In industrial production, laser is one of the most popular marking technologies. Metals, plastics, ceramics, and other materials can be marked with laser radiation at 1064 nm (infrared), 532nm (green), and 355nm (ultraviolet). As a tool, lasers can be applied to numbers, text, trademarks, or machine-readable codes, such as data matrix codes with high information density, arranged in tight spaces on different scales.
In this way, high-speed marking can shorten the cycle time of the production process, and at the same time, does not require expensive preliminary work and final finishing work. In addition, lasers can be easily integrated into automated production lines. Through the user-friendly program interface, you can quickly switch to new processing; the result is a product with good repeatability and resistance to aging and wear.
In the application of plastic marking, the potential of laser is far from being realized. In addition to the benefits mentioned above, the increasing promotion of frequency-doubled lasers and triple-frequency lasers, as well as the diversity of combinations of materials and processes, are opening up more new areas for laser applications in the plastics industry.
Precision marking
For plastic marking, Q-switched short-pulse solid-state lasers or fiber lasers are commonly used types. These lasers usually have an average power of less than 100W, and the pulse duration is between 10ns and 100ns. The pulse frequency reaches 120kHz, and in the case of fiber lasers, it even reaches 1MHz. In this way, the interaction with the material to be marked can be fine-tuned. The short pulse time results in a peak pulse power of several tens of kilowatts reaching an average value of 10W.
The laser is a diode pumped laser with high energy efficiency. They have good focusing ability, so they are very suitable for fine marking. The diode-pumped solid-state laser has a very high beam quality, so that the laser beam during the marking process has a small focusing diameter. In this way, precise marking track widths as small as 30 m can be achieved on small parts.
Adaptation of absorption characteristics
Lasers used for marking usually produce radiation in the infrared wavelength range. Green lasers and ultraviolet lasers target plastics and semiconductor materials. In special marking applications, the use of UV wavelengths has opened up new possibilities for laser marking on plastics. The short wavelength directly produces a photochemical reaction with the plastic compound without heating, so as not to damage the material (Figure 1); especially some of the more critical materials, such as plastics containing flame retardants, or sensitive electronic components. These lasers perform high-contrast marking at very high speeds without any negative impact on surface quality.
Figure 1. The so-called cold marking of hearing aids-using UV wavelengths to directly produce photochemical reactions with plastic surfaces,
There is no heating material.
The most important point is that the plastic must absorb laser radiation to a great extent. Plastic biomolecule structures usually only absorb light in the ultraviolet and far infrared (IR) range (wavelength 10.6 m). Additives, fillers and pigments in engineering plastics have a great influence on the absorption characteristics of the material, so that plastics can better absorb laser beams in the near infrared range or visible green laser range. Through this method, higher marking speed and better contrast can be obtained.
Figure 2. Different additives and pigments in plastics,
It can have a positive effect on the absorption of laser by the material.
Some plastics, such as polyethylene (PE), polyoxymethylene (POM) or polyurethane (PU), only show very weak contrast (Figure 3). Therefore, they cannot meet the high requirements of laser marking. In order to obtain durable, high-definition and high-quality marking results in a short processing time, laser-sensitive additives need to be specially formulated. They greatly improve the marking ability of the material.
Figure 3. In order to obtain high contrast using laser marking with a wavelength of 1064 nm,
Special laser additives are added to these, as well as some other materials (such as PS, PVC). [next]
Four crafts
In general, plastics can be marked in four different ways (Figure 4).
Figure 4. In theory, plastics can be marked through four different processes. The figure lists the application cases in each case.
Figure 4a and b: Color changing process. Used in the cosmetics industry, such as eyeliner and mascara containers made of PE materials.
Figure 4c and d: Engraving process. Used in electronic components industry, such as microelectronic printed circuit made with EP.
Figure 4e and f: foaming process. Used in automotive engineering, such as ignition coils produced with POM.
Figure 4g and h: Ablation process. Used in automotive engineering, such as tachometer.
In the discoloration process, color changes occur on the surface of the material (carbonization). The application of this color change is used in serial numbers and batch numbers in large-scale production of beauty products and so on. Because it is customized, this logo is used for anti-counterfeiting protection (Figure 4, a and b).
The engraving process is used to remove materials by melting and gasification (Figure 4, c and d). This process can be used for the identification of electronic components without damage.
In the Foaming process, the laser melts the surface (Figure 4, e and f), creating bubbles, and the light is reflected on the tiny bubbles to make the mark white visible. Just like the numbers seen on the surface of the ignition coil.
In the ablation process, the coating is removed (Figure 4, g and h). This process is mainly used in switches and some operating devices called day and night designs to obtain high contrast. Base coatings, or translucent base materials must not be injured. Lasers with very good pulses can ensure this with pulse stability. The high focusing ability of the laser beam is also very important, so that the laser ablation mark has a sharp edge.
The best marking method in each case is determined by the special requirements of each marking, the plastic marked, and the laser wavelength. For most thermoplastics, the discoloration process is used. In the case of darker materials (mostly black), the foaming process is more commonly used (1064 nm wavelength has an advantage). The thermosetting plastics and elastomers mainly use engraving technology.
in conclusion
Although lasers can be widely used in marking, a comprehensive solution is still very unique. The best marking process can only be verified through close cooperation between the customer and the laser supplier. Parameters such as wavelength, quality (contrast, homogeneity, resolution and clarity), marking speed, and customer requirements play a decisive role in finding the most correct laser and the best settings. This choice, as well as its integration as a step in the production process, determines the production cycle, as well as the quantity and quality of the parts produced.
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