In the production of industrial gravure printing plates, a large surface area requires a high spatial resolution. The rapid workflow cycle of printing rollers requires effective engraving of an area of several square meters with micron-level accuracy in a short period of time. The application of laser in this field has the following characteristics: high processing rate, precise focusing, and the advantages of digital modulation. Due to increased precision, repeatability, flexibility, and productivity, direct Laser Engraver microstructuring is replacing traditional gravure platemaking techniques (such as mechanical engraving with diamond pens or chemical etching).
Rotary gravure printing plate consists of a uniform copper or galvanized steel roller. The image information is engraved into tiny cavities in copper or galvanized layers to transfer the ink to the substrate (see Figure 1). A thin layer of chrome ensures a long service life of the printer under severe grinding conditions. By using a doctor blade, it is possible to ensure that only the ink amount determined by the cell size is delivered.
The gravure printing cylinder is 0.3-4.4 meters long, the circumference is 0.3-2.2 meters, and the surface area can reach 10 square meters. When the screen resolution is 60-400 lines / cm, the number of cells on the drum is usually 108 to 1010. In order to do image processing in the most economical time, Laser Engraver is required to have high pulse repetition rate and high average power.
For large-scale micro-engraving by thermo-optical ablation, the most effective method is to use a pulsed laser beam, whose single laser pulse creates a complete mesh cavity. A Q-switched Nd: YAG laser system with an average focus power of 500 watts and a repetition rate of 70 kHz (see Figure 3) can achieve a volumetric ablation rate of zinc of 1 cm / min and an area ablation rate of 0.1 M / min. The shape of the cells is determined by the intensity waveform of the laser beam.
Half-autotypical cells (both depth and diameter are variable in grayscale) can be generated by a laser with a Gaussian beam waveform, while traditional cells (with constant depth-changing diameter at each gray value) are generated by using flat-bottomed waveforms (see figure 2). The size of the mesh cavity depends on the pulse energy and is controlled by the digital image data set by using an acousto-optic modulator. The diameter ranges from 25 meters to 150 meters, which can define the screen resolution of the image; the depth ranges from 1 meter to 40 meters, which can define the gray value of the printed dots.
The heat transfer and convection of the melt must be minimized. Therefore, Daetwyler has developed a special electro-galvanized material with organic additives, which has a lower thermal conductivity than ordinary zinc structures. By vaporizing and ablating this special zinc, the melting area and burrs can be reduced to a thin layer of sediment (within 2-3 meters around the cell).