Key Takeaways
- The colourant frequently decides the mark, more than the base polymer does.
- Carbon black absorbs strongly at 1064 nm, so black grades often mark with no additive at all.
- Some organic pigments bleach or shift colour under the beam, producing marks nobody specified.
- Every colour is a separate qualification. A recipe proven on one colour of a grade does not transfer to another.
Ask why a laser mark failed and the answer usually offered is the polymer. Ask what
actually changed between the sample that worked and the production run that did not, and the
answer is far more often the colour. Colourants are present at low loadings and rarely
discussed in marking specifications, yet they routinely dominate the result.
Three Ways a Colourant Changes the Outcome
| Mechanism | What happens | Typical consequence |
|---|---|---|
| Absorption | The pigment absorbs at the laser wavelength and converts energy to local heat | Carbon black and some inorganic pigments make an otherwise unmarkable resin mark well |
| Scattering and opacity | The pigment scatters incident and reflected light | Titanium dioxide raises apparent contrast on a dark mark, but reduces beam penetration |
| Thermal decomposition | The pigment itself degrades, bleaches or changes colour under heat | Marks that come out an unexpected colour, or contrast that reverses from the intended direction |
The third mechanism is the one that surprises people. A mark is normally expected to darken
or lighten the substrate; where the pigment itself decomposes, the mark colour is set by what
the pigment turns into, which may be neither.
Behaviour by Colourant Type
- Carbon black. A strong absorber at 1064 nm even at low loading. Black
grades therefore frequently mark without a
laser additive — worth
checking before one is specified. The difficulty on black is the opposite: producing a
light mark, which requires foaming
and usually a MOPA source. - Titanium dioxide (white). Highly opaque and scattering. It provides
excellent background contrast for a dark mark, but it reflects and scatters the beam, so more
energy may be needed to reach the marking threshold. High TiO2 loadings can make a
grade harder to mark despite the attractive contrast. - Inorganic pigments — iron oxides, ultramarines, mixed metal oxides.
Generally thermally stable, so they survive the marking event and behave predictably. Many
absorb usefully. - Organic pigments and dyes. The least predictable group. Many decompose at
marking temperatures, bleaching to a lighter shade or shifting hue. This can be exploited
deliberately, but it far more often produces an unwanted result on a colour that was chosen
for appearance rather than for processing. - Pearlescent and effect pigments. Mica-based effects both scatter light and
respond to the beam, and they interfere with contrast measurement because the reading changes
with viewing angle. - Fluorescent colourants. Frequently heat-sensitive, and the fluorescence
may be lost in and around the mark, leaving a visible halo well beyond the intended feature.
Why Each Colour Needs Its Own Qualification
It is common to qualify a marking recipe on one colour of a grade and release it for the
whole colour family. That practice reliably produces problems, because the pigment package
differs between colours in loading, chemistry and thermal stability. A recipe tuned for a
natural grade is applied to a blue one and produces a weak mark; tuned for black, applied to
white, it burns.
What a defensible approach looks like:
- Treat colour as part of the material specification, not a cosmetic
variant. The marking recipe is qualified against grade and colour. - Screen every production colour with a power-speed matrix, measuring L*
rather than judging by eye. - Record a separate recipe per colour and recall it, rather than adjusting
by hand at changeover. - Re-match the colour standard with any laser additive present. Colour
approved on unmodified resin will not hold once an absorber is compounded in. - Name colourant change as a requalification trigger, including a change of
pigment supplier at nominally the same colour.
Diagnosing a Colour-Driven Marking Problem
| Symptom | Likely colourant cause | Action |
|---|---|---|
| Mark weak on a pale colour, good on natural | Opaque pigment scattering the beam before it couples | Raise energy modestly, or specify an additive for that colour specifically |
| Mark burns on a black grade at settings that suit white | Carbon black absorbing far more strongly | Separate recipe per colour; this is expected behaviour, not a fault |
| Mark comes out an unexpected hue | Organic pigment decomposing rather than the polymer | Substitute a thermally stable inorganic pigment if the colour permits |
| Coloured halo around the mark | Heat-sensitive colourant degrading outside the intended feature | Reduce energy and shorten pulse width to tighten the heat-affected zone |
| Contrast reading varies with viewing angle | Pearlescent or effect pigment | Fix measurement geometry in the inspection method before setting a tolerance |
| Contrast drifts within one colour over months | Pigment lot variation or dispersion change | Check the colourant certificate before re-tuning the laser |
Related Reading
- Laser Color Matching
- Laser Marking Additives
- Laser Marking Masterbatch: Complete Technical Guide
- Laser-Markable Plastics
- Process Parameters Affecting Mark Quality
Need help with this?
The Sabreen Group provides independent engineering support for colourant screening, marking recipe development and contrast qualification. Our engineering services team works with manufacturers on process development, material qualification and production troubleshooting. Contact us to discuss your application.
Frequently Asked Questions
Why does the same resin mark differently in different colours?
Because the pigment package differs in loading, chemistry and thermal stability between colours, and pigments interact with the beam directly. Carbon black absorbs strongly, titanium dioxide scatters, and many organic pigments decompose at marking temperatures. The base polymer is identical; what the laser actually interacts with is not.
Do black plastics need a laser additive?
Often not. Carbon black is a strong absorber at 1064 nm even at low loading, so black grades frequently mark well with no additive at all — worth screening before paying for one. The harder problem on black is producing a light mark, which needs foaming rather than carbonization and usually a MOPA source with short pulse width.
Can a marking recipe be shared across colours of the same grade?
It should not be. Qualifying on one colour and releasing across the family is a common practice and a reliable source of production problems, because a recipe tuned for black will burn white and one tuned for natural will barely mark a pigmented grade. Record and recall a separate recipe per colour, and treat colourant change — including a supplier change at the same nominal colour — as a requalification trigger.
Why has my mark come out the wrong colour entirely?
Most likely an organic pigment decomposing under the beam rather than the polymer degrading. The mark colour is then determined by what the pigment turns into, which may be lighter, a different hue, or simply unpredictable. Where colour permits, substituting a thermally stable inorganic pigment resolves it; where it does not, the marking energy has to be reduced enough to leave the pigment intact.
How should contrast be measured on a pearlescent or metallic-effect grade?
With the measurement geometry fixed and stated in the inspection method, because effect pigments change appearance with viewing angle and illumination. Without that, supplier and customer will legitimately obtain different readings from the same part. Fix the geometry before agreeing a tolerance, not after the first dispute.
