Carbonisation

Carbonisation

Key Takeaways

• Carbonisation 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.

Carbonisation

In plastics laser marking, carbonisation 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 carbonisation, 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.

The wider material-science background is set out in basic material science for plastics laser marking.

Carbonisation Versus Foaming

Carbonisation is one of two competing thermal outcomes, and which one dominates is a matter of how fast energy is delivered:

• Carbonisation produces a dark mark and is favored 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 favored 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 carbonisation 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 Carbonised Marks Last

Carbonisation 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.

That last row is why carbonised 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

Carbonisation is thermal decomposition, so it necessarily produces decomposition products. Handling them properly is what separates a clean mark from a gray, sooty one:

• Redeposited soot is the usual cause of a dull mark. Ejected particulate settling back onto the surface reads as a hazy gray 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 Carbonised Marks

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 carbonisation 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 carbonisation residue on the surface?

By optimising the laser setup. With correct parameters there is minimal surface carbonisation residue; poor control leaves visible soot-like deposit that compromises appearance and can affect code readability.

When is carbonisation 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 carbonised 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 carbonisation is the default mechanism for traceability, serialisation and anti-counterfeiting work.

Why does the mark come out gray and hazy instead of black?

Soot redeposition. Ejected particulate settles back onto the surface as a gray 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 carbonise 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.

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