Key Takeaways
- Filler usually improves contrast, because it absorbs and scatters energy that the base polymer would transmit.
- Fibre emergence is the characteristic defect — the polymer vaporises at an energy the glass does not.
- Lower energy and more passes is the standard correction, not more power.
- Filled and unfilled grades of the same resin need separate recipes, and separate qualification.
A great deal of laser marking guidance is written as though a polymer were a single
material. In production it rarely is. Most engineering thermoplastics reaching a marking
cell carry 10–50% glass fibre, glass bead, talc, mica or wollastonite, and that filler
changes the marking result more than almost any other formulation variable — often
for the better, occasionally in ways that ruin an otherwise good process.
Why Filler Helps More Often Than It Hurts
The fundamental difficulty in marking
plastics with a 1064 nm fiber
laser is that most polymers barely absorb at that wavelength. The beam passes through
rather than depositing energy at the surface. Anything in the formulation that absorbs or
scatters therefore works in the marker’s favour, and filler does both.
This is why filled engineering resins so frequently mark acceptably straight out of the
mould while their natural, unfilled equivalents need a compounded
laser additive. Before specifying
an additive for a filled grade, mark it as it is — the cost may already have been paid
by the filler.
| Filler | Typical effect on marking | What to watch |
|---|---|---|
| Glass fibre | Markedly improved contrast; the most common reinforcement | Fibre emergence at the mark surface; anisotropic surface finish |
| Glass bead | Improved contrast with a more uniform surface than fibre | Less reinforcement, so it is rarely the structural choice |
| Talc and mica | Good contrast improvement; fine particle size gives a smoother mark | Mica can produce a pearlescent sheen that affects contrast measurement |
| Carbon fibre | Strong absorption — marks very readily | Substrate already dark, so contrast range is limited; consider a light foamed mark |
| Wollastonite | Moderate improvement, smooth surface | Generally well behaved |
| Flame retardant packages | Highly variable — some absorb strongly, some char and discolour | Qualify separately; never infer from the base resin |
Fibre Emergence: The Defining Defect
Glass and polymer do not respond to a laser at the same energy. The polymer reaches its
decomposition temperature and vaporises while the glass, with a far higher melting point,
does not. What remains is a mark with fibre ends standing proud of the surrounding surface
— visually rough, tactile, and in code applications a readability problem because the
raised fibres scatter the reader’s illumination.
The instinct is to raise power to “clean it up”. That makes it worse, removing more
polymer and exposing more fibre. The corrections that work:
- Reduce energy per pass and add passes. Several light passes remove
polymer more gradually and leave less relief than one aggressive pass. - Shorten pulse width where the source allows it. A
MOPA source set short reduces the
thermal depth and therefore the amount of polymer lost around each fibre. - Aim for a colour-change mechanism rather than removal. Where
carbonization can produce
adequate contrast without ablating material, fibre emergence largely disappears. - Consider mark location. Fibre orientation follows flow, so a mark placed
where fibres run parallel to the surface looks better than one where they present end-on. - Accept it where it is cosmetic only. On a hidden surface carrying a
traceability code, a slightly rough mark that grades acceptably is not a defect.
Filler Content Is Not Uniform Through the Part
The most commonly missed factor is that a “30% glass” moulding is not 30% glass
everywhere. Injection moulding produces a skin-core structure: a resin-rich skin where the
melt froze against the tool, and a core where fibre concentrates. Marking removes or alters
that skin.
The practical consequences are worth designing around:
- Mark depth changes what the laser is interacting with. A very shallow
mark sees mostly resin; a deeper one reaches the fibre-rich core and behaves differently. - Contrast varies with wall thickness, because skin thickness does.
- Gate proximity matters. Fibre orientation and concentration differ near
the gate, and marks placed there can read differently from the same mark elsewhere. - Moulding conditions move the result. Melt temperature and injection
speed change skin thickness, so a marking recipe can drift because the moulding process
drifted, with no material change at all.
Qualifying a Filled Grade
- Mark the production material, at production filler loading, colour and
the maximum permitted regrind fraction. Regrind matters more in filled resins, because
repeated processing shortens fibres and changes both appearance and flow. - Run a power-speed matrix on a moulded plaque, measuring L* in each cell
rather than judging by eye. - Choose the middle of the widest acceptable region, not the highest
contrast cell — filled grades vary lot to lot and the plateau is what survives it. - Examine the surface under magnification, not just for contrast. Fibre
emergence is often invisible to the naked eye and obvious to a code reader. - Grade any machine-readable code to ISO/IEC 15415 rather than
inspecting it visually. - Qualify filled and unfilled variants separately. They are different
materials for marking purposes, whatever the datasheet calls them.
Related Reading
- Glass Fiber Emergence: Solving Laser Marking Problems in Plastics
- Laser-Markable Plastics
- Process Parameters Affecting Mark Quality
- Laser Marking Glass-Filled Polymers and Injection Molding
- Plastics Laser Marking Solutions
Need help with this?
The Sabreen Group provides independent engineering support for marking development on filled and reinforced engineering resins. Our engineering services team works with manufacturers on process development, material qualification and production troubleshooting. Contact us to discuss your application.
Frequently Asked Questions
Does glass filler make plastics easier or harder to laser mark?
Easier, in terms of getting contrast. Filler absorbs and scatters energy the base polymer would transmit, which is why filled engineering resins frequently mark well with no laser additive while their unfilled equivalents need one. The trade-off is surface quality: filler introduces fibre emergence and a rougher mark, so the difficulty shifts from getting a mark to getting a clean one.
How do I stop glass fibres appearing in the mark?
Reduce energy per pass and use more passes, rather than raising power. Fibres appear because the polymer vaporises at an energy the glass does not, so removing more polymer exposes more fibre. Shortening pulse width helps where the source allows it, and aiming for a colour-change mechanism instead of material removal largely avoids the problem where contrast permits.
Why does mark quality vary across one filled moulding?
Because filler content is not uniform through the part. Injection moulding produces a resin-rich skin over a fibre-rich core, and skin thickness varies with wall section, gate proximity and cooling. Marking cuts into that structure to different depths in different places, so the laser is effectively interacting with a different material composition depending on where the mark sits.
Can I use the same recipe for filled and unfilled grades of one resin?
No. They behave as different materials for marking purposes regardless of what the datasheet calls them, because the filler changes absorption, thermal conductivity and surface behaviour. Qualify each separately, and treat a change in filler loading or type as a change requiring requalification rather than a minor substitution.
Does regrind affect marking on filled resins?
More than on unfilled ones. Repeated processing breaks fibres shorter, which changes flow, surface appearance and the way filler distributes through the skin. Screen at the maximum regrind fraction the line is permitted to run, and treat an increase in the permitted fraction as a process change rather than a purchasing decision.
