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.
Related Terms and Reading
- Plastics laser marking solutions
- Ablation
- Laser marking additives
- Process parameters affecting mark quality
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.
