Black Mark on Stainless Steel
The classic MOPA showcase: deep, true black on raw stainless that won't rub off. The technique is short pulses, very fast speed, and slightly defocused beam.
Why it works (the science in one paragraph)
Short pulses (1-4 ns) with high frequency deposit just enough energy to grow a microscopic layer of chromium and iron oxides on the surface without ablating the metal. The oxide layer traps light, so it reads as black. Defocus spreads energy slightly to widen the heat-affected zone evenly.
This defocused short-pulse approach is one route to black. There is also a focused annealed black (slow speed, high frequency ~325 kHz, ~20ns pulse, very tight interval) that grows the same oxide family as color annealing, see Color Annealing on Stainless / Titanium. Use the annealed route when you want black that matches annealed colors on the same piece.
Starting settings
| Param | 70mm small parts | 150mm large parts |
|---|---|---|
| Speed | 3000-5000 mm/s | 3000-5000 mm/s |
| Power | 10-15% | 15-25% |
| Frequency | 35-60 kHz | 35-60 kHz |
| Pulse width | 2 ns | 2 ns |
| Defocus (raise Z) | +1.5 to +2.5 mm | +2.5 to +4.0 mm |
| Hatch interval | 0.01 mm | 0.01 mm |
| Passes | 1-2 | 1-2 |
Test grid procedure
- Sticker chart: rows = power, columns = speed.
- 9 squares (3×3) varying ±20% around starting values.
- Run grid, photograph under daylight LED at 45°.
- Pick the square with no rainbow tint and smooth black surface. Black should not wipe with alcohol.