Cold Laser Marking
Key Takeaways
- Two distinct routes share the name: ultraviolet photochemical marking, and ultrashort-pulse athermal ablation.
- For plastics the mainstream route is UV at 355 nm, which breaks polymer bonds directly instead of heating the material.
- Micron-scale resolution with no charring, discolouration or soot.
- Essential where heat would be unacceptable — medical implants, PCBs, thin films and precision plastics.
Cold Laser Marking
Cold laser marking uses short-wavelength ultraviolet light — typically 355 nm — to mark a polymer by breaking chemical bonds directly, rather than by heating the material until it degrades. The process is often called athermal or photochemical marking. It is not literally cold, but the heat-affected zone is small enough that thermally sensitive parts can be marked without melting, charring or distortion.
The Physical Basis
Photon energy rises as wavelength falls. A 1064 nm infrared photon carries roughly 1.2 eV — far too little to break a covalent bond, so its energy simply becomes heat. A 355 nm ultraviolet photon carries about 3.5 eV, which is comparable to the dissociation energy of a carbon–carbon bond in a polymer backbone.
At that energy the absorbed photon can sever the polymer chain outright. Material is removed or altered by photolytic bond scission, with only a modest thermal component. Two consequences follow, and they are the whole reason the technique exists:
- A very small heat-affected zone, so no melt rim, no charred halo, no crazing and no dimensional distortion.
- Strong absorption by most polymers. Nearly all organic polymers absorb well in the UV, even those that are effectively transparent at 1064 nm. That means high-contrast marks on natural, clear and light-coloured resins without compounding in a laser additive.
Cold Marking Versus Infrared Marking
| UV / cold marking (355 nm) | Fiber / infrared (1064 nm) | |
|---|---|---|
| Mechanism | Photochemical bond scission | Thermal degradation |
| Heat-affected zone | Minimal | Moderate to significant |
| Additive needed on natural resins | Often not | Usually yes |
| Feature resolution | Finer — shorter wavelength focuses smaller | Coarser |
| Marking speed | Lower | Higher |
| Capital and running cost | High | Moderate |
Where It Is Specified
- Medical devices — UDI marks that must survive autoclave, gamma or EtO sterilisation without compromising the substrate, and without an additive that would complicate biocompatibility approval.
- Thin-walled and film components, where infrared marking would deform or perforate the part.
- Transparent and light-coloured polymers, where infrared marking gives little or no contrast — see laser marking clear and transparent plastics.
- Fine codes and micro-marking, where a tight heat-affected zone preserves the cell definition that machine vision depends on.
- Additive-free applications, where regulatory, food-contact or optical requirements rule out compounding in an absorber.
The source is normally a frequency-tripled vanadate (Nd:YVO4) laser. Against its advantages, weigh higher capital cost, lower throughput and the finite service life of frequency-conversion optics. For pigmented and filled engineering resins that already mark well thermally, a fiber laser remains the more economical answer — the comparison is developed in UV laser marking vs fiber laser marking.
Two Routes to a Cold Mark
The term covers two different technologies, and confusing them leads to specifying an
expensive machine for a job the cheaper one does well. Both avoid a thermal mark; they
avoid it by different physics:
| Ultraviolet photochemical (355 nm) | Ultrashort pulse (ps / fs) | |
|---|---|---|
| How heat is avoided | Photon energy breaks bonds directly, so little energy becomes heat | Energy is delivered faster than heat can diffuse out of the interaction zone |
| Typical source | Frequency-tripled vanadate | Picosecond or femtosecond solid-state |
| Depends on ultraviolet absorption | Yes — the polymer must absorb at 355 nm | No — works through nonlinear absorption on almost anything |
| Relative cost | High | Very high |
| Throughput | Moderate | Low |
| Where it is chosen for plastics | The mainstream route: medical, clear and thin-walled parts | Where even the ultraviolet heat-affected zone is unacceptable, or on substrates that do not absorb in the ultraviolet |
For the great majority of polymer work the ultraviolet route is the right answer. An
ultrashort-pulse system is justified by a specific requirement — a sub-micron
feature, a multilayer film where one layer must be marked without disturbing the next, or
an optical component where any subsurface damage is disqualifying.
Qualifying a Cold Mark on a Medical Device
Cold marking is specified for medical devices more than for any other sector, because it
delivers a permanent unique device identifier without an additive that would reopen a
biocompatibility question. That places the mark inside the device master record, so the
qualification has to be documented rather than merely successful:
- Mark on the finished device material, in its final colour and with the
regrind fraction production will actually run. A qualification on a natural plaque proves
very little about a pigmented moulding. - Grade the code, do not inspect it. Unique device identifiers are
machine-read, so the acceptance criterion belongs in ISO/IEC 15415 terms for a
two-dimensional symbol. Fix the minimum grade, the aperture and the illumination in the
inspection method, not just in the operator instruction. - Re-grade after sterilisation. Steam autoclave, gamma and ethylene
oxide each affect polymer surfaces differently. Grade the symbol after the full validated
cycle count, not after one pass. - Confirm no subsurface damage. Section and examine samples for crazing
or haze under the mark, particularly on polycarbonate and acrylic where damage may not be
visible from the surface. - Bound the requalification trigger. Resin lot change, colourant change,
source power drift and optic replacement should each be named as events that require the
mark to be re-graded.
Troubleshooting Cold Marks
| Symptom | Likely cause | Correction |
|---|---|---|
| Weak contrast on a natural resin | The polymer absorbs poorly at 355 nm, or power has drifted down | Measure source power against its baseline before changing the recipe. If power is on target, the grade may genuinely need a laser additive despite the ultraviolet source. |
| Contrast falling gradually over months | Frequency-conversion crystal ageing | Trend measured power on a control chart. Index or replace the crystal on the trend, not on a fixed calendar interval. |
| Mark quality varies across the field | Optic contamination, or focus not held across the scan field | Clean the final optic and check purge air. Ultraviolet fixes airborne organics onto lens surfaces faster than infrared does. |
| Faint yellowing at mark edges | Residual thermal component — energy above what the photochemical process needs | Reduce power and recover contrast by slowing the scan rather than by adding energy. |
| Code grades well at inspection, fails on the line | Different illumination geometry, not a different mark | Match the line reader illumination and angle to the qualification method, and specify both. |
Related Terms and Reading
- UV laser marking vs fiber laser marking
- Vanadate laser
- Pharmaceutical and medical manufacturing
- Plastics laser marking solutions
- Accelerated aging
Applying this in production
The Sabreen Group provides independent engineering support for UV cold marking qualification for medical, optical and heat-sensitive components. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
What makes cold laser marking “cold”?
That the mark is made without meaningful heat. Two different mechanisms achieve this. Ultraviolet marking at 355 nm carries enough photon energy to break polymer bonds directly, so the change is photochemical rather than thermal. Ultrashort-pulse marking, using picosecond or femtosecond sources, delivers energy faster than heat can diffuse out of the interaction zone. Neither is literally cold, but in both the heat-affected zone is small enough that thermally sensitive parts survive unmarked outside the intended feature.
Is cold laser marking better than traditional laser marking?
It depends on the application. Cold laser marking is superior when working with delicate, heat-sensitive or micro-scale parts where thermal damage would be unacceptable. For general production marking on robust substrates, conventional thermal marking is faster and far more economical.
What lasers are used for cold marking?
For plastics, overwhelmingly a frequency-tripled vanadate source at 355 nm — it is the economical route and it marks most polymers without an additive. Picosecond and femtosecond sources also produce cold marks, by outrunning heat diffusion rather than by photochemistry, and are specified where even the small ultraviolet heat-affected zone is too much, or where the substrate does not absorb usefully in the ultraviolet. They cost substantially more.
Can cold laser marking be used on plastics?
Yes, and it is particularly suited to plastics prone to melting or discolouration under heat. It produces sharp, high-contrast marking on materials such as polyimide, polycarbonate and ABS that are sensitive to thermal input.
Is cold laser marking permanent?
Yes. The marks created are permanent and abrasion resistant, and are unaffected by moisture, chemicals or UV exposure, because the change is made in the material itself rather than applied to its surface.
Is cold marking the same as ultrashort-pulse marking?
Not quite — ultrashort-pulse marking is one of two routes that produce a cold mark. For plastics the more common route is ultraviolet marking at 355 nm, which avoids heat by breaking bonds photochemically rather than by outrunning heat diffusion. Picosecond and femtosecond systems achieve a similar result at substantially higher cost, and are specified when the application genuinely needs them.
Does cold marking avoid the need for a laser additive?
Often, but not always. Most polymers absorb far more strongly in the ultraviolet than at 1064 nm, which is why clear and light-coloured resins that will not mark under a fiber laser mark readily under ultraviolet. Contrast on natural polyolefins is still modest, so where a high-contrast code is required an additive may remain necessary. Screen the actual production grade rather than assuming.
How is a cold mark qualified for a medical device UDI?
Grade the symbol to ISO/IEC 15415 rather than inspecting it visually, mark on the finished device material in its production colour, and re-grade after the full validated sterilisation cycle count. Name resin lot change, colourant change, source power drift and optic replacement as requalification triggers, since all four move mark contrast without any alarm being raised.
