In the production of industrial gravure printing plates, a wide surface area requires a high degree of spatial resolution. The rapid working cycle of the printing roller requires that several square meters of area be effectively engraved with micron-level accuracy within 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 the increase in precision, repeatability, flexibility and productivity, direct laser microstructuring technology is replacing traditional intaglio platemaking technology (such as mechanical engraving with diamond pen or chemical etching).
The rotary gravure printing plate consists of a uniform copper or galvanized steel roller. The image information is carved into tiny holes in the copper or galvanized layer, transferring the ink to the substrate (see Figure 1). A thin chromium layer can ensure that the printing press still has a long service life under severe grinding conditions. By using a doctor blade, it is possible to ensure that only the amount of ink determined by the size of the cells is transferred.
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 mesh holes on the drum is usually 108-1010. In order to do image processing in the most economical time, the laser is required to have a high pulse repetition rate and a high average power.
For large-scale micro-engraving by means of thermo-optic ablation, the most effective method is to use a pulsed laser beam with a single laser pulse to create a complete network cavity. A Q-switched Nd: YAG laser system with a working focus average power of 500 watts and a repetition rate of 70 kHz (see Figure 3) can achieve a volume ablation rate of 1cm3 / min for zinc and an area ablation rate of 0.1m2 / minute. The shape of the cell is determined by the intensity waveform of the laser beam.
Half-autotypical cells (the depth and diameter of which are variable in gray scale) can be generated by lasers with Gaussian beam waveforms, while traditional cells (the diameter of which changes in depth at each gray value does not change) are generated by using flat-bottom waveforms (see figure 2). The size of the net hole depends on the pulse energy and is controlled by the digital image data set by using an acousto-optic modulator. The range is 25 meters to 150 meters in diameter, which can define the screen resolution of the image; the range is 1 meter to 40 meters in depth, which can define the gray value of the printed dots.
The heat conduction 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 structure. By vaporizing and ablating this special zinc, the melting area and burrs can be reduced to a thin layer of sediment (within 2 to 3 meters around the net hole). [next]
The entire drum surface is alternately engraved by a continuous spiral mesh track. When the drum speed reaches 20 rpm, the processing head moves parallel to the axis of the drum at a traverse feed of 15 to 150 microns / rev (depending on the screen resolution). The thickness of the mesh wall between the mesh holes is only 4 to 6 microns when the tone value is maximum. This requires the aiming accuracy of the beam to illuminate the drum to reach about 1 micron.
Another method is to use a pulse-modulated high-power fiber laser (average power 500 watts) whose pulse repetition frequency can be modulated in the range of 30-100 kHz. At a frequency of 35 kHz, there is more energy per pulse, which allows a single firing to drill a large mesh hole (for example, a screen with 70 lines / cm and a diameter of 140 microns). When the frequency is 100 kHz, the energy per pulse becomes less, so small holes are carved (for example, the screen is 400 lines / cm and the diameter is 25 microns).
The operation of the laser beam is non-contact, which is a key advantage compared to electromechanical engraving using a diamond pen. As long as the printing process is predictable and repeatable, the engraving uniformity can be ensured over the entire width of the cylinder. Because of the high repeatability, the single-shot single-hole laser process is about 10 times faster than electromechanical engraving.
Beam intensity waveform modulation
There are many different substrate materials (such as paper or elastic foil) in the printing market, and each material has different surface characteristics. The optimization method of ink transfer depends on: substrate surface (such as roughness, ink absorption capacity), ink parameters (such as viscosity or type of pigment), printing plate. For each different situation, different shapes of carved nets can be used to achieve the best.
In addition to heat conduction and convection, the cell accurately represents the focus intensity waveform of the laser beam. In order to achieve a specific shape for each cell, the three-dimensional intensity waveform of the beam is actively formed in real time, and the frequency controlled by the image data is up to 100 kHz. The overall scheme of this stereo modulation technique is shown in Figure 4.
Through the active modulation of the intensity waveform and the independent change of the energy of each laser pulse, the shape, diameter and depth of each single cell can be independently determined. This new type of cell in the plate-making process is called the Super Halfautotypical cell (SHC), which is an extension of the Halfautotypical cell (the depth and diameter of the semi-automatic cell varies, but it cannot be controlled independently).
The SHC modulation makes it possible to engrave all kinds of net holes (traditional, Autotypical, Halfautotypical) with one laser system. In the past, different processes (electromechanical engraving, chemical etching) were required. It is now possible to generate completely new cell shapes to optimize the ink transfer characteristics and printability for each color% -tone value and printing substrate.
Strategy and Application
In addition to the "single shot and single hole" method of SHC beam waveform modulation, it is also possible to design engraved meshes by superimposing continuous laser pulses, except that the spot diameter is smaller than the required mesh size (for example, the spot diameter is 10 to 15 Micron, the cell size is 100 microns). The shape and internal structure of the formed cells depend on the modulation, overlapping, and laser pulse scanning schemes (such as the scanning algorithm of the image layout machine).
CW lasers are switched or gray-scale modulated, and they can engrave small overlapping stripes to form diamond-shaped holes. The advantage lies in the high resolution of the image (for example, the resolution is 1000 lines / cm when the forward feed step is 10 microns, and the spot diameter is 15-20 microns). The disadvantage lies in the loss of production capacity, which needs to be compensated by using a higher modulation frequency (about 1 MHz) and a multi-beam engraving head.
Due to its high peak power during focusing, high-brightness fiber lasers (200 to 600 watts, continuous wave, pulse modulation) or ultrashort pulse lasers can achieve this advanced engraving method. In addition to zinc, this high brightness can also be used to engrave other materials, such as copper and ceramics.
The scanning process algorithm of the image typesetting machine is suitable for many high-resolution two-dimensional (printing) applications and three-dimensional (printing) applications. For example, engraving RFID gravure roller.
Printed electronic technology is an upcoming new technology. The high precision required by electronic components and circuits will set a new benchmark for the accuracy and uniformity of printouts. Most of the organic and inorganic inks of conductors and semiconductors are paste-like, which is very laborious to print.
For the uniform non-porous layering of these inks, precise control of the mesh geometry and surface texture of the gravure printing plate is critical. Fig. 5C shows the engraving test of the RFID tag antenna, and the outline width is only 10 microns.
Summary and outlook
Laser technology combined with digital imaging methods improves the traditional printing plate-making process and improves the efficiency, screen range, accuracy and quality of printouts. Corresponding algorithms can be used to use different laser types. Using the modulated laser beam waveform, the current single shot single cavity SHC process is the fastest process for gravure, which can be used for various substrates, inks and printing. A new engraving algorithm using a high-power TEM00 source extends the application of laser ablation methods to a range of industrial applications, such as anilox rollers for large-area material transfer, high-precision gravure printing patterns for printing electronics, Used for 3D printing tools. When both the necessary laser power and the new engraving mature algorithm are satisfied, the ultrashort pulse laser will be able to promote and improve the above methods. The challenge ahead will be to use picosecond ultrashort pulse lasers to optimize the ablation process.
Stainless Steel Gn Pan,Stainless Steel Gastronorm Pan,Stainless Steel Gastronorm Trays,Stainless Steel Gastronorm Containers
Jiangmen Junerte Stainless Steel Kitchenware Co.,Ltd , https://www.jetkitchenware.com