Laser Marking FAQs: Metals and Particularly Stainless Steel

After 12 years experience of laser marking stainless steel I thought that it might be helpful to write something on this subject. This is not meant to be particularly technical, but more as a guide to our customers so that they can choose what kind of mark they would like. I do not cover the use of proprietary pastes which are sometimes used to blacken the surface when combined with CO2 marking lasers. These pastes certainly have their place, but add a process that is usually unnecessary with a YAG laser and are incompatible with any of the medical components that we mark.

Which effects can be achieved with Nd:YAG lasers, how, and with which metals?

Different effects can be achieved with a Nd:YAG marking laser on metals and this is done by optimising the laser power, speed, pulse frequency and focus. Stainless steel, however, allows a greater variation in visual effect than most metals and in our experience titanium is the only other metal that comes close. With titanium, colour is fairly easy to achieve and even stainless steel can be coloured under certain conditions. Colouring of stainless steel is covered later on in the article.

Which colours can be marked on which materials?

Broadly speaking stainless steel can be marked black, different shades of brown (which are not very visually appealing), gold, white and with a polished mark where the surface appears smoother than the surrounding area and so reflects light differently. These are surface marks of almost no depth. It is also possible to engrave the surface although the material removal rate is very low compared to mechanical methods.

How do you choose or decide on a mark?

The mark chosen is usually dictated by cost, appearance, permanence and other technical considerations. For example, to achieve a black surface mark we have to run the lasers at a fifth of the speed used to achieve a good white mark. This means that large black logos can take a long time. It is important to remember that laser marking is not a printing process and involves moving the laser beam over the entire area to be marked. To double the logo size quadruples the marked area and therefore the marking time.

Can you give me some examples of different markings?

See below:

Black surface mark:

Good for text, small logos, general identification, Datamatrix codes and serial numbers. As a surface mark there is no biological trap and so this mark is often used for medical implants and devices. It gives a good aesthetic appearance.

This type of marking is bad for large logos (e.g. over 40mm in size). Some forms of passivation can remove the mark and it can sometimes be hard to read from certain angles where the surrounding surface reflects dark. The mark can be removed with aggressive surface abrasion although this is not usually an issue as in most instances the mark is considered permanent enough. (Experience has shown that where a customer wishes to remove a black surface mark, abrasion of the surface is required with emery paper or Scrotchbrite and it can take some effort.)

Matt black engraved mark:

Good for extra permanence where there is abrasion (i.e. kiosk keyboard buttons.) Some material removal during laser process. Good for high contrast. Lower processing temperatures.

Bad for biological traps and where a surface burr can pick up contamination or damage surgical gloves.

White mark:

Good for speed, appearance, (particularly where the surface is polished), low processing temperatures. Subtle effect on brushed surface.

Bad for contrast where the mark is overly subtle.

Combination of the above:

Good for when logos cannot be easily represented monochromatically. Gives a good aesthetic effect. Where only the outline is black the logo can still be marked quite quickly in comparison to a full black logo. This is particularly useful for larger logos and graphics.

Gold mark:

This mark is similar to a white mark, but the surface is slightly oxidised by using more laser energy. This mark provides more contrast. Some stainless steel does not give a good white mark and a light gold is the best that can be achieved, particularly when extra power is needed for curved surfaces.

Polished mark:

This mark can be very effective against a brushed surface or a plain 2B or rolled surface. The laser very slightly melts and smoothes the surface and this reflects light in a different way to the surrounding metal. We use this type of marking for signs, where a large area needs to be marked (up to 900mm x 600mm.) This is an aesthetic mark, and would not normally be used for serial numbers or part marking.

Engraved mark:

Good for very precise material removal. Effectively the process is like milling with a 50 micron cutter with minimal aspect ratio considerations. Depth can be 1mm plus. The laser gives very sharp corners, little or no draft angle and is non-contact. There is no tool to blunt or break and no steel is too hard.

Bad for multiple components where large volumes of material need to be removed. The removal rate, in our experience, is at a maximum of 13 cubic mm/minute for steel. We have heard of rates as high as 30-40, but have never actually witnessed it or achieved it. With a 50 micron beam the removal rate is closer to 3 cubic mm/minute. Some metals give a higher removal rate and certainly silver and aluminium are better than steel.

Colour marking:

We are not aware of colour laser marking being used commercially although there are people trying to make the process more repeatable and reliable. Material surface finish, material batch variations and material thickness all prevent this from being an easy, repeatable process and many factors affect the end result including beam stability. In our experience the laser has to be run very slowly, and even if the resulting colour is distinct and homogeneous it may not be the shade of colour that is required. Discussion of Pantone is pointless, for example.

We would be interested to hear from anyone who sees colour laser marking being used commercially at the moment.