Application Note
Advantages of Cutting and Perforating Plastics
with a 10.2 µm Wavelength CO2 Laser
CO2 lasers offer numerous advantages over
traditional mechanical methods for cutting, scoring
and perforating of the plastic films and other
flexible materials widely used in the packaging of
food and other consumer goods. While lasers
operating at the traditional CO2 wavelength of
10.6 µm are most commonly used for these
applications, CO2 lasers with output at 10.2 µm
have proven to deliver higher processing speeds
and superior results with certain plastic materials,
in particular, polypropylene and bi-axially oriented
polypropylene (BOPP). This document provides
some background and context on laser processing
in general, and presents specific applications
results for processing at 10.2 µm.
Modern Food Packaging
Increasing consumer demand for high quality and
freshness in packaged foods have led to some very
sophisticated packaging techniques based around the
use of plastics and plastic films. For example,
perishable items and baked goods are often sealed in
flexible plastic films, which may themselves consist of
multiple layers of different materials. These material
combinations are engineered to deliver the optimum
combination of physical strength and handling
characteristics (to withstand production and transport to
the consumer), to provide a barrier to moisture and
oxygen (the prime contributors to product spoilage),
and to optimize printability (in order to support attractive
and eye catching graphics).
The single layer films used for wrapping items such as
cookies and candy usually consist of polypropylene,
typically in the 30 to 100 µm thickness range.
Polypropylene is also often a very significant
component in multilayer films. Frequently, this material
is thermally extruded from a homopolymer, and is then
stretched, or “blown,” in two directions, normal to each
other, to improve strength even further. This material is
known as bi-axially oriented polypropylene (BOPP).
BOPP, and the other plastic films, papers and foils
used to create packaging, are typically supplied in the
form of large rolls. These rolls are mounted onto
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machines which continuously unroll webs of film and
transform them into individual, finished packages. This
process often requires that the material be cut, slit,
perforated or scored (cut to a finite depth, rather than
completely through) while it is moving through the
machinery.
Laser Processing Advantages
Traditionally, mechanical means, such as blades and
punches have been employed to perform converting
processes such as cutting and perforating. However,
lasers are now replacing mechanical tooling for several
reasons. First, non-contact laser tools do not wear
over time. This eliminates the need to stop machinery
in order to replace cutting tools, and also delivers
results which are consistent over time. Additionally,
laser converting enables high precision control over
scoring depth and micro-perforation hole size, both of
which are extremely hard to achieve with mechanical
tooling. Non-contact laser processing also doesn’t
impart any force to the plastic films which could distort
or tear them.
Laser processing is cleaner than mechanical methods.
Specifically, it produces very little dust and debris, and
leaves a very clean, processed edge. Cutting debris
can be problematic because it settles on other parts of
the machinery, and can affect their efficiency. For
example, debris which lands on print rollers or pads
can degrade graphics reproduction.
Another significant advantage of laser processing is its
flexibility. Laser cutting parameters are varied simply
by changing values in software, making it easy to
switch between jobs, or to individualize packaging on
the fly. Furthermore, these changes don’t incur any
tooling or setup charges.
The web processing equipment used for packaging
converting often runs at speeds of up to several
hundred meters per minute. Laser converting is fast,
making it compatible with the speed of existing
production lines. In addition, several manufacturers
produce laser-based tools that can easily be integrated
with existing converting equipment.
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Why 10.2 µm?
In order to accomplish CO2 laser processing, the laser
beam is focused down to a small spot on the workpiece.
The laser energy is absorbed by the material, causing it
to heat up above its melting point. The resulting molten
material is then cleared away by either a gas jet or the
induced vapor pressure to form the kerf.
Galvanometer mirror scanners are used to sweep the
laser beam over the surface of the plastic film in order
to produce the desired processing pattern. The laser is
directly modulated (on/off pulsing) in order to stop and
start laser processing as needed, as well as to change
overall output power in order to vary the depth of
processing when scoring.
Most polymer films have an infrared absorption spectrum
that consists of numerous sharp peaks. Consequently,
small shifts in laser wavelength can have a dramatic
impact on absorption efficiency. Since flexible
packaging involves very thin films, these films often only
absorb a small percentage of the incident laser power, in
contrast to applications such as metal cutting where all
the laser power is typically utilized. For this reason,
increasing the absorption coefficient of the thin film(s) by
optimizing the laser wavelength can dramatically
increase the processing speed and efficiency for a given
laser power level. Polypropylene and BOPP, in
particular, have been found to have significantly greater
absorption at 10.2 µm, rather than the traditional 10.6
µm output of the CO2 laser, making it sometimes
advantageous to work at this shorter wavelength.
Absorption of BOPP at 10.2 µm and 10.6 µm
Thickness (µm)
Absorption (%)
P orientation
T orientation
58.6
83.7
50
λ = 10.2 µm
λ = 10.6 µm
100
18.5
18.5
180
92.4
92.4
50
7.0
14.7
100
28.5
28.5
180
46.2
46.2
Figure 1. The 10.2 µm wavelength delivers significantly higher
cut speeds with polypropylene for film thicknesses below 100 µm.
Cut speed (m/s)
Polypropylene Cut Speed
6.0
4.0
Applications Results
The Coherent applications laboratory has investigated
the processing of plastic films at 10.2 µm under a
variety of process conditions. Some of the most
significant results are presented in this section.
Hole drilling in 30 µm thick BOPP film
The production of large numbers of small diameter
(~50 µm) through-holes is critical to packaging
techniques such as micro-perforating for fresh produce.
In this case, the small holes yield a breathable package
that avoids the build-up of anaerobic conditions leading
to product spoilage. Precise control of hole size is
necessary in order to provide the precise oxygen
transmission rate (OTR) for a specific application. This
study compared hole diameters that can be readily
achieved using 10.2 µm and 10.6 µm CO2 lasers (both
from the Coherent GEM series). Specifically, a series
of single shot drilling trials were performed with both
lasers on 30 µm thick BOPP film.
For the 10.2 µm laser, it was possible to fully perforate
the film and create a 45 µm diameter hole with a 15 µs
pulse, having a pulse energy of 150 µJ. In contrast, the
smallest hole that could be achieved using a 10.6 µm
laser and similar optics was 60 µm in diameter. And,
this required a pulse energy of 1.7 mJ, a tenfold
increase in pulse energy.
The ability to completely pierce through the film with a
short duration pulse is essential in this application
because of the high web speeds. For example, at a
web speed of 5 m/s, a 50 µm hole drilled in 20 µs
becomes an ellipse that is 100 µm long. This increase
in area can raise OTR’s to unacceptably high levels.
Hence, shorter pulsewidths enable the production of
smaller diameter holes or ellipses without the need for
complex scanning systems to compensate for web
movement.
The 10.2 µm drilled holes produced in this testing were
very clean, with some re-melted reinforcement of the
hole. Plus, the pulse duration necessary to drill these
small holes is short enough such that, at normal web
speeds, no significant ‘stretching’ of the hole occurs
along the direction of travel.
10.2 µm
10.6 µm
2.0
0.0
25
75
125
Film thickness* (microns)
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Cutting 4 in x 2 in labels from 95 µm thick, white
polypropylene film
The goal of this test was to prove that a CO2 laser
operating at 10.2 µm can cut 95 µm thick polypropylene
film at a high speed and with a clean edge.
Processing parameter summary
Material
Polypropylene Film
Material Thickness
Laser
0.95 µm
Coherent C-40 CO2
Wavelength
To explore this, the beam from a Coherent C-40
10.2 μm CO2 laser was directed into a 3.3X telescope,
and then delivered a distance of 1.5 meters to a
Scanlab HurryScan II galvanometer scanner equipped
with a 200 mm focal length meniscus scan lens. The
theoretical focused spot diameter for this configuration
is 220 μm. The scanner was controlled by SCAPS
SAM Lite software.
10.2 µm
Average power
12 W
24 W
Pulse Repetition
Rate
10 kHz
20 kHz
30 µs
40 µs
Pulse Width
Pulse energy
1.2 mJ
1.2 mJ
Pulse spacing
33 µm
30 µm
Theoretical spot
size
220 µm
220 µm
325 mm/sec
600 mm/sec
85 %
86 %
Scan speed
Pulse overlap
.
Figure 3. Magnified edge detail of a 95 µm thick polypropylene film cut with a 10.2 µm CO2 laser operating at 12 W.
A plate with a 4 in x 2 in rectangular aperture was set
beneath the scanner so that its upper surface was in
the focal plane of the scan lens. A sheet of 95 µm
thick, white polypropylene film was set on the plate and
held down with metal bars. Circular cuts were made at
a variety of scan speeds, laser pulse repetition rate and
laser pulse durations. The cuts were examined under a
binocular microscope.
Results showed that the material could be cleanly cut
at a speed of 325 mm/sec using 12W of laser power.
Cutting could also be accomplished at a speed of
650 mm/sec using 24W of power. The cut edge had
some taper but was clean
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Perforation of 0.0018 in (45 µm) thick laminated
polypropylene film
This experiment was conducted to prove that a CO2
laser operating at 10.2 µm can cleanly perforate 45 µm
thick, laminated polypropylene film at a processing
speed of 600 feet/min.
To assess this, the beam from a Coherent C-40
10.2 μm CO2 laser was passed through a ScanLab
HurryScan 14 galvanometer scanner equipped with a
100 mm focal length, flat field lens, to yield a nominal
working spot diameter of 125 µm. The scanner was
controlled by Scaps Samlite software, which also
commanded laser pulsing. The pulse repetition rate
was set at 1250 Hz in order to achieve the desired
perforation length and spacing at the scanning speed of
500 to 600 feet/min. The photo shows that clean
perforations were obtained under these conditions.
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Conclusion
Compact, sealed CO2 lasers offer an ideal combination
of low cost, high reliability and small footprint for
converting applications such as cutting, scoring and
perforating of plastic films. For polypropylene and
BOPP, in particular, increased processing speed and
improved results can be obtained with lasers operating
at 10.2 µm, making these even more attractive tools for
systems integrators and converters alike. And, since
Coherent offers the highest output power lasers
available at 10.2 µm, these products are the ideal
solution for high efficiency converting applications.
Figure 4. Perforations produced in 0.018 in thick polypropylene
film with a 25W, 10.2 µm CO2 laser.
Processing parameter summary
Material
Polypropylene Film
Material Thickness
45 µm
Laser
Coherent C-40 CO2
Wavelength
10.2 µm
Average power
24.9 W
Pulse Repetition Rate
1250 Hz
Pulse Width
500 µs
Pulse energy
20 mJ
Pulse spacing
250 µm
Theoretical spot size
125 µm
Scan speed
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