Engraving, cutting, marking: laser processes explained

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A laser can remove material, cut through it or simply discolour the surface. Which result you get depends on laser type, power, speed and material. Here you can see what happens in each process and which laser can do what.

Engraving: removing material

When engraving, the laser removes material at the surface. Wood burns and vaporises at the surface, so the engraving turns dark and slightly recessed. With acrylic, the material vaporises and the engraving turns matt and white. On slate and stone, the surface becomes lighter.

The laser engraves areas line by line, much like a printer. The head traces one line, moves on by a small distance and traces the next. This line interval determines the resolution. In LightBurn, DPI = 25.4 divided by the line interval in mm. An interval of 0.1 mm therefore gives 254 DPI.

The laser can also trace lines and contours directly along the path. This is called vector engraving or line mode. Low power produces a fine line, while more power and several passes produce a cut.

Cutting: separating material

When cutting, the laser traces the contour slowly until the material is cut through. Thick material needs several passes. What matters here:

• Power: More watts cut thicker material or cut faster. See Understanding laser power.
• Focus: The spot must be small and in the right place, often slightly inside the material for thick stock.
• Air assist: An airflow blows smoke and embers out of the kerf. Without it there are more burn marks and flaring.

On hobby lasers the kerf is usually only a few tenths of a millimetre wide. For precisely fitting joints you have to allow for it in the design.

What can be cut depends heavily on the laser type. Diode lasers cut wood, plywood, leather, felt, paper and dark acrylic. They do not cut clear acrylic, because the blue light passes straight through. CO₂ lasers cut almost all non-metals, including clear acrylic. Fibre lasers only cut thin sheet metal, usually under 1 mm. UV lasers are not intended for cutting.

Marking: discolouring the surface

Marking removes hardly any material. The laser changes the surface so that contrast appears. This is especially important for metal and plastic.

Annealing on stainless steel and titanium: The laser heats the surface without removing it. A thin oxide layer forms, which appears dark or coloured depending on the temperature. The surface stays smooth. This is typical for fibre lasers. A MOPA fibre laser, as used by the xTool F2 Ultra, for example, can produce colours on stainless steel in a controlled way via the pulse duration.

Colour change in plastics: Many plastics change colour under a fibre or UV laser without the surface changing noticeably. Whether and how well this works depends heavily on the plastic and its additives. Note: PVC, polycarbonate and ABS do not belong under a hobby laser. More on this under Prohibited materials.

Removing coatings: With anodised aluminium, painted metal or coated tumblers, the laser removes only the top coating. The bright metal underneath is revealed. Diode lasers and CO₂ lasers can do this too.

Marking spray: On bare metal, special marking sprays or pastes help diode lasers and CO₂ lasers. The laser fuses the layer onto the metal, and you wash off the rest. The result is dark and durable, but needs one more working step.

Deep engraving

For a noticeably deep engraving, the laser runs over the same area many times. In wood this is quite quick with diode and CO₂, and you have to brush off smoke and soot afterwards. In metal you need a fibre laser and many passes. Depending on power and depth, that can be dozens, often with a changing fill direction so that the bottom stays level.

A special form is 3D or relief engraving. Here a greyscale image controls the power: light areas get little energy, dark areas get a lot. This produces a relief with different depths. It works mainly in wood and takes a lot of testing.

Photo engraving

A laser basically knows only on and off. To give a photo shades of grey, the software converts it into a dot pattern. This is called dithering. LightBurn offers several image modes for it:

• Jarvis and Stucki: error diffusion, usually the best choice for photos.
• Dither and Atkinson: also error diffusion, and Atkinson suits flat-colour motifs better.
• Grayscale: the laser varies the power instead of placing dots. This needs a material that responds evenly to power.
• Threshold: black and white only, for logos and line art.

For good photo engravings, the line interval should roughly match the size of your laser spot. If it is much smaller, the lines overlap and the image clogs up. If it is larger, the result looks pale. Light, even material such as maple or linden, anodised aluminium or slate gives the best results.

Which laser can do what?

The table is a rough guide. Results vary depending on device, power and material. With every new material, first run a test grid with different powers and speeds. How to set one up is explained in the article Material test.

Safety for all processes

Whether you engrave, cut or mark: smoke is produced, and the material can catch fire, more so when cutting than when engraving. Never leave the laser unattended, keep a fire extinguisher ready and, with open devices, wear safety glasses that match the wavelength.

Continue with: Laser types compared and Laser parameters