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Application of laser technology in printing and relief
Reading:5757     Date:2017-11-24
Since the 1980s, the industrial market for processing large films has undergone tremendous changes, almost entirely replaced by lasers and digital processing. Since then, a common technique in the printing industry has been the use of RF-excited CO2 lasers with a power of up to 1 kW, which can be adjusted according to the pattern carved.

The mesh is covered with a thin polymer layer, which is carved with modulated laser beams, and holes in the mesh will be opened where they are carved. This is a very effective way to produce printing plates and rollers, especially when mass printing is involved. This technique can be used for printing almost all the functions of textiles, carpets, wallpapers and banknotes.

The direct modulation of CO2 laser is limited to about 10 kHz, mainly due to metastable nitrogen, which is a major part of laser gas mixture. At present, the technical requirements for printing in pipes and cans have higher pulse frequencies, about hundreds of kHz. This is mainly due to the requirement of higher resolution rather than the real 3D structure of the material. Engraving mesh is basically a 2D process, while engraving printing plate and polymer or rubber roller is a 3D process with complex structure. Each directly engraved structure requires a solid base to maintain stability during printing, and they may have complex geometries at the top, such as a clear-cut pattern and a bite to compensate for dot enlargement.

In the future, high-security printing (banknotes, security documents, passports, etc., as shown in Figure 2) will require at least 500 kHz or higher frequencies, while the industry now wants to achieve photographic printing in packaging design, which requires similar performance.

Acousto-optic modulators (AOMs) can control laser beams with much faster modulation frequencies than direct modulation of RF laser discharges. However, the acoustooptic modulator is limited due to the absorption and damage threshold of germanium crystal. In order to get the best output results, the acoustooptic modulator, laser source and beam path must be carefully designed.

All advanced lasers are tested, especially their pulse behavior, power stability, directional stability and mode. The rise and fall time determines the pulse behavior, and therefore determines the speed of carving. Nitrogen in mixed gas will reduce the pulse frequency to about 10kHz. This is enough for many applications in the past, but it is not enough for future needs. A typical laser power and time diagram shows a deviation of + 5~10%.

This is absolutely not suitable for controlling 3D engraving materials. The laser pointing stability of the tested lasers is surprisingly good, which will have a direct impact on the use of the acoustooptic modulator (which is very sensitive to the incident angle).

Germanium crystals are sensitive to the bad laser field mode when approaching the power limit of acoustooptic modulator. The hot spot will lead to the deformation of the exit beam and easily destroy the bad laser mode. Usually, the distance between the output coupler and the acoustooptic modulator should be about 2 m or more, so there will be a better laser field mode.

Sometimes this is hard to achieve, especially in compact carving equipment.

New CO2 laser project

The wise choice is to use modern materials (such as carbon fiber) in the classical folded CO2 laser resonator structure to achieve a highly stable resonator and near perfect beam mode. The coefficient of thermal expansion of carbon fibre tubes is very small (less than 1 micron per meter and Kelvin), especially when well designed, such as the design of a reinforced finite element method (FEM) to optimize thermodynamic behavior.

Beam path optimization. Customized carbon fiber optics are used to achieve high precision laser resonators, and to set the beam path of acoustooptic modulators and infrared (IR) cameras (PyroCams), so that beam patterns can be visualized online. Two PyroCams infrared cameras placed at the front and back of the acoustooptic modulator accurately measure the effect (especially the deformation) of germanium crystals.

The six axis. Germanium crystal acoustooptic modulator can provide better performance, and can achieve or even exceed the power of 600W CO2 laser, provided that the laser beam mode is close to Gaussian shape. If the power is too high, especially if there is a hot spot on the crystal surface, it will be easily damaged.

Optimizing the acoustooptic modulator means shaping the laser beam to achieve a balance between the small spot and the intensity that still fits the crystal. The smaller the spot, the higher the pulse frequency. Optimizing in both transverse and rotational directions will play a significant role unless the pivots of two translational and three rotational motions can be transferred to the center of mass of the incident beam on the crystal surface. The six-axis achieves this function in a perfect way and allows movement within a tenth of a nanometer range. Figure 5 shows the central cavity of a carbon fiber CO2 laser with six axis and quick replacement lenses. A green laser beam is illuminated on the beam path for pre calibration.

Experiments prove that the acoustooptic modulator has large shift and tilt tolerances and is highly sensitive to the beam mode. Poor modes immediately cause severe beam distortion, which can be detected by comparing the PyroCam results after the acoustooptic modulator with the previous results.
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