Laser processing of transparent plastics: using CO2 and fibre lasers correctly

Laser processing of transparent plastics: applications and limits

Transparent plastics such as PC, PMMA, PETG, COC or PSU offer optical clarity, dimensional stability and low density. Laser processing allows burr-free contours, tight tolerances and flexible geometries without tooling costs. For optically demanding applications in medical technology, electronics, mechanical engineering or advertising, edge quality and process stability are decisive.

The suitable wavelength determines whether the laser beam is absorbed, transmitted or reflected. CO2 lasers at 10.6 µm are strongly absorbed near the surface by most polymers and are suitable for cutting and engraving. Fibre lasers in the near infrared at around 1.06 µm are mostly transmitted by transparent thermoplastics. They are therefore ideal for marking with additives, structuring surface processing, micro-drilling or for transmission welding in combination with absorber concepts.

In production practice, three factors count: the right laser source, a robust process window and material-specific presets such as drying, protective film, focal position and gas guidance.

Physics and absorption: wavelength determines the process

• CO2 laser 10.6 µm: strong surface absorption, thermal cut, potentially smooth edges with suitable parameters.
• Fibre laser 1.06 µm: high transmission in clear thermoplastics, suitable for marking with additives, structuring, micro-drilling and welding with absorbers.
• Green laser 532 nm and UV laser 355 nm: higher photon energy, fine micro-structuring, high-contrast marking, low heat input, rather for thin layers and surface processes.
• Laser power, spot size, pulse duration, pulse frequency and scan speed determine energy density and heat input. The ratio of these variables defines the process window between insufficient absorption and damage.

CO2 laser and fibre laser compared for transparent plastics

CO2 laser at 10.6 µm

Typical applications are laser cutting, contours, cut-outs and engravings. PMMA often shows an optically appealing, almost polished cut edge. Polycarbonate and PETG require tighter process control, as yellowing, soot or micro-cracks can occur. A shielding gas feed with nitrogen reduces oxidation and bubble formation. Several shallow passes with a slightly positive focus offset improve edge quality and reduce the heat-affected zone.

Fibre laser at around 1.06 µm and variants

For clear material without additives, the fibre laser is primarily suitable for labelling, micro-structuring or transmission welding with absorbers. Transparent to transparent is usually not possible without special wavelengths or material modification. Markings succeed with laser-active additives, doped masterbatches or lightly coloured substrates. Ultra-short pulse lasers enable crack-free micro-processing with a minimal heat-affected zone in individual cases.

UV and green lasers

UV and green are valuable for transparent plastics for fine markings, DataMatrix codes, micro-drillings or matting with low thermal load. They complement CO2 and fibre lasers where optical surface quality and small structures are prioritised.

Edge quality and heat-affected zone

In transparent plastics, edge quality, optical clarity, haze and stress cracks are central quality characteristics. The cut edge should be smooth, without white break, without fume deposits and without micro-bubbles. Causes of poor edge quality are excessive linear energy density, damp semi-finished products, wrong focal position, insufficient extraction or wrong beam overlap.

• Set the focus offset slightly positive in order to shift the highest energy density just below the surface. This can reduce bubbles at the edge.
• Multi-pass strategy with lower power instead of a single cut with high power, in order to lower the heat peak.
• Direct shielding gas nitrogen or technical compressed air specifically into the kerf to minimise oxidation and soot.
• Dry semi-finished products beforehand, in particular PC, PETG and amorphous high-performance materials, to avoid bubble formation and stress cracks.
• Leave protective films on where possible. They reduce fume deposits and scratches. Before series start, check whether the film peels off with low residue.

Defining the process window: parameters and methodology

A stable process window is created through systematic screening. Procedure for repeatable results:

1. Record material data: polymer, modifications, residual moisture, extrusion direction, protective film, optical requirements.
2. Define starting parameters: spot size, focus offset, pulse mode, approved power and speed ranges.
3. Set up the test plan: matrix of power, speed, pulse frequency and track overlap. Choose step sizes pragmatically.
4. Carry out evaluation metrically: edge Ra, haze, kerf width, geometric deviation, micro-cracks under the polarising microscope, discolouration via colour measurement.
5. Narrow down the window: define the working range between burr, haze increase and crack formation. Then long-term test under alternating climate.
6. Series release: document parameters, nozzle distance, gas pressure, nozzle geometry, extraction volume flow and test routines.

Material-specific recommendations

The following overview summarises tried-and-tested tendencies. Values serve as guidance, preliminary tests remain indispensable.

Material	Suitable lasers	Expected edge quality	Aids	Notes
Polycarbonate PC	CO2 cutting, UV marking, fibre with additives	Good to very good with tight parameter setting	Drying, N2, multi-pass	Reduce tendency to yellowing and micro-cracks
PMMA acrylic glass	CO2 cutting and engraving	Very good, virtually polished	Leave thin film on, fine spot position	Sensitive to stress cracks if stored incorrectly
PETG	CO2 cutting, UV marking	Good, avoid bubbles	Drying, moderate energy density	Softer than PMMA, tends to melt threads
COC, COP	CO2 thin, UV finely structuring	Good with low thermal load	N2, multi-pass	Very clear, low haze tolerated
PSU, PESU, PEI	CO2 cutting, UV marking, fibre with additives	Good, slightly amber-coloured	Drying, N2	High Tg, define optical tolerances
PS	CO2 cutting	Good, brittle risk of fracture	Reduced energy density	Have supplied stress-relieved
PMP TPX	CO2 thin, UV fine	Medium to good	N2, sharp focus	Low density, high light transmission
PVC clear	Not recommended	Corrosion and health risk	Extraction is not sufficient as protection	Choose alternative processes

Polycarbonate PC

PC is tough and heat-resistant. For clean cut edges with CO2, drying, nitrogen and multi-pass strategies are effective. Yellowing arises from excessive heat input. UV is suitable for fine markings. You can find material information under Polycarbonate.

PMMA acrylic glass

With CO2, PMMA often delivers a crystal-clear, polished cut edge. Avoid standing edges with heat accumulation. Remove the protective film only after processing. For thick sheets, several passes at higher speed.

PETG

PETG tends to form bubbles with residual moisture. Therefore dry beforehand. CO2 cutting with moderate heat input leads to clear edges. UV is suitable for fine markings and matting.

COC and COP

Very clear, amorphous cycloolefins require low thermal load. CO2 can be used for thin cut pieces, UV for micro-structures. Pay particular attention to haze and stress birefringence.

PSU, PESU and PEI

High-temperature plastics with amber-coloured transparency. CO2 cuts stably, though with tight parameter setting due to the increased glass transition temperature. Drying is mandatory. Further material pages: Polysulfone, Polyethersulfone, Polyetherimide.

Polystyrene PS

PS is clear but brittle. CO2 cuts cleanly, but the risk of micro-cracks increases with mechanical reworking. Choose parameters with a low heat peak and good extraction. Product information under Polystyrene.

Polymethylpentene PMP

PMP is extremely light-transmissive and very light. CO2 is practicable with thin wall thicknesses, UV for fine structures. Details under Polymethylpentene.

PVC transparent

Laser processing is not recommended. Corrosive and health-endangering gases are produced. Use alternative methods.

Labelling and engraving transparent plastics

For durable, high-contrast markings on transparent plastics, three routes are common: UV or green lasers for photochemical effects, fibre lasers in combination with laser-active additives, or fine CO2 engraving for matting. Detailed background is provided by our article Laser labelling of plastics.

• Contrast through additives: additive packages create light-dark contrast without coatings.
• Gentle surface modification: UV reduces heat input and preserves transparency.
• Traceability: 2D codes with low depth for medical technology and electronic components.

Transmission welding of transparent plastics

As standard, a clear upper part is joined with an absorption-modified lower part. The fibre laser passes through the upper part and is absorbed at the contact joint. For transparent to transparent, special approaches exist with wavelengths around 2 µm, internal material absorption or nanodot-coated layers. Decisive are the joint geometry, joining pressure, gap dimension below 50 µm and homogeneous energy distribution.

• Preconditioning: dry and anneal components to minimise stresses.
• Ensure optical quality: clean the joint, choose a low-particle environment.
• Verify the process window: tensile test, leak test, micrograph.

Quality assurance and measured variables

• Haze and transmission: spectrophotometer for T values and degree of turbidity.
• Edge smoothness Ra: tactile profilometer or white-light interferometer.
• Microstructural damage: polarising microscope for stress birefringence.
• Geometry and tolerances: camera-based measurement directly on the system.
• Discolouration: colorimetric evaluation in L a b with acceptance window.

Process technology from practice

• Position extraction close to the kerf to avoid streaks and recondensation on the optics.
• Choose nozzle distance and gas pressure so that the melt is carried away and not blown back into the joint.
• Smooth contour splines, limit corner speed, to avoid heat peaks at edges.
• Document the material batch, as additives and residual moisture shift the window.
• Clean protective glass and F-theta lenses regularly, check the focal position cyclically.

Alternatives and combination processes

For highly heat-sensitive geometries or PVC, waterjet cutting or CNC milling with subsequent polishing is recommended. A comparison of the processes can be found under Waterjet cutting vs. laser processing. Further manufacturing options are given in our service pages Laser cutting and Milling.

Project checklist for a quick start

1. Define the requirement: optical quality, edge appearance, tolerances, release tests.
2. Choose the material: clarity, Tg, additives, moisture behaviour. Basic article: Transparent engineering plastics.
3. Define the laser source: CO2 for cutting, UV for fine marking, fibre for additive marking or welding.
4. Determine the process window: parameter matrix, metric evaluation, documentation.
5. Safeguard series processes: SPC, regular optics cleaning, batch traceability.

Frequently asked questions

Which laser is best suited for cutting transparent plastics?

For cut pieces and contours, the CO2 laser at 10.6 µm is the first choice. It is absorbed near the surface by most clear thermoplastics and delivers high edge quality with the right parameter setting.

How do I achieve a polished edge on PMMA?

With a CO2 laser, sharp focus, high cutting speed, low energy density and, where possible, in a single pass with thin sheets. Leave the protective film on, extraction close to the joint, minimise run-out times.

Why does polycarbonate yellow during laser processing and how do I avoid it?

Yellowing arises from excessive heat input. Avoid by drying, nitrogen as shielding gas, multi-pass strategies with lower power, a slight positive focus offset and a sufficiently high cutting speed.

Is transparent to transparent laser welding possible?

Yes, but only with special wavelengths around 2 µm, internal material absorption or thin absorber layers. Standard fibre lasers usually require an absorption-modified lower part.

What role does material drying play?

Residual moisture causes bubbles, haze and micro-cracks. PC, PETG and amorphous high-performance thermoplastics should be conditioned or dried before laser processing in order to safeguard edge quality.

When are alternative processes advisable?

For heat-sensitive parts, for PVC or when no discolouration is tolerated. Here waterjet or CNC milling with polishing are often superior, if necessary in combination with a laser for fine details.

How do I safeguard the process window in series production?

With documented parameters, regulated gas guidance, SPC checks of haze, edge Ra and geometry, regular optics care as well as batch traceability. Define limit samples and test plans.