Getting the most out of marking

Andrew May, director of ES Precision, shares five tips to get the best laser marking results 

ES Precision is only two years old, however the three staff that program our eight laser marking workstations have been working with industrial lasers much longer than that – around 75 years! – which is certainly enough time to have developed a few ‘tricks of the trade’.

Our firm’s business is a job shop – we undertake the subcontract laser marking of components or finished goods on behalf of our customers. We are able to mark tiny ID matrices and alphanumerics barely visible to the eye, cut and mark tamper-evident labels, and produce scales or other functional marks on medical devices and automotive parts. We’ve done this on hundreds of different materials: metals, plastics, coated products, ceramics, wood, leather, and more.

Laser marks are permanent, high resolution, can have excellent contrast, and can be produced in a variety of ways depending on the material. Metals such as stainless steel, for example, can either be engraved with depth, which provides little contrast, or instead annealed to generate a smooth, black oxidised surface, providing high contrast. Plastics on the other hand can either be carbonised, producing a dark mark, or foamed, producing a white mark. For coated materials – such as back-illuminated car dash switchgear or laminated labels – the underlying layer can be seen even after the top layer has been cleanly ablated.

^{Figure 1: A test matrix showing the range of possible marks on stainless steel by varying Q-switch frequency (x-axis) against galvo speed (y-axis). (Credit: ES Precision)}

The action of the laser, delivered at high speed by galvo mirrors, depends not only on the material but also on the parameters of the laser selected. Lasers commonly used in laser marking and engraving applications can be solid state (Nd:YAG, fibre; Vanadate) or gas (CO₂). Most operate in the invisible infrared part of the spectrum, but some can be frequency-doubled or even tripled to generate laser pulses in the green or UV part of the spectrum. While certain parameters are intrinsic to the type of laser source being used – the wavelength and maximum power available, for example – others will be programmable during testing – such as the peak power, Q-switch frequency and galvo speed.

Whether you want to get the most out of a laser you already have, or are considering buying a new laser for a particular task, the following tips might help you to improve the results you get:

1. Focus is key. The laser will usually give the crispest, cleanest results only when perfectly in focus, and the tolerance to getting this wrong (‘depth of focus’) can be as little as 1mm. Double-check the laser is properly in focus if you are unhappy with your results.
2. Select the right lens for the task. It is tempting to use a large marking field – why not take a lens so that you can mark anywhere over a larger area? In addition, a larger lens gives the user a bigger depth of focus (see tip 1) – what’s not to like? The reason is down to basic optics: doubling the focal length of the lens not only doubles the field dimension, but also doubles the focus diameter of the spot. For a pulsed laser that will mean that each pulse only has one quarter of the peak power density. That can easily make the difference between an acceptable result and an unacceptable one.
3. Mark a matrix to vary one parameter against another to efficiently test a material’s response to the chosen laser. Several different test matrices can be created, but one that is particularly useful is to vary pulse- or Q-switch frequency against galvo speed – see figure 1 for an example on stainless steel.
4. Faster marking can be better – two or more passes, sometimes each with different parameters, can yield better results than a single slower pass in the same total mark time. Deeper marks can be produced with many repeated rapid passes – erosion engraving – rather than one slow, thermally-damaging pass. Another trick useful when good contrast is essential, for example when marking a barcode onto bare metal, is to etch the whole region of the code with a mark to lighten up the metal and then mark the dark bars on top. See figure 2 for an example.
5. Try out as many different laser types as possible! At ES we have five laser types – Nd:YAG, Vanadate, fibre, CO₂ and frequency-tripled Vanadate. We can therefore choose from a wavelength range of 355nm to 10,600nm and from outputs which are either continuous wave, pulsed or Q-switched. We regularly surprise customers with what we can achieve after they advised that they had tried laser marking and ‘it didn’t work’ – not all lasers are equal!

Laser marking is ubiquitous – even the keys I’m hitting to type this are laser marked (NB – if your keyboard graphics wear off, they’ll be screen-printed, not laser foamed). At its best it is an elegant and even beautiful way to finish products and it is unmatched for resolution and permanence. However, we do see sub-optimal laser marking – results that could have been improved with a little extra care and attention, or by choosing a better suited laser for the job.

It’s not easy to select from the burgeoning range of laser marking technologies out there; today’s engineers can even use picosecond and femtosecond laser marking to eliminate thermal damage, if budgets permit.

^{Figure 2: ID Matrix contrast on metals can be improved by using two passes; one to create a light background and the second to mark the dark code. (Credit: ES Precision)}

Anyone considering using lasers on their production line or via a subcontract service such as ES provides should pay particular attention to tip 5 above. Choosing the best laser is critical and best quality and throughput are likely to be worth paying a bit more for.