Laser Engraving Overview

Introduction

There are a lot of different laser engraver (marker) systems available to buy. To select the right one, one needs to answer a few questions.

• What material(s) need(s) to be engraved?
• How large should be the engraving area?
• Laser safety?
• At what accuracy and depth is the engraving required?

The working principle of a laser engraver is basically always the same: the laser energy is focused onto a small spot using a lens, the spot is moved around, and the laser is turned off/on according to the pattern to be engraved. The material in the spot will be burned away, oxidized or violently evaporated. For deep engraving one has to define engraving layers – like a reverse 3D printer or CNC milling machine.

Safety (with enclosure and without enclosure)

Laser engraving needs a lot of concentrated light. If it's enough to burn wood, plastics, metal (1000-2000°C!) than it's more than enough to burn living tissues (skin, eye..etc). That means, the user and the people around the machine must be protected from this very intense light. Laser engraving machines come in two different Laser Safety Classes: Laser Class I and Laser Class IV.

Class 1 machines are enclosed, the light which is (eventually) visible through a laser safety widow is not intense (and coherent) enough to cause any harm to the user. Also, if the user opens the enclosure intentionally or unintentionally, the laser must be off.

Class 4 machines must be put in a closed (controlled) area, for instance a lab, where only those people can enter who have the right laser protection (no cheapy laser goggles!) and are trained how to use the machine safely. Also, the area should be locked so no one can enter without laser protection while the laser is on.

These are only guidelines, so if you are planning to operate a Class 4 Laser, check your local regulations and get protection equipment according to the laser protection standards.

Laser types

There are a hundred of different laser types which can be used for engraving. The most popular types are CO2 lasers, Visible or Near-Infrared diode lasers, Fiber lasers (e.g. Yb-Fiber), Solid-State Lasers (e.g. Nd:YAG).

CO2 lasers operate at the far infrared wavelength (usually 10.6um or 10600nm)). The ones used for marking and end engraving have usually 20-60W power. Other wavelengths (9.3um, 10.2um) are also available, but from the engraving or marking point of view it won't make a difference.

Diode Lasers when they were first developed had near-infrared wavelengths (700-1000nm) and these is also today the ones available for relatively high power and low price. Often used also for pumping other lasers (solid-state and fiber lasers). The consumer grade or Do-It-Yourself laser engravers use 455nm (royal blue) or 405nm (blue) diodes. These diodes have relatively low power (2W, 5W, 15W) and are used in projectors and blue-ray devices so they are again relatively low price and at least visible wavelength.

Fiber lasers usually operate around 1060-1080nm (not visible) wavelength. For engraving purposes pulsed, 20-60W lasers are used. Higher power would need water cooling. These lasers have a very good beam quality and can be focused with a 160mm focal length lens to 30-50um spot size. Compared to CO2 and diode laser spots (0.1-0.2mm) this is a 4x – 36x increase in the intensity (light power per area). Also, the pulsed operation packs all the lasers energy into a several 10 thousand pulses per second. A 20W laser operated at 20k pulses per second, 100 nanoseconds pulse duration is equivalent (for the short time of the pulse) to a 10 000 W diode laser. Fiber lasers are also reliable, can operate 10000s of hours without noticeable power loss / degradation.

Solid State Lasers. The most popular solid state laser is Nd:YAG. It operates at 1064nm wavelength and has very good beam quality. The wavelength of these lasers are sometimes converted with a special crystal to 632nm (1064/2, green) or 355nm (1064/3, ultraviolet). The shorter wavelength will enable even smaller spot size and the absorption of some transparent materials (like glass) will be higher for green and UV, so it makes it possible to engrave. These lasers are unfortunately pricy and have a limited lifetime (few thousand hours).

Machine types by beam guidance

The focused laser beam is moved around on the workpiece using two methods or the combination of these two.

To engrave a small area precisely, laser galvanometer scanners are used. The laser is reflected on two mirrors which are mounted on little actuators. By tilting the one mirror the beam moves in X direction, by tilting the other mirror the beam moves in the Y direction. The beam can be moved like this very quickly, sometimes at 5-10 meters (!) per second speed and positioning accuracy of +/-2um. The marking area will depend on what kind of lens is used. Usual work area sizes are 100x100mm 250x250mm or 350x350mm. The larger the work area, the further it will be from the mirrors and the spot size will be larger. The focusing lens used in this arrangement is called f-theta objective lens. (It's a flat field projection lens). There are a lot of different options and properties to consider when using the laser galvo scanner method (beam angle, variable focal length in Z direction, beam expanders, correction files) but about these maybe in a different article.

To engrave a large area, a XY(Z) gantry system is used. The positioning accuracy and the travel speed is smaller compared to the galvo-scanner method, but in exchange a relatively large area can be engraved/marked. Systems like that work very similar to CNC-Milling and 3D Printing machines so no wonder sometimes these products are modular: just replace to laser head to a drill or extruder and you have a different processing capability. The laser is mounted either directly on the gantry system including its cooling and power supply (diode lasers) or only the mirrors and lenses are mounted on the stages and the laser source is fixed to the base (CO2 lasers). The latter method needs to be used with CO2 lasers as these are relatively large and have several connections (water cooling, high voltage connectors) which is hard to mount then directly on the moving stages. In this case the moving mirrors must be aligned perfectly and must be cleaned regularly to deliver constant engraving performance. Diode lasers are small so they can be moved directly above the workpiece.

The combination of the two methods is also used, but similar machines are large, expensive, and mostly used in industrial environment. These kinds of products use a laser galvo scanner mounted on a XY(Z) gantry system. This method combines the precision and speed of the galvo-system with the large area processing capability.

Popular systems

The most popular types of laser marking and engraving systems:

• Fiber laser with galvo scanner
• CO2 laser with gantry
• Diode laser with gantry or galvo scanner

Fiber Laser with Galvo Scanner

Not the most used and popular, but it's my favourite so I will start with this. The most basic fiber laser engravers start at about 2kUSD (Class 4, without enclosure, from China). The price might be the largest disadvantage of this type. Enclosed types start at about 10kUSD, 3 axis types start at about 20kUSD roughly. Premium systems sell for 20-60kUSD.

The laser powers are 20-40-60W (air cooled), above that usually 100W water cooled. There are usually two types of systems available: variable pulse length (5-100ns / MOPA) and fixed 100-120ns pulse length. The longer the pulse length, the more the workpiece will heat up locally. This can cause bending, discolouring, warping around the lasered area. Variable pulse length makes finer edges, and a larger variety of materials can be processed.

The laser has a wavelength around 1064nm, the pulses per second (repetition rate) can be adjusted from 20kHz to 200kHz. Power can be adjusted between 10-100%. Beam travel speed, number of loops (engraving cycles at the same position) have to be adjusted to the chosen materials and desired results.

Given the wavelength, only those materials can be processed effectively which absorb at least a small amount of the light at this wavelength. Metals (copper, brass, bronze, steel, stainless steel, magnesium, aluminium, bismuth, manganese, nickel, silver, gold, titanium ..) can be processed but some of them will need more power than the others. Copper for instance reflects >94% of the light so it won't be processed very easily. A low power (20W) laser without Z axis focus adjustment will engrave maximum (maximum) 2-3mm of aluminium or titanium, 4-5mm of bismuth, 0.2mm of copper within a reasonable time.   

Plastics are usually transparent at this wavelength and they melt and decompose very easily. Marking might be possible if the plastic has the right colour additive, but engraving is not possible with a 1064nm fiber laser at all.

The commonly available glasses are also mostly transparent and can't be processed for the same reason. There are some companies who sell frequency converted (Green or UV) fiber lasers, these would work of course, but are also very expensive (start price at about 50kUSD), usually need water cooling and by there is significant power loss during conversion.  

Wood and textiles will have low absorption or variable absorption along the workpiece and for this reason they can't be processed. Surprisingly leather can be engraved with fiber lasers very well, this material has a consistent absorption and thickness.  The same with rocks – every rock which absorbs the near-infrared light can be engraved 3-4 mm deep very effectively.

Due to the different absorption of paint/dirt/rust and metals, these lasers can be efficiently used to remove rust, selectively remove paint from metal surfaces and to clean metal objects (coins).

CO2 laser with XY gantry

CO2 Lasers are somewhat cheaper than fiber lasers, a small one would cost only a few hundred USD, larger ones could sell for thousands of dollars. These systems are heavy, need regular cleaning of the mirrors, need compressed air to blow away material from the focus and optics, the CO2 laser tube has a limited lifetime (3000h or more advanced ones 8000h). The mirrors are gold coated and the lenses are made of a special material called zinc-selenide (ZnSe).

CO2 lasers work at far infrared wavelength, this is the same as radiated heat from the fireplace for instance. Plastics, wood, glass, organic materials have a large absorption at this wavelength and can be processed very easily. However, metals are still highly reflective even in the far infrared and can't be processed. Removing of paint would work, but engraving is not possible.

The wavelength of the laser is very large, this will also make the spot size relatively large (0.1-0.2mm).

Diode lasers on gantry or mini-galvo laser scanner

Diodes lasers with gantry system or attached to mini galvo-scanner are recently more and more popular. They are very compact, reliable, easy to use and cheap. You can get a DIY kit for less than 100 USD and even a good quality galvo scanning will only set you back a few hundred dollars.

Unfortunately, there is a compromise for the low price. The beam quality is not very good, spot size is similar to the CO2 laser's, but processing speed and achievable engraving depth will be smaller. Also, there is usually no gas assistance (compressed air) so the burned material won't be removed from the marked positions.

Some of the transparent materials (plastics, glass) will not absorb enough light for processing. The average power is anyway to low and not even pulsed. (By pulsing the 5W laser at 0.1s on 0.1s off, the power will be lower (2.5W) – compared to the fiber laser where the power of the off-time is packed into the on-time.)

Wood, leather, painted surfaces can be marked but deeper engraving will take a lot of time or won't work at all.

After all this, you might ask if there was a laser system which can engrave everything? Yes, there is:

Special laser engraving systems

The key to engraving everything is making the pulses shorter and packing as much energy as possible in this shortest time / pulse.

Some fiber and solid-state lasers have pulses 0.5-1ns long. This is about the time when the thermal effects are getting negligible, there won't be any heating and the intensities are getting in the range where even glass can be marked. However, this is still not enough. To be able to engrave everything, one need to go down below 0.1ns (100ps). The usual pulse duration of picosecond lasers is around 20picoseconds, this combined with about 4W average power and few 10s of kHz repetition rate will be enough to engrave any material. So why are not too many of these around? Picosecond laser systems still cost in 2021 around 60-100kUSD at least (yes, even from China). Special optics, clean room required to build these, somewhat special optics required to guide the light, special knowledge required to design such a system. They are getting more and more available, but it will take still a long time until everyone can engrave any material with just one laser.