指南
Synrad - Optimal
Polycarbonate Processing
with CO2 Lasers
Introduction to Polycarbonate and its Applications
Polycarbonate (PC) is a material that, due to its excellent physical and chemical properties, isin high global market demand. The advantage of this thermoplastic lies in its higher toughness,breaking strength and transparency as well as high heat resistance. These attributes makepolycarbonate attractive in the automotive, medical and mechanical engineering industries. Additives
can be used to further improve the properties of polycarbonate and allow its use in different environments.

Chemical Properties and Absorption Characteristics of PC
In general, CO2 lasers are particularly suitable for cutting plastics due to their long wavelength and the corresponding
absorption curves of the plastics. Despite the high absorption of polycarbonate, when cutting it with a CO2 laser
the cutting quality is low. Although it is easy and fast to cut, the resulting cutting edge often shows yellow to brown
discolorations. Due to its optical properties PC is an excellent substitute for traditional material such as glass. But
applications in headlamps and displays require a clean cutting edge since every discoloration can induce unwanted
color effects in the material. Further, due to its high toughness and stability it is hard to cut using mechanical processes,
therefore the aim is to find the optimal laser processing parameters to achieve the best possible cutting quality. In the end
we want to help PC manufacturers and users, to better understand the CO2 laser cutting process of polycarbonate andhow to use this for their advantage.
0.8
0.7

9.3 µm

10.6 µm

Absorbance (AU)0.60.5
0.4
0.3
0.2
0.1
0910
Wavelength (µm)

An experimental measurement of a Polycarbonate sample using an FTIR spectrometershows a strong peak at the expected position©2020 Novanta Corporation. All Rights Reserved.

11Absorption describes theinteraction between material andlaser and is one important factor
that influences the cutting quality.
When a polymer absorbs infrared
radiation, its chemical bonds will
vibrate, resulting in stretching andbending of those bonds. To achieve
an efficient absorption the energyof the photons must match thedistinct vibrational energy of themolecule bond. Polycarbonateshows an increased absorption
at 1080 cm-1 which corresponds
to a wavelength of 9.26 µm. Thisabsorption peak represents theCarbon-Carbon bending vibrationalmodes in the backbone of thepolycarbonate polymer chain. The
calculation of this bending mode iscomplex and outside the scope ofthis paper.

Novanta | Optimal Processing of Polycarbonate with CO2 Lasers

Polycarbonate cutting examples
How does the absorption help to improve the cutting quality? To evaluate this behavior three different polycarbonate
types, with a thickness of 2mm, were tested. The materials are described as Makrolon® 2405 (standard grade), Makrolon®
AL 2447 (UV-stabilized), and Makrolon® 6555 (flame-retardant).
For the cutting tests two Synrad CO2 lasers were used, one at the standard wavelength of 10.6 µm and another at thealternate 9.3 µm wavelength. In addition to wavelength, assist gas type, assist gas pressure, and laser frequency variations
were also tested to determine impact on cut quality.

Summarizing the processing scheme
One parameter variable was tested while all other parameters remained fixed. The influence of the gas pressure and thelaser frequency was varied within a certain parameter range. All tests were performed with pressured air and nitrogen as
an assist gas. Additionally, every cut was repeated with a laser wavelength of 9.3 µm and 10.6 µm. The cutting speed wasselected to be as high as possible, while still achieving a complete cut-through. The laser power was kept at a constantaverage power of 100W for all cuts.

Influence of the assist gas
The assist gas is an important factor for the cutting quality. It pushes the melt out of the laser cutting zone. For
polycarbonate this means a high gas pressure is needed to expel the discolored melt and decomposition products as fast
as possible. The difference between 1 bar and 4 bar of air pressure are shown in the pictures below. While the pressure of
the gas is important, the gas type seems to have no effect on the cutting quality.

Air

Nitrogen

1 bar

4 bar
2 mm
Standard grade PC cut with pressured air and nitrogen at 1 bar and 4 bar

Influence of the frequency
Frequency testing was done in incremental 1 kHz to 50 kHz steps, revealing linear behavior in cut edge quality. As thetesting frequency increased, discoloration of the cut edge was reduced. This observation indicates that pulsed laser
operation, especially at low frequencies, results in a more discolored cutting edge. The low heat affected zone in thesurrounding area in combination with the long off-time between the pulses results in the quick solidification of the
discolored melt and decomposition products, before they can be expelled by the assist gas. In order to achieve a clean
cutting edge, cutting in a quasi-continuous mode is required.

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Novanta | Optimal Processing of Polycarbonate with CO2 Lasers

1 kHz

10 kHz

50 kHz2 mmStandard grade PC cut with different frequencies

Influence of the material composition
As mentioned above three polycarbonates with different additives were provided. An absorption measurement with thespectrometer did not show a difference in the absorption behavior, but a difference in cutting quality is visible.

Standard Grade

UV Stabilized

Flame Retardant

2 mmThree different types of PC cut with the same processing parameters

For all tests the standard grade and the UV-stabilized polycarbonate show similar results. These two materials can be
cut close to perfection with the right choice of parameters. The polycarbonate with the flame-retardant additive behavesdifferently and no suitable parameter was found during our tests. In the flame retardant image above the cutting edge
shows (red arrow) distinct black ridges, most likely the result of carbonization that occured during the laser cuttingprocess.

Influence of the laser wavelength
Both the 9.3 µm and 10.6 µm wavelengths were tested at the same average laser power. Further a high pressure of 4 barand a frequency of 50 kHz was used, as in the previous tests these parameters show the best cutting results. Due to thedifference in absorption, the maximum cutting speed at 10.6 µm is 50 mm/s instead of 70 mm/s at 9.3 µm. The cut with
10.6 µm produces a completely colorless edge but a very rough surface. The surface produced with 9.3 µm is muchsmoother but show some residual tint on the edge.

9.3 µm

10.6 µm

2 mm
70 mm/s

50 mm/sStandard grade PC cut with the non-standard wavelength of 9.3 µm and the standard wavelength of 10.6 µm

At 10.6 µm wavelength laser energy scatters inside the material due to lower absorption. Dissipation of the laser energy
into the material causes a larger melting zone resulting in higher surface roughness. The higher surface roughness
suggests that decomposition particles could be expelled more efficiently with an assist gas due to the larger meling zone.
Further, the 10.6 µm wavelength test showed less discoloration due to lower absorption and subsequent laser energy
scattering, yielding less energy for chemical decomposition.

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Novanta | Optimal Processing of Polycarbonate with CO2 Lasers

Conclusion
This paper provides an overview of the influences of several processing parameters on the cutting quality ofpolycarbonate. The table below summarizes the results and offers the best parameter to use. Our findings lead to theconclusion that there is an optimal temperature regime, where the material melts without decomposition. Cutting PC witha CO2 laser above the optimal temperature regime results in decomposition and thus discoloration takes place.
Parameters

Best Parameter

Result9.3 µm for highest speed

Better absorption for higher speed but more decomposition

10.6 µm for best quality

Lower absorption results in more melting than decomposition

Wavelength

Frequency

50 kHz or higher(quasi-continuous)

Pulsed operation results in better cooling effect of the gas
discolored melt gets solidified inside the cut area

Assist gas typePressured air

No advantage of nitrogen over air

Gas pressure

4 bar or higher

Stronger gas flow casts out decomposition products more quickly

It was observed that the additives have a significant influence on the cutting result, therefore knowledge about theexact composition of the material is crucial for industrial applications. Utilizing a high frequency results in a quasicontinuous wave mode, and leads to much better cutting results in comparison to pulsed operation.
When using a CO2 laser for polycarbonate cutting, the quality of the cut is affected by assist gas pressure. Using thehighest possible assist gas pressure (> 4bar) results in cleaner edge cuts. High pressure assist gas quickly casts out melt
particles to yield a clean, polished edge, making CO2 laser cutting more efficient.
A comparison between the two wavelengths 9.3 and 10.6 µm reveals a difference in cutting quality and speed. The best
result, in terms of avoiding discoloration, was achieved in quasi-continuous mode with a wavelength of 10.6 µm and ahigh assist gas pressure. Overall the weaker absorption leads to a reduced cutting speed by approximately 30%. Theeconomically most viable choice is a 9.3 µm laser in quasi-continuous mode. The resulting cutting edge suffers only froma very small discoloration but allows a higher cutting speed and surface smoothness at the same average output power.
Furthermore, as nitrogen and pressured air result in a similar edge quality, pressured air should be chosen to reduce
cost.

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