problem is modulating the laser power as the function of the scanner velocity during
the acceleration or deceleration periods. Another strategy is using a smart control algorithm to maintain
constant speed throughout the job. This is done by dynamically planning the scanning trajectory andcoordinating the laser on and off operations.
In this paper, we demonstrate the high speed, high accuracy and low drift digital scanning technology thatincorporates both techniques to achieve uniform laser density with minimal additional process development.
We also demonstrate the benefits of using a 9.3µm laser for creating a smaller spot size and give examples
for other benefits of non-standard CO2-laser wavelengths.

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2. Experimental setup
A Synrad i401-9.3µm laser was used in combination with a Cambridge Technology Lightning TM II 50mm
digital scan head. The laser shows a Gaussian beam shape with a measured beam diameter of 5mm (1/e 2)
that is coupled into a 2.0x 9.3µm BSL17-series ULO beam expander and then into the 3-axis Lightning II scanhead with a 17mm clear input aperture and 50mm mirror apertures. The lens distance between the dynamic
focusing lens and the objective lens of the scan head was set to 189mm creating a 200mm x 254mm usable
field size in a working distance of 191mm below the scanning head. The maximum optical power of the laser
was measured to be above 400W while optical loss through all optical elements of about 12% was observed.
A power stability test at 45% duty cycle showed a constant power of 220W with excellent power stability
over a frequency range from 2kHz to 100kHz.
The 9.3µm CO2 laser was chosen as it is particularly useful for processing many thin plastic films, which
often have much higher absorption compared to 10.6µm. Specifically, PET films, often found in flexible
packaging and sleeve type labels, have cleaner and smoother cut and perforation edges when processedwith a 9.3µm CO2 laser. In addition to PET, PVA and PC films are often used in optical film stacks inserted into
display panels for mobile devices, where any degradation to the optical clarity or heat-affected zone on thestack edge may adversely affect performance and prohibit accurate fitting into the device.
Furthermore, polyester-based fabrics and high performance fabrics like Lycra have higher absorption
peaks at 9.3µm. This not only increases the laser process speed but it also minimizes excessive melting,leading to smoother edges. For white or lighter colored polyester fabrics, there is also a noticeable reduction
in yellowing of the cut which can be a problem at 10.6µm.
In addition to the absorption advantages in those specific materials, the 9.3µm wavelength allows thelaser beam to be focused to a spot size which is 12% smaller than 10.6µm due to the shorter wavelength
decreasing the diffraction limits. This allows more flexibility to do fine detail processes, especially withinsome micro-perforation applications.
The Lightning II scan head with 50mm mirror aperture was chosen in this setup for its high performance in
speed, accuracy, stability and reliability. The light weight, low inertia and high stiffness advantages ofberyllium mirrors allow the use of large mirrors that are needed to generate a small focal spot diameter
while still being able to achieve high processing speed. As it can be seen in Section 3, the resulting maximum
speed and spot size satisfy requirements even for very demanding applications.
High accuracy and stability of a Lightning II scan head is achieved by using a 24-bit position encoder,predictive state-space servo control and the 24-bit command resolution. In a 300mm x 300mm field, thesmallest step command is ~4.5µm using 16-bit command resolution as compared to 0.017µm using 24-bit
command resolution. This ultra-fine command resolution combined with the repeatability of 2µrad allows
the integrator to achieve <10µm patterning accuracy in a 300mm x 300mm work field with the digital scanhead.
Low drift and thermal stability is another attribute of the Lightning II digital scan head that benefits thetargeted laser converting applications. A production environment usually requires long machine up-time,sometimes 24/7 operation. It is essential that the scan head performs with constant high quality during thislong period in order to produce high quality parts for high yield. The digital encoder embedded in theLightning galvo motor has very low drift. The long term drift for Lightning II scan head is as small as 10µrad

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and the thermal drift is only 2µrad/°C. The Lightning servo also uses an adaptive thermal model that
constantly adjusts the motor control to maintain the same level of performance.
2.1. 9.3µm CO2-Lasers
While CO2 lasers are capable of operating at many discrete points within the 9.3-10.6µm wavelength
range, the principal output bands are centered at ~10.6µm and ~9.3µm, depending on laser design. By far
the most common is 10.6µm, owing to the output gain spectrum of typical CO 2 laser gas mixes.
To preferentially generate light around 9µm, the optical gain of the 10.6µm band must be suppressed.
One tactic is to use an isotopic gas mixture which has higher gain at 9.4µm versus 10.6µm. While thismethod does achieve the desired outcome, the output power tends to be lower compared to the same laser
cavity with a standard gas mixture emitting at 10.6µm. This isotopic gas mixture is less stable and can alsodegrade over time, resulting in output power loss and wavelength shift.
To produce 9.3µm lasers, Synrad has chosen to use inter-cavity optical filtering techniques. The benefit ofthis approach is that a standard CO2 laser gas mix can be used. When this method is coupled with an allmetal laser tube design such as Synrad’s field proven and market leading i401, there is no power deratingwhen comparing 9.3µm and 10.6µm versions of the same laser model. For ceramic-based tube designs, thesame does not hold true as ceramic tends to have higher absorption at 9.3µm versus 10.6µm and istherefore much less efficient at 9.3µm.
2.2. Smart control techniques for uniform power density
Although state-of-the-art digital scan head technologies discussed above can enable fast, accurate andreliable laser processing, digital galvanometer scanners, being a physical system, still need to accelerate anddecelerate whenever its velocity or motion direction is changed. With conventional control methods, eachcommanded vector is constructed such that the scanner starts its motion from a static position and finishes
the vector returning to zero speed. So there is always an acceleration phase and a deceleration phase in each
vector where the scanner moves slower than the commanded velocity. Often the laser power is kept
constant, which results in more laser energy deposition in these phases. This non-uniform laser energy
deposition impacts the cut depth in applications like kiss-cutting, either resulting in insufficient cutting at
some locations or over-burning at other locations in the cut contour.
There are two methods to maintain uniform laser density. One is to keep the scanning speed and lasing
power constant during the process. The other is to modulate the laser as a function of the actual scanningspeed. Cambridge Technology has developed smart control techniques that implement both methods.
2.2.1. Constant velocity control
The constant velocity control is one of the features included in the smart control algorithm called
ScanPack that is developed by Cambridge Technology. ScanPack is an adaptive scan control based on an
advanced servo model. It uses the accurately predicted position information by the servo and dynamically
adapts the scanner and laser commands to the immediate situation and job requirements. In short, ScanPack
control algorithm plans the scanning trajectory automatically in coordination with laser firing based on thescanning system capability as well as the job requirements. The job requirements here not only refer to theaccuracy requirements of pattern shape such as corner sharpness, but also include process requirements likethe required velocity at both ends of a vector, which impacts the processing quality.
As illustrated in Fig. 1, with a conventional control scheme, the commanded trajectories are independentof the scanner capabilities, and the scanner always has to stop at the vector ends for either mark or jump

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vectors, resulting in more burning at the mark vector ends or joints (corners). In contrast, ScanPack allows
the user to set the desired velocity at the end of mark vectors (end velocity) based on their process needs. It
also asks the user to set the required sharpness of corners when applicable. During marking when the laser ison, ScanPack commands the scanner to strictly follow these requirements. When the laser is off and scanner
acceleration/deceleration does not affect processing, ScanPack plans the jump trajectory to help meet themarking requirements, while ensuring such a jump is still within the scanner capability to prevent instabilities
or inaccuracies.
In the right example shown in Fig. 1, ScanPack will mark lines A-B and C-D with the set mark speed andrequired end velocity. A jump trajectory is planned to move from point B to C: for a smaller end velocity, ajump with shorter turns and path length (like the red path) is possible; if a higher end velocity is required, thesame scanner would need a longer path length with larger turns (like the blue path). ScanPack will determine
the most accurate and efficient way to mark a corner, like A-B-C shown. This can include simply completingthe corner (like the green vectors), or leaving and re-entering the corner in a “skywriting” loop (the red andblue paths). When a “skywriting” loop is triggered, the scanner will enter and exit the loop at the same
velocity, and the marked corner is always sharp.

Fig. 1. Conventional control commands (left); ScanPack trajectory planning (right)

2.2.2. Velocity based laser modulation
The other control method to achieve uniform laser density is to modulate the laser power as a function of
the actual scanning speed. Modulation can be done on laser frequency, laser power or laser pulse width.
When the scanner accelerates or decelerates, the laser frequency, power, or pulse width are modulated
accordingly to compensate for a slower scanning speed. Illustrated in Fig. 2 is the example of laser frequency
modulation in response to actual scanner velocity.

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Fig. 2. Velocity Based Laser Modulation based on frequency. When the mark vector command (1 st curve) starts and the laser is turnedon (3rd curve), the galvo actual velocity (2nd curve) increases from zero and gradually reaches a constant value. The laser modulation
signal (4th curve) is increased from 10kHz to 100kHz accordingly.

For the scanning controller to estimate the actual acceleration or deceleration behavior of a scanner, itneeds user input on the bandwidth of the scanner. The user can also define modulation range. For example,
you can choose to vary laser power from a minimum non-zero value to the nominal value when the velocity
varies from zero to the mark speed. In Fig. 2, the modulation range set for laser frequency is from 10kHz to
100kHz. Identifying the optimum parameter set of bandwidth and modulation range usually requires
iteration, but the high power stability over a wide frequency range and the linear relationship between
pulse-width and power of the i401-laser used in this paper is a key factor for best performance.
Fig. 3 presents the marking results of a corner with no laser modulation (left) and with velocity-basedlaser modulation (right). The laser power is kept constant and modulation is on laser frequency. Using
velocity based laser modulation to compensate for scanner deceleration and acceleration at the cornerindeed shows much more uniform laser pulse density throughout the corner!

Fig. 3. Marking results of a corner with conventional control with no laser modulation (left) and with velocity-based laser modulation on
frequency (right). Velocity-based laser modulation clearly improves the uniformity of laser density at the corner.

In summary, both constant velocity control in ScanPack and velocity based laser modulation can be used
to achieve uniform laser deposition in laser additive manufacturing process. Cambridge Technology’s
ScanMaster Controller is equipped with both techniques.

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3. Results
3.1. Beam caustic and velocity measurements
To demonstrate the advantages of the laser and the scanner discussed above, the focal spot wasmeasured using a Primes focus monitor and actual marking speed tests were performed.
The result of the beam caustic measurement shown in Fig. 4 clearly demonstrates the small spot size
obtained with the shorter laser wavelength and the large mirrors. The laser beam was measured at thecenter of the marking field at high optical power in order to detect any potential thermal lensing effect in the
beam guiding system. The excellent beam quality of the laser was maintained through five ZnSe optics andtwo Beryllium mirrors resulting in a focus diameter of only 140μm.

Fig. 4. Caustic measurement result of the focal spot at a working distance of 191mm below the Lightning II 50mm scan head using a
9.3µm laser beam from a i401-Laser

In order to test the maximum speed of the setup, different speed values were commanded in traditionalscanning mode while the PWM was set to 75µs pulse period and a low pulse width. Using this setting, a full
field size pattern consisted of straight lines and sharp corners as shown left in Fig. 5 was marked. Afterwards
the marked pattern was analyzed for corner sharpness and line width changes. This test pattern was marked
on paper with black ink coating, with marking speeds starting at 1m/s and increasing in 1m/s increments
until the radial error. The radial error is defined as the difference of the actual marked corner compared toan ideal sharp corner and the max allowed error was chosen to be 100µm.

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Fig. 5. (Left) Commanded marking pattern for the speed limit test; (Right) Magnified view on the centered top square resulting from a
12m/s marking test

On the right side in Fig. 5, a section of the marking result at the speed of 12m/s under conventional
control mode is shown. No significant variation in the line thickness across the field was observed, while aradial error approaching the 100µm limit and decreased dot spacing was observed at the corners. A
significant variation of the line thickness was only observed for speeds above 20m/s.

Fig. 6. Marking results with conventional and ScanPack control. The comparison is focused on a corner in the outline pattern (top) andthe edge of lines (bottom).

The observed large radial error at the corners and the decreased spot distance were further investigatedthrough perforation tests. The results comparing conventional and ScanPack control are shown in Fig. 6. The
two top pictures show the marking result at a sharp corner. The conventional control on the left side again
shows a small radial error and more densely packed spots at the corner. In contrast, the ScanPack result on
the right side shows an absolutely sharp corner and a constant spot distance due to the triggered skywriting
jump loop. The bottom group of Fig. 6 shows pictures of the ends of perforation lines. The conventionallycontrolled lines show more densely packed and enlarged holes at the end of the lines while the ScanPack

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controlled lines do not show a changed hole distance or size. This test demonstrates that the radial error canbe significantly improved by using ScanPack control.
3.2. Kiss-Cutting test
Another test using the ScanPack control was performed on a kiss-cutting application by using a Lightning II
30mm scan head and a Synrad Firestar ti60 laser. As necessary for label kiss-cutting applications, the task
was to thoroughly cut a circle on the top layer without perforating or even weaken the base liner
underneath.

Fig. 7. Marking results of a circle on burn paper with conventional (left) and ScanPack control (right).

Initial tests showed that a conventional controller is not able to perfectly close a shape on a very sensitive
application. Depending on the delay settings, the shape will either not be fully closed or it will perforate theliner. Shown on the left side in Fig. 7 is the result of a mark on burn paper after carefully optimizing the delay
settings in conventional control mode. An obvious line width change at the start and end point can be seen
here. The ScanPack controlled result on the right side of Fig. 7 is nearly perfect—and it was not necessary to
optimize any delay settings since those are not used for this control mode.

Fig. 8. Kiss-cutting on double-layer paper with ScanPack shows thorough cutting on the top layer without damaging the bottom layer

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The actual kiss-cutting result using ScanPack control is shown in Fig. 8. It is demonstrated that the cut at theclosings of the square and circle is smooth and consistent on the front view and without burn or damage on
the backside – a successful kiss-cut.
4. Conclusion
A fully digital galvanometer scan head with 50mm mirror aperture was combined with a 400W CO 2 laser
at 9.3µm in this work. We have demonstrated a focal spot size of 140µm on a field size of 200mm x 254mm
and great marking quality at a scanning speed of 12m/s. The high speed and high accuracy of the scan head
and the controller are all critical to achieve consistent, high quality results for converting applications.
Combined with a 9.3µm laser with best-in-class average power, this integrated solution can be an ideal
solution for many demanding applications.
More generally, for digital converting to continue to gain traction in a market dominated by mechanicalprocesses, high processing throughput and consistent quality will be essential. The non-contact, change onthe-fly nature of digital converting will only grow in value, but must be balanced against application-specifictechnical aptitude. Choosing the appropriate laser wavelength matching the material’s absorption
characteristics, optimizing power density and scan speeds, and optimizing a converting process with
controller-enabled skywriting or laser frequency modulation are all cornerstones for success in converting.
The demonstrated integrated setup is therefore an ideal solution for highly demanding converting
applications such as digital printing machines, kiss-cutting of labels, fabric processing, or thin film cutting. It
provides the flexibility of a digital process, including cutting, perforating, kiss-cutting, scoring, and more
within a single system that has been engineered to deliver consistent, high quality results with a wide rangeof cut patterns or designs.