Of all the industrial applications of lasers, marking materials is one of the broadest and most important. Business has been brisk for the past 15 years or so, and there is steady growth in demand – whether it be for marking, coding, or engraving – driven by many disparate needs ranging from product identification, to safety of medical instruments, marketing, consumer demand, and legislation.
Because the technology is so widespread and because price is often the over-riding consideration, it is difficult for individual laser manufacturers to stand out from the crowd. But some have been trying to differentiate themselves by adjusting the duration of the pulses that their lasers produce; tweaking the wavelengths to adapt the beams to a specific application; and modifying the quality of the beam. Even sacrificing the speed of marking in favour of a highly efficient and therefore compact and lightweight laser package may be more appropriate to some applications.
Recently, consumer electronics has been outstripping the medical market as a driver of laser-marking technology – especially in the demand for high-quality marks on plastic. People want their mobile phones to be fashion accessories, not just useful items of technology, so consumers opt for attractive designs on the covers of their mobile phones but, at the same time, they want the surface to be smooth to the touch. Lasers can produce these marks, but it has to be done without carbonising the material or producing bubbles on the surface – so-called ‘cold marking’.
According to Frank Gaebler, director of marketing at Coherent, this has led to a ‘trend for UV lasers in marking – they have the advantage that they do not carbonise and do not heat-affect the material at all. You have a white plastic – the white pigment is titanium dioxide – and this turns a dark grey, but you don’t destroy the plastic matrix itself. It is a real cold mark, creating a nice contrast.’ Coherent offers a solid-state, Q-switched, laser – Matrix 355 – which operates in the ultraviolet at a wavelength of 355nm.
For Jack Gabzdyl, product line manager for pulsed fibre lasers at SPI Lasers, on the other hand, the solution for marking mobile phones is to control the bulk heating by hitting little but often – pulsing the laser beam. In his view, the technique will work for both metal and plastic casings: ‘If you are going for colour change in more sensitive materials, more decorative marking, very short pulse durations – 3 to 10ns – give the kind of decorative marking these companies are looking for.’ If the aim is a subsurface mark without bubbling or blistering on white plastic that has a UV cure on the surface: ‘You need to peck away at it with very short, controlled pulses so there is no bulk heating but there is sufficient energy to do the colour change and generate the contrast in the material.’
SPI, he said, offers the flexibility to tailor the pulses to the application: ‘We can change pulse duration from 3ns to 500ns on some applications with certain lasers. That’s what makes our lasers unique.’ SPI produces master-oscillator power-amplifier lasers that use a directly modulated semiconductor seed as the source and then a fibre amplifier-chain to increase the power, with the company’s redENERGY lasers being suitable for marking applications. He pointed out that most of the solid-state marking lasers have been Q-switched lasers, which are less flexible in terms of the pulse duration.
Gabzdyl drew an analogy with using a hammer – a sledge-hammer is the equivalent of the single long pulse and the rate that anyone can swing a sledge-hammer is quite low. However, with a small jeweller’s hammer ‘you can be pinging it at a rate of knots. There are certain materials that behave better depending the duration of pulses. Some metals benefit from a longer pulse duration. If you want to form the melt, then the material is removed by a splash generated by recoil pressure due to vaporisation on the surface of the material.’
Coherent is interested in even shorter pulse durations and has come up with a small package in the sub-nanosecond range (600ps), in particular for marking glass and marking or annealing anodised aluminium. According to Frank Gaebler: ‘Brittle materials are a challenging area for lasers. Carbon dioxide lasers have good absorption in glass but that would make a crack in the surface that would grow and that is not what is wanted. One micron lasers [the typical wavelength of fibre lasers] are not absorbed by glass, so they would go straight through.’
The company has therefore come up with a diode-pumped solid-state laser that is actively Q-switched and radiates in the green. ‘When you use these for marking glass, you have a broad process window to process the glass nicely,’ Gaebler said. ‘You can make a mark inside the glass – like a 3D image – but the laser can also do shallow engravings that do not chip; deep engravings that do not chip; and diffractive marking, where you achieve a mark that is shallow but it changes the diffractive index of the glass so you can read text or a logo. Just a few Watts is enough, then you have enough peak power to create a beautiful mark.’ Previously, such work had been done with huge femtosecond lasers which are not viable commercially – but Coherent has now developed a process with sub-nanosecond lasers, from 600ps to 800ps. ‘That is interesting to the medical industry – glass ampoules; lab on a chip; anti-counterfeiting,’ he pointed out.
‘These are not typical lasers,’ he continued. ‘Typically you have 20-30ns DPSS or fibre lasers and then the next one is 10-15ps. There are not a lot of products in between, but we identified this as an interesting parameter space, because you can have picosecond performance for nanosecond cost.’
As well as working on the pulse durations for marking glass, Coherent has been looking at the advantages of slightly modifying the wavelength of more traditional carbon dioxide lasers, in response to changes in practices within the packaging industry. CO2 lasers work in the infrared band at 10.6µm, which is fine for making ablation marks on paper and cardboard packaging and even plastic yoghurt pots. But, Gaebler explained: ‘We have seen in the packaging industry – for example for cigarettes and medical – the paper packages sometimes have a polypropylene layer on top as a water protection. These need to be marked, and it is difficult to do with regular CO2 lasers.’ If there is a plastic film on top, it is more difficult to achieve a clear ablation mark. ‘But if you use a slightly different wavelength of the laser – change it to 10.2µm, which is no big deal for us as laser manufacturers – then you can achieve a really good contrast. That is an area where customers in the packaging industry are driving demand.’
Although it may sound slightly counter-intuitive, SPI Lasers thinks that a good way to differentiate its lasers from the competition is to offer customers what purists would regard as ‘poorer’ beam quality. Gabzdyl explains: ‘In the past, people have always viewed beam quality as going in only one direction – the sharper the better. From a physics definition, I kind of agree; but from a materials-processing perspective, there are huge benefits in being able to tailor the beam quality to get a different effect. A good example of that would be black marking on stainless steel – the intention is to grow a black oxide film but without any melting of the substrate. For these kinds of marks, the very best beam-quality lasers do not always give the best results. You have to throttle them back but with M2=3, you can go full throttle.’ The process is sometimes called anneal marking (even though it is not annealing but rather black-oxide marking). If the laser heats the steel so that it begins to melt, then the result will be roughness on the surface, which may be undesirable in itself (say, for medical instruments) and it may adversely affect how the material resists corrosion. The medical industry uses these kinds of marks on surgical instruments and even very slight melting may disrupt the surface and so lead to potential for films to develop that can harbour bacteria.
Although the diameter of a laser beam can be changed using the optics, this does not change the energy distribution across the spot. SPI Lasers ‘can control the beam quality by design of the fibre’. Low M2 means that the beam profile is truly Gaussian and its energy distribution has a high central peak. SPI can offer lasers with M2 at <1.3; <1.6 and <2 and one at 3. According to Gabzdyl: ‘The M2=3 product, using the same optics, will have a spot size that is effectively three times greater than the highest quality beam – and a broader, or more even energy distribution throughout that spot. When you are area-marking – say, you want to do logos – working with a bigger pen is a useful thing.’
One of the most recent additions to the set of laser-marking tools seeks to differentiate itself in a different direction. The Nexgen laser marker from World Star Tech uses a high intensity, very small spot size – around 9µm – produced by a highly efficient fibre laser and optics package that allows the ensemble to be compact, lightweight – and portable. The company has been working on the system for about four years, and started shipping units to customers within the past few months.
According to Cetin Karakus, the founder of World Star Tech: ‘We sacrifice the speed of marking to focus on the efficiency of the laser and the scanner. We combine these to make a really compact laser to mark small areas – 50mm by 50mm. The final product weighs less than 1kg for the whole system, whereas others are 20 to 30kg and are therefore quite heavy and bulky.’ The system also costs much less than more elaborate packages – with prices below $10,000 compared to $50k to $100k for larger systems. They will even work off a 6V battery, he said.
The small spot size means a high-intensity beam, allowing the system to mark anodised aluminium even with a 5W laser. But in addition to direct marking, Karakus sees novel applications when the technology is combined with what he describes as ‘laser ink’. He said: ‘We have also developed special dyes – you can call it “laser ink” – for this wavelength to mark on different materials including glass, ceramic, and carbide. It’s very difficult to mark on carbide with conventional lasers but using this special laser marking ink you can mark on this material.’
He pointed out that currently, once a carbide cutting tool is removed from its packaging there are no identifying marks on the piece itself. He believes his system will allow manufacturers to mark their products and thus protect themselves against counterfeiting – ‘this will be a completely new area’. He also sees applications in the jewellery market: ‘We can do nice black marking on gold which will be fantastic for hand-made jewellery’. Because of the compactness and lightweight nature of the system, he envisages jewellery workshops using the system to provide decorative markings on individual pieces of jewellery, replacing some of the older techniques used by craftsmen.
The small spot size and high resolution also mean that the laser is suitable for the small-area work required by jewellers. It fits with the general strategy: ‘We are targeting small to medium manufacturers with few thousands of units a day.’ The slower marking speed means that it will take on to two seconds to mark a lot number or barcode, so ‘not a high-production environment but small parts and a volume of up to 50,000 a day’.
What of the future? Coherent’s Gaebler points out: ‘In China alone, you will find many laser-marking companies. The general market is price-driven. Laser marking systems are becoming a commodity so in order to compete you need to find differentiation.’
For SPI’s Gabzdyl, the demand for product identification is not diminishing. In his view the ‘war’ with other marking technologies is being won and he too focuses on cost. ‘Printing has its place,’ he said, ‘but the issue is one of cost. The difference between ink-jet marking and laser marking is basically that lasers have a high initial, capital cost but low operational costs, whereas printing tends to be low initial cost, high operating cost. And people don’t want to buy consumables.’
He pointed out that maintenance is also an issue, as the nozzles of ink-jet printers can become clogged. ‘Lasers are eating into their cake,’ he concluded.
Dr Tom Wilkie is the editor for Scientific Computing World. 
You can contact him at tom.wilkie@europascience.com.
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