The approach to laser marking should begin by considering customer requirements
For many years, laser marking has made inroads in almost every manufacturing environment. The evolution in technology can best be seen in the laser marking performance and cost-of-ownership of the machine. This article concentrates on pulsed solid state laser marking technology and applications. For marking, CW solid state lasers (continuous wave) are rarely used. CO2 lasers are widely used in packaging, marking on organic materials (e.g. wood, leather etc.), and applications with lower contrast and larger feature size than the solid-state laser. There is no significant overlap between solid-state and CO2 laser applications.
What is the right laser?
Most people would agree with the statement: The more horsepower I have in my car, the faster it is. Simple enough, but unfortunately not true. Other factors might be more important. What about the weight of the car or the quality of tires for traction? It is very possible that a car with less horsepower will outperform another that has more. The same applies to laser marking, and choosing the appropriate laser for the task at hand is not just a question of horsepower, or in the case of a laser, wattage. Other factors to consider include pulse frequencies, pulse durations, and peak power.
The approach to laser marking should begin by taking into consideration customer requirements. There is always a trade-off between mark quality, mark time and depth. The time it takes to create the mark and the depth of the mark are easy to quantify. But when it comes to mark quality, every customer has different expectations in areas such as color contrast, edge quality (sharp, clean, straight, burr), surface finish, line width, resolution, mark consistency on curved surfaces, scratch resistance (plastics only), heat affect zone (HAZ), and pulse to pulse stability – just to name a few. In addition to the technical mark specifications, some customers require versatility – such as job shops marking on a variety of materials or repeatedly marking the same materials.
|FIGURE 1. Pulse overlap-engraving on stainless steel.|
Laser marking technologies
The complexity of requirements makes it impossible to have simple rules because every application is different. The only way to narrow the parameters is to have an understanding of the specific pros and cons of different laser marking technologies, which will then make it possible to find the appropriate match for a specific need. The final proof of an application is always done in an application lab with experienced personnel. Today three different laser marking types are utilized: Nd:YAG (rod laser), Nd:YVO4 (rod laser), and Ytterbium (fiber laser). The lamp pumped lasers (Nd:YAG) have been almost entirely replaced by diode pumped laser technology. High-performance YAG and vanadate lasers are water-cooled but are also available air-cooled for lower performance levels, as are most fiber lasers. Assuming a state-of-the art design, the running costs of a YAG, vanadate or fiber laser are almost identical, considering consumables, power requirements, and the lifetime of the pump diodes. Therefore, the customer has the benefit of choosing the right laser for their application needs without worrying about cost considerations.
Relevant beam properties
Engraving on metals requires the material to melt and to evaporate. A simplified way of illustrating this process is to explain that the average laser power melts and the peak power evaporates the material. There is a key point with an optimum engraving rate where there is just enough melted material to be evaporated by the available peak power. An excess of melted material results in a lower engraving rate and a lot of recast material and burr. Not enough melted material and a lot of peak power results in a lower engraving rate. It is only with a high peak power that nice, high contrast marks can be achieved on some ceramics and plastics.
All solid state laser marking systems are 'nanosecond' lasers. Nd:YAG typically has 10 to 150 ns and vanadate 5 to 30 ns. In both cases, the pulse duration becomes longer as the frequency becomes higher. Fiber lasers have either fixed pulse duration in the 100 ns range, or with the master oscillator fiber power amplifier (MOFPA) design, a variable pulse duration from 10 to 200 ns. Pulse duration influences the heat penetration depth into the material. Short pulses are desired for sensitive applications, such as day-and-night design for clean ablation of ink layers or metal marking with low HAZ. One example of this occurs when marking on stress parts for the aerospace industry. On plastics, there is no general rule; some work better with a longer pulse, some with shorter pulses.
The metric for beam quality is M2. The smaller the M2 value, the better the beam quality, whereas M2 = 1 is the ideal laser beam. A laser with good beam quality can be focused to a small spot size, which leads to a high energy density which is, for many applications, desirable or even required. Two additional effects are associated with beam quality:
- First is the mark field size. The larger the mark field, the bigger the spot size. To maximize the mark field while keeping the spot size in a reasonable range, the beam quality needs to be good.
- Second, for depth of focus, the better the beam quality, the more forgiving the marking process if one gets out of focus. To cite one example, this is very important for marking on cylindrical parts.
|FIGURE 2. Typical frequency response Nd-YAG and vanadate lasers.|
For smooth surfaces and good edge quality, the overlap from one pulse to the next is important. The faster the spot travels over the surface (velocity), the higher the frequencies required to get a certain overlap shown in FIGURE 1. Unfortunately on every laser the pulse peak power and pulse energy gets smaller with higher pulse frequency, but some do better than others. This explains why the fiber and vanadate laser is the technology of choice in fast ablation processes (e.g. thin layer ablation).
Every laser type has a different frequency response. FIGURE 2 shows a typical behavior of a YAG and vanadate laser.
As shown in TABLE 1, fiber lasers are considerably different from the YAG and vanadate laser. Generally, they start with considerably lower peak power but maintain it better toward higher frequencies. Fiber lasers with the MOFPA principle allow changing the pulse duration, which is advantageous in optimizing laser marking parameters. As an example in an engraving process, the optimum ratio between melted and evaporated material can be found by adjusting the pulse duration.
|Table 1: Typical values of laser marking systems|
|TYPICAL VALUES||YAG||VANADATE||YTTERBIUM (FIBER)|
|Pulse duration||10 to 150 ns||5 to 30 ns||10 to 200 ns|
|Beam quality||M2 < 1.2||M2 < 1.2||M2 < 2|
|Peak power||High, 100 kW range||Medium, 80 kW range||Low, 10 kW range|
|Average power||5 to 30 W||5 to 40 W||10 to 50 W|
|Frequency range||5 to 80 kHz||20 to 120 kHz||20 kHz to 1 MHz|
For solid state laser marking systems, all three technologies (Nd:YAG, vanadate, Ytterbium/fiber) have and will continue to have their place in industrial manufacturing. All three have the potential to be further developed in order to increase performance and lower prices. Customers want to have the opportunity to perform unbiased application trials with their parts, given all technologies available. In addition to the straightforward laser marking performance, such as contrast, speed and depths, other factors in the use of laser marking might be equally or even more important. These include variable data management, handling of peripheral accessories such as bar code readers, field maintainable design, and telediagnostics. Finally, laser power calibration for constant laser marking results from cradle to grave is an important consideration in the use of laser marking.
Peter Grollmann (email@example.com) is national product manager for laser marking at TRUMPF Inc.