The holes are covered by a thin polymer layer. When this layer is carved with a modulated laser beam, the laser engraver will open a hole in the mesh where it is carved. This is a very effective method for producing printing plates and cylinders, especially when it comes to high-volume printing. This technology can be used in almost all printing of some functions for textiles, carpets, wallpaper and banknotes.
The direct modulation of CO2 lasers is limited to about 10 kHz, mainly due to metastable nitrogen, which is a major part of the laser gas mixture. Laser pointer technology currently used in tube and tank printing requires higher pulse frequencies, on the order of hundreds of kilohertz. This is mainly due to higher resolution, not due to the true 3D structure of the material. Engraving mesh is basically a 2D process, while engraving printing plates and polymer or rubber rollers is a 3D engraving process with a complex structure. Each directly sculpted structure requires a solid base to remain stable during printing. They may have complex geometries on the top, such as a well-defined pattern and undercuts to compensate for dot expansion.
In the future, high security printing (banknotes, security documents, passports, etc., as shown in Figure 2) will require a frequency of at least 500kHz or higher. At the same time, the industry now wants to achieve photo-like printing in packaging design, which also requires similar performance.
Acousto-optic modulators (AOMs) can control the laser beam with a much faster modulation frequency than the discharge of a directly modulated RF laser. However, the acousto-optic modulator is limited due to its absorption in the germanium crystal and its damage threshold. For best output results, the acousto-optic modulator, green laser pointer laser source, and beam path must be carefully designed.
Test all advanced lasers, especially their pulse behavior, power stability, pointing stability and mode. The rise and fall times determine the pulse behavior and therefore the engraving speed. Nitrogen in the mixed gas will reduce the pulse frequency to about 10 kHz. This is enough for many applications in the past, but not enough for future needs. Typical laser power vs. time plots show a deviation of ± 5 to 10%.
This is definitely not suitable for controlling 3D sculpting materials. The laser pointing stability of the various lasers tested is surprisingly good, which will have a direct impact on the use of acousto-optic modulators, which are very sensitive to the angle of incidence. Near the power limit of the acousto-optic modulator, germanium crystals are very sensitive to poor laser field modes. Hot spots can cause distortion of the outgoing beam and can easily damage the laser mode. Generally, the distance between the output coupler and the acousto-optic modulator should be about 2m or more, so that there will be a better laser field mode.
The wise choice is to use modern materials (like carbon fiber) in the cavity structure of a classic folded CO2 laser, which can achieve a highly stable cavity and a nearly perfect beam mode. The thermal expansion coefficient of carbon fiber tubes is very small (less than 1 μm per meter and Kelvin), especially when it is well designed, such as the design of enhanced finite element calculation (FEM) to optimize the thermodynamic behavior.
Optimized beam path. Custom-made carbon fiber optics are used to achieve the high precision of the laser cavity, and set the beam path of the acousto-optic modulator and infrared (IR) camera (PyroCams) so that the beam pattern can be visualized online. Two PyroCams infrared cameras placed in front and behind the acousto-optic modulator can accurately measure the effects (especially deformation) of germanium crystals.
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