In industrial gravure plate production, a large surface area requires a high degree of spatial resolution. The fast working cycle of the printing roller requires an effective engraving of several square meters in micrometer precision in a short period of time. The application of lasers in this field has the following characteristics: high processing rate, precise focusing and the advantage of digital modulation. Direct laser micro-structuring technology is replacing traditional gravure-making techniques (such as mechanical engraving with diamond pens or chemical etching) because of advances in precision, repeatability, flexibility, and yield.
The rotary gravure plate consists of an average copper or galvanized steel roll. The image information is engraved into tiny cells in a copper or galvanized layer to transfer the ink to the substrate (see Figure 1). A thin layer of chrome ensures a long service life of the press under intense grinding. By using a doctor blade, it is possible to ensure that only the amount of ink determined by the size of the cells is transmitted.
The gravure printing cylinder has a length of 0.3 to 4.4 meters, a circumference of 0.3 to 2.2 meters, and a surface area of ​​up to 10 square meters. When the screen resolution is 60 to 400 lines/cm, the number of cells on the drum is usually 108 to 1010. In order to perform image processing in the most economical time, the laser is required to have a high pulse repetition rate and a high uniform power.
For large-scale micro-engraving in the form of thermal ablation, the most effective method is to use a pulsed laser beam with a single laser pulse to create a complete cell. A Q-switched Nd:YAG laser system with a working power of 500 watts and a repetition rate of 70 kHz (see Figure 3) enables a volume ablation rate of zinc to reach 1 cm3/min and an area ablation rate of 0.1 m2/ minute. The shape of the cell 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, whereas conventional cells (with varying depths per gray value) are generated by using a flat-bottom waveform (see figure 2). The size of the cells depends on the pulse energy and is controlled by the digital image data set by using an acousto-optic modulator. A diameter ranging from 25 meters to 150 meters, which defines the screen resolution of the image; a depth ranging from 1 meter to 40 meters, which defines the gray value of the printed dots.
The heat transfer and heat convection of the melt must be minimized. Therefore, Daetwyler has developed a special electrogalvanized material with organic additives that has lower thermal conductivity than ordinary zinc structures. By ablating this special zinc by vaporization, the melting zone and burrs can be reduced to a thin layer of deposit (within 2 to 3 meters near the cell). [next]
The entire drum surface is alternately engraved by a continuous spiral mesh track. When the drum speed reaches 20 rpm, the machining head moves at a traverse feed of 15 to 150 μm/rev, parallel to the drum axis (depending on the screen resolution). The thickness of the mesh wall between the cells 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 approximately 1 micron.
Another method is to use a pulse-modulated high-power fiber laser (uniform power of 500 watts) with a pulse repetition frequency modulation range of 30-100 kHz. When the frequency is 35 kHz, there is more energy per pulse, so that a single shot can drill a large cell (such as a screen with a diameter of 140 microns when the screen is 70 lines/cm). When the frequency is 100 kHz, the energy per pulse becomes less, and thus a small cell is engraved (for example, a screen having a diameter of 25 μm when the screen is 400 lines/cm).
The operation of the laser beam is long and short contact type, compared with the electromechanical engraving using a diamond pen, which is the advantage of a hub. As long as the printing process is predictable and repeatable, the engraving average can be guaranteed over the entire width of the drum. Just because of the high repeatability, the single-shot single-hole laser process is about 10 times faster than electromechanical engraving.
Beam intensity waveform modulation printing has many different substrate materials (like paper or elastic foil), each with different surface characteristics. The optimization of ink transfer depends on: the surface of the substrate (such as roughness, ink absorption), ink parameters (such as the viscosity or model of the pigment), printing plates. For each different case, engraved cells of different shapes can be used to achieve optimum.
In addition to heat conduction and convection, the cells accurately represent the focus intensity waveform of the laser beam. In order to achieve a specific shape for each cell, the stereoscopic intensity waveform of the beam is actively formed in real time, and the frequency controlled by the image data is as high as 100 kHz.
The shape, diameter and depth of each single cell can be determined independently by the active modulation of the intensity waveform and the independent change in the energy of each laser pulse. 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-automated cell are variable, but cannot be controlled independently).
SHC modulation allows a laser system to engrave various cells (traditional, Autotypical, Halfautotypical). In the past, different processes (electromechanical engraving, chemical etching) were required. Now, it is possible to create a new cavity shape to optimize ink transfer characteristics and printability for each color %-tone value and print substrate.
Strategy and application In addition to the “single-shot single-hole†method of SHC beam waveform modulation, it is also possible to design engraved cells by superimposing continuous laser pulses, except that the diameter of the spot is smaller than the required cell size (just like the spot) The diameter is 10-15 microns, and the cell size is 100 microns). The shape and internal structure of the formed cells depend on modulation, overlap, and scanning schemes of laser pulses (such as image typesetting machine scanning algorithms).
Continuous wave lasers are switched or gray scale modulated and can engrave fine overlapping stripes to form diamond-shaped cells. The upper hand is the high resolution of the image (for example, a resolution of 1000 lines/cm with a forward transport step size of 10 microns and a spot diameter of 15-20 microns). The disadvantage is 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.
Because of its high peak power during focusing, high-brightness fiber lasers (200-600 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, like copper and ceramics.
The image layout machine scanning process algorithm is used in many high resolution two-dimensional (printing) applications and three-dimensional (printing) applications. For example, engraving an RFID gravure roll.
Printed electronics is an upcoming new technology, and the high precision required for electronic components and circuits will set a new benchmark for printout accuracy and averaging. Most of the organic or inorganic inks of conductors and semiconductors are paste-like, and printing is very laborious.
For the average non-porous delamination of these inks, precise control of the geometry of the cells and the surface texture of the gravure plates are often used. Figure 5C shows an engraving test of an RFID tag antenna with a contour width of only 10 microns.
Summary and prospective laser technology combined with digital imaging methods, improved the traditional printing platemaking process, promoted the efficiency of print output, screen range, accuracy and quality. Corresponding algorithms can be used to utilize different laser types. Utilizing the modulated laser beam waveform, the single-shot single-hole SHC process is currently the fastest process for gravure and can be used for a variety of substrates, inks and printing. The new engraving algorithm using high-power TEM00 sources extends the range of laser ablation methods to a range of industrial applications, such as anilox rolls for large-area material transfer, high-precision gravure printing for printed electronics, and Used for 3D printing tools. Ultrashort pulse lasers will be able to drive and improve the above methods when both the necessary laser power and the new engraving mature algorithm are satisfied. The challenge for the future will be to use a picosecond ultrashort pulse laser to optimize the ablation process.
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