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MOPA colour marking on stainless and titanium
Colour on bare metal is not ink and not a coating. It is a controlled oxide layer a few hundred nanometres thick, and the colour you see is light interfering with itself inside that layer. Get the layer thickness right and you choose the colour. Get it wrong and you get brown.
What makes the colour
When you heat stainless or titanium in air, it grows a transparent oxide film. Light reflects off both the top of that film and the metal underneath, and the two reflections interfere. Depending on how thick the film is, some wavelengths cancel and others reinforce - so the surface reads as gold, purple, blue or green. It is the same physics as the colours in an oil slick or a soap bubble.
That means colour marking is really thickness control. The laser is a very precise way of depositing exactly enough heat to grow a film of a chosen depth, and colour is the read-out that tells you how you did.
It also means the colour is not a pigment. There is nothing to fade in the usual sense - but there is a thin film that can be scratched through or thickened further by more heat.
Why it takes a MOPA
A standard Q-switched fiber laser has a fixed pulse width. You can change power, speed and frequency, but not how long each pulse lasts. A MOPA (Master Oscillator Power Amplifier) separates the seed from the amplifier, which lets you set pulse width independently - typically anywhere from about 2 ns to 500 ns, with repetition rates in the 20-1000 kHz range.
That independence is the whole point. Short pulses at high frequency deposit heat in small, controlled doses - which is exactly what growing a thin, even oxide requires. A fixed-pulse fiber tends to dump too much energy per pulse, and instead of a tuned film you get the deep grey-black of ordinary annealing.
| Knob | Typical range | What it changes |
|---|---|---|
| Pulse width | 2-500 ns | Energy per pulse. The primary colour control. |
| Frequency | 20-1000 kHz | Pulse overlap and total heat input. |
| Speed | 200-3,000 mm/s | Dwell time per unit area. |
| Line interval | 0.005-0.02 mm | How evenly the film builds. |
| Power | 10-60% | Coarse level. Usually not where you tune. |
Titanium is the easy one
If you are starting, start on titanium. It grows a clean, stable, strongly-coloured oxide across a wide parameter window, and the colours are vivid - the blues and purples in particular are hard to get on anything else. Grade 2 and Grade 5 both behave well.
Stainless is harder. The window is narrower, the colours are more muted, and the alloy matters: 304 and 316 do not respond identically, and a mirror-polished surface behaves differently from a brushed one. Do not assume a parameter set from one supplier's sheet transfers to another's.
Why your colours drift
This is the complaint that brings people to forums, and the causes are boringly consistent:
- Surface prep is not identical. Colour is a surface-film effect, so anything on the surface changes it. Fingerprints, residual oil, a slightly different brush finish - all shift the result. Degrease with isopropyl alcohol and handle parts by the edges.
- Focus is off by a fraction. Colour marking is far more focus-sensitive than engraving. A part sitting a half-millimetre higher than the last one will come out a different colour, not just a slightly softer one.
- The part is heating up. Marking a large area, or several parts in quick succession, raises bulk temperature and the film grows faster. Colours drift across a batch. Let parts cool, or space the passes out.
- Air movement over the part. Strong air assist cools the surface and changes oxide growth. Consistency matters more than the setting itself - pick one and keep it.
The practical answer is a colour swatch card. Mark a grid on your actual stock stepping pulse width against speed, label every cell, and keep it. That card is worth more than any published parameter table, because it was made on your machine with your material.
Durability, and what colour is not for
Be straight with customers about this. The oxide film is thin - a few hundred nanometres - so:
- It resists handling and indoor use well.
- It scratches. Abrasion goes straight through the film and takes the colour with it.
- Sustained outdoor UV and weather will dull it over time.
- Anything that reheats the part - welding nearby, a hot dishwasher cycle repeated for years - can shift the colour.
Do not promise colour marking on anything that will be tumbled, sandblasted, or subject to real wear. For those parts, a deep annealed black or a proper engraved mark survives where colour will not.
On food contact and medical parts, check the requirement rather than assuming. The film is the metal's own oxide, which is often exactly why colour marking is chosen for surgical instruments - but "often" is not a specification, and the buyer's standard is the one that counts.
Is it worth the machine?
A MOPA costs meaningfully more than a fixed-pulse fiber of the same wattage. The question is whether colour is a product feature you can charge for - branded tooling, anodised-look parts without anodising, knife and EDC work, medical instrument coding - or a novelty you will use twice.
Work out the per-part number before deciding. Our fiber marking calculator gives cost per part at your real cycle time, and the ROI calculator turns that into a payback period at your actual volume. If colour is a genuine line of work, a MOPA pays back quickly. If it is a maybe, a standard fiber marks everything else just as well.
FAQ
How does a MOPA laser make colours on metal?
It grows a transparent oxide film on the surface a few hundred nanometres thick. Light reflecting off the top of that film interferes with light reflecting off the metal underneath, and the film thickness decides which wavelengths reinforce. The colour is interference, not pigment.
Can a normal fiber laser do colour marking?
Not reliably. A standard Q-switched fiber has a fixed pulse width, so you cannot control energy per pulse independently of frequency. It tends to deposit too much heat per pulse and produces annealed grey-black rather than a tuned oxide film.
Why do my MOPA colours change between parts?
Usually surface prep, focus height, or heat build-up. Colour is a surface-film effect, so fingerprints and oil shift it, a half-millimetre focus error changes it outright, and parts marked in quick succession run hotter and grow thicker films. Degrease, re-focus per part, and let parts cool.
Is laser colour marking durable?
Indoors and with normal handling, yes. It scratches though - abrasion goes through the thin film and removes the colour - and sustained outdoor UV dulls it over time. Do not specify it for parts that will be tumbled, blasted, or heavily worn.