Carbonization
Key Takeaways
- Carbonization is the most common laser marking mechanism on plastics, producing dark or black contrast.
- Absorbed energy raises local temperature enough to thermally degrade the polymer around the absorption site.
- Limited oxygen inside the substrate causes charring rather than burning, which is what forms the dark mark.
- Mark darkness depends on two things — the energy absorbed, and the material’s unique thermal degradation pathway.
Carbonization
In plastics laser marking, carbonization is the thermal degradation of a polymer at the point of laser absorption, leaving a carbon-rich residue that appears as a dark or black mark. It is the most common marking mechanism on plastics, and it is the reason a laser can produce a permanent, high-contrast mark on a light-coloured part without adding any ink, label or coating.
The sequence is short and entirely localised. Laser energy is absorbed within a very small volume at or just below the surface. Local temperature rises past the polymer’s decomposition threshold. Chemical bonds break, volatile fragments are driven off, and what remains behind is a residue enriched in carbon — dark, chemically stable and integral to the part rather than applied on top of it.
Why the Polymer Chars Instead of Burning
Burning requires oxygen. Inside the bulk of a moulded part, oxygen supply is severely limited, so combustion cannot proceed to completion. Instead the polymer pyrolyses in an oxygen-starved condition, which is precisely the condition that favours char formation. The surface itself can burn where it is exposed to air — and visible sooting or a brown halo around the mark is a common symptom of exactly that — but the mark itself is formed by charring beneath.
Polymer Chemistry Governs Char Yield
This is the single most useful thing to understand about carbonization, because it predicts which materials will mark well before any trial is run. Polymers with aromatic rings in the backbone char readily; aliphatic polymers tend to depolymerise into volatiles and leave little behind.
| Polymer family | Backbone | Char tendency | Practical consequence |
|---|---|---|---|
| Polycarbonate, PPS, PSU, PEI | Aromatic | High | Mark dark and cleanly, often with no additive |
| ABS, PET / PBT | Partly aromatic | Moderate to high | Generally good dark contrast |
| Polypropylene, polyethylene | Aliphatic | Low | Depolymerise to volatiles; weak or absent dark mark without additives |
| POM (acetal) | Aliphatic, unzips readily | Very low | Notoriously difficult; tends to ablate rather than char |
The wider material-science background is set out in basic material science for plastics laser marking.
Carbonization Versus Foaming
Carbonization is one of two competing thermal outcomes, and which one dominates is a matter of how fast energy is delivered:
- Carbonization produces a dark mark and is favoured by longer pulses and higher cumulative heat input. It gives excellent contrast on light-coloured substrates.
- Foaming produces a light mark. Gas evolved during degradation is trapped as microbubbles that scatter light, and is favoured by shorter pulses and lower net heat. It is how legible marks are produced on dark and black plastics.
A MOPA laser, with independently adjustable pulse width, is the usual means of steering deliberately between the two on the same material.
Controlling the Result
Two variables govern mark darkness: the energy actually absorbed, and the specific degradation pathway of that resin grade. The second is why two grades of nominally the same polymer, from different suppliers or with different additive packages, will not mark identically at the same settings. Colourants, fillers, flame retardants, stabilisers and regrind content all shift the outcome.
Practical levers, in the order usually worth trying:
- Pulse width and repetition rate — the primary control over whether energy accumulates as heat or is deposited and dissipated.
- Marking speed — governs dwell and therefore local temperature rise.
- Average power — raise last, not first; excess power is the usual cause of burning and surface damage.
- Line spacing and hatch — controls cumulative heat in filled areas.
Excessive carbonization residue on the surface is a symptom of an unoptimised process, not an inevitability. With correct parameters, surface soot is minimal; where a manufacturing process leaves residue, it points back to the settings rather than the material. Related failure modes are catalogued in troubleshooting common laser engraving problems on plastics.
Mark Durability and Why Carbonized Marks Last
Carbonization is specified for permanence more than for appearance, and the reason is
structural: the contrast is a chemically altered region of the part itself, extending
below the surface, rather than a layer sitting on it.
| Exposure | Carbonized mark | Foamed mark | Printed or inked mark |
|---|---|---|---|
| Abrasion and handling | Excellent — extends below the surface | Moderate; foam is mechanically weak | Poor to moderate; depends on ink and pretreatment |
| Solvents and cleaning chemistry | Excellent | Good | Often the failure mode |
| Steam autoclave | Excellent | Good | Variable; frequently fails |
| Ultraviolet and weathering | Excellent — carbon does not fade | Good, though the surrounding polymer may yellow | Pigment-dependent; fading is common |
| Deliberate removal | Very difficult without visible damage | Can be abraded away | Removable |
That last row is why carbonized marking is the default for traceability, serialisation
and anti-counterfeiting work: a mark that cannot be removed without leaving evidence of
tampering carries a security value that an applied mark does not.
Managing Residue and Emissions
Carbonization is thermal decomposition, so it necessarily produces decomposition
products. Handling them properly is what separates a clean mark from a grey, sooty one:
- Redeposited soot is the usual cause of a dull mark. Ejected
particulate settling back onto the surface reads as a hazy grey halo rather than a crisp
black mark. This is an extraction problem, not a parameter problem, and adding power makes
it worse. - Extract at the work point, not at the enclosure. Capture velocity at
the mark is what removes particulate before it settles; a general enclosure extract
removes it after it has already landed. - The same particulate fouls the final optic, which reduces delivered
energy and causes the slow contrast decline that is often misread as material variation. - Match filtration to the polymer. Particulate filtration alone does not
address organic vapour, and flame-retardant packages can produce decomposition products
quite different from the base resin. - PVC must not be laser processed — it evolves corrosive hydrogen
chloride. Fluoropolymers warrant specific assessment. These are material exclusions rather
than ventilation problems. - Post-process cleaning may be legitimate on cosmetic parts, but if a
cleaning step is needed to make the mark acceptable, the extraction is under-specified.
Troubleshooting Carbonized Marks
| Symptom | Likely cause | Correction |
|---|---|---|
| Mark grey and hazy rather than crisp black | Soot redeposition from inadequate extraction | Improve capture velocity at the work point. Adding power deepens the haze rather than the contrast. |
| Little or no darkening on a natural resin | Aliphatic backbone with low char yield, such as a polyolefin or acetal | These depolymerise to volatiles rather than charring. A laser additive is effectively required. |
| Mark dark but surrounded by a brown halo | Heat spreading beyond the intended feature | Raise speed or shorten pulse width. The halo is degradation outside the mark, and it will worsen with more power. |
| Blistering or raised texture in the mark | Gas evolution trapped below the surface | Reduce energy per pass. Also check that the resin was properly dried before moulding. |
| Contrast declining gradually across a production run | Optic fouling from marking fume | Measure power at the workpiece and clean the protective window. This is usually mistaken for material drift. |
| Cracks appearing around the mark days later | Thermal stress added to residual moulding stress | Move the mark away from gates and weld lines, reduce energy, and consider annealing before marking. |
| Good contrast, poor machine readability | Heat-affected zone bleeding into adjacent code cells | Shorten pulse width to tighten the affected zone, and verify by grading to ISO/IEC 15415 rather than by eye. |
Related Terms and Reading
- Plastics laser marking solutions
- Ablation
- Laser marking additives
- Process parameters affecting mark quality
- Digital Product Passport
Applying this in production
The Sabreen Group provides independent engineering support for laser marking process development and resin selection for reliable mark contrast. 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 is carbonization in the context of laser marking?
A thermal chemical reaction, also called charring, where laser energy absorbed by the substrate raises the local temperature high enough to thermally degrade the polymer. The carbon-rich residue left behind appears as a dark or black mark. It is the most common surface reaction in plastics laser marking.
Why does the polymer char rather than burn?
Because oxygen is limited inside the substrate. Burning requires oxygen, and while the surface can burn in its presence, the restricted supply within the material means the polymer chars instead, forming dark marking contrast.
What controls how dark the mark comes out?
The energy absorbed, and the material’s own thermal degradation pathway. Two grades of the same polymer family can degrade at different temperatures, so the same laser settings will produce different mark darkness on each.
How do you avoid carbonization residue on the surface?
By optimising the laser setup. With correct parameters there is minimal surface carbonization residue; poor control leaves visible soot-like deposit that compromises appearance and can affect code readability.
When is carbonization the wrong mechanism to use?
When you need a light or white mark on a dark substrate. That requires foaming, where an additive releases steam during degradation to create a light-scattering structure — and the polymer must degrade at a higher temperature than the foaming additive to prevent charring taking over.
Why is a carbonized mark harder to remove than a printed one?
Because it is a chemically altered region of the part extending below the surface, not a layer applied to it. Abrasion that would remove ink only exposes more of the same altered material, and removing it entirely means removing part geometry, which leaves visible evidence. That is why carbonization is the default mechanism for traceability, serialisation and anti-counterfeiting work.
Why does the mark come out grey and hazy instead of black?
Soot redeposition. Ejected particulate settles back onto the surface as a grey halo, which is an extraction problem rather than a parameter problem — adding power increases the particulate and deepens the haze. Improve capture velocity at the work point rather than at the enclosure, since the point is to remove particulate before it lands.
Which polymers will not carbonize usefully?
Aliphatic ones with low char yield — polyolefins and acetal in particular depolymerise into volatiles rather than leaving a carbon residue, so there is little to darken. Aromatic backbones such as polycarbonate, PPS, PSU and PEI char readily and mark dark with no help. For the low-char families a laser additive is effectively mandatory, or the job moves to a different marking mechanism.
