Gravure printing is a relatively simple process with few variable factors, so the quality control is simple and the repeatability is high. With the wide application of gravure printing machines in the printing field, people have put forward higher requirements for the adaptability of the gravure printing process.
For different products or substrates, we often use different processes, especially for the printing of paper substrates, especially for the printing of fine products, gravure printing increasingly uses electrostatic assisted ink removal systems ( ESA).
Why use an electrostatic ink transfer device?
We all know that the advantages of printing methods such as gravure and flat printing or embossing are that gravure printing can not only control the tone on the printed matter through the area ratio of the dots, but also can express the image layer on the printed matter through the thickness of the ink layer It is precisely because of this that gravure is better able to express some patterns with rich tones, and also has advantages in the performance of some special requirements. Similarly, our use of gravure printing is actually because of its stable printing quality.
But in the printing process of paper, we often have the problem that we can not completely release the ink in the mesh, and sometimes only reach 70%, so it often causes our gravure products to fail to reach the gravure printing. Requirements, then a process came into being, the ESA system (electrostatic ink transfer device) we mentioned earlier. Through such a device, we can solve the problem of residual ink in the net hole.
Working principle of electrostatic ink removal device
The electrostatic ink removal device is composed of electrostatic pressure roller, conductive aluminum roller (or conductive brush), control cabinet (including high voltage electrostatic generator), as shown in Figure 1.
figure 1
Its working principle is shown in Figure 2. It generates high voltage and low current through the control cabinet, and transmits the current to the electrostatic pressure roller through the conductive aluminum roller or conductive brush, so that the electrostatic pressure roller is charged with a positive (or negative) charge, so that the electrostatic pressure roller is in contact with the paper A high voltage electrostatic area is established between a conductive surface, paper and gravure plate. Under the effect of static electricity, the ink in the gravure printing plate is transferred to the surface of the paper uniformly, so that the transfer rate of the ink reaches or approaches 100%.
figure 2
In the process of electrostatic loading, the positive or negative charge should be selected according to the color of the ink, the composition of the ink, and the positive and negative charges on the surface of the paper.
Some experiences in assembling static electricity removing device
A. It ’s important to choose the right product for your company. Sometimes this choice is really a headache, but you must make a choice between price and quality. For imported products from Germany and Japan, the price of each electrostatic pressure roller is about 8 thousand to 15,000 yuan. Each color group needs 200,000 yuan (excluding electrostatic pressure roller) to complete the matching. Each gravure production line must be equipped with at least For groups 3-4, this requires several hundred thousand yuan of capital, but the overall quality can be said to be quite good. Domestic devices. It is understood that the only successful companies are Beijing and Tianjin. The price of each color group is about 35,000 to 40,000 yuan (including electrostatic pressure rollers). Such a production line only costs 15 Ten thousand yuan is enough, which is equivalent to the price of one color group of similar products abroad, but there is a certain gap between the quality of use and the life of the product.
B. After the product is selected, some parameters and auxiliary facilities of the device should be determined, such as the way of electrostatic loading, electrostatic pressure roller, the width of the printing machine, the width of the printed product, etc.
(1) There are two main ways of electrostatic loading, that is, there is a gap between the aluminum roller and the electrostatic pressure roller; the brush is easy to wear and deform when the brush is loaded, which causes uneven electrostatic loading.
(2) When selecting an electrostatic pressure roller, distinguish it from the pressure roller. The surface of the electrostatic pressure roller must be conductive, and the hardness should be different from the pressure roller. The hardness of the general pressure roller should reach HS90, and the hardness of the electrostatic pressure roller should be controlled at HS80 ~ 85 according to the different equipment. , Durability is poor; too hard and easy to cause incomplete ink transfer, static effect is not obvious.
(3) The width of the printing machine and the width of the printed product should be considered together to solve the production of the electrostatic pressure roller and the length of the conductive part. In the past, the length of the electrostatic pressure roller was the length of the conductive part, which easily caused the paper to break, or the edge of the paper was damaged, and could not be used on the machine. The electrostatic pressure roller is now divided into a conductive part and an insulating part in the longitudinal direction, just like the pressure roller.
Problems that should be paid attention to in the use of electrostatic ink transfer device
Since the electrostatic ink removal device is mainly used for printing screen-mesh adjustment products, an electrostatic pressure roller must be installed every time it is used to readjust the pressure, and the fund accumulation time is longer. In addition, in order to ensure a good ink transfer effect and safety in printing, the following must be noted.
①Check the installation position of the electrostatic pressure roller, the pressure of the pressure roller and the pressure at both ends of the pressure roller. ② Check the installation position of the conductive aluminum roller and electrostatic brush and the wires connected to the electrostatic control cabinet. ③ Check the working state of the pressure switch or speed controller. ④ The loading of electrostatic voltage should be carried out when the speed of the gravure printing machine reaches more than 30m / min, and gradually increase the voltage from zero according to the improvement of the printing effect. If increasing the voltage has no obvious effect, stop increasing the voltage. Change the polarity of the loading voltage and check the existing problems in time. ⑤ If the screen printing effect is not improved after using the electrostatic ink transfer device, you should also check the surface smoothness of the paper used and the engraving quality of the printing plate on the net hole.
Shortcomings of electrostatic ink transfer device
Due to the relatively narrow range of use of electrostatic ink transfer devices in China, the research on this technology has just begun. There are still some deficiencies in the domestic devices that are currently used more. I hope that colleagues can use this as a mirror.
â‘ The number of engraved lines of the gravure screen mesh product can be up to 175 lines / inch, which makes the fineness of the gravure mesh hole greatly restricted, so it is difficult to meet the level and fineness of the product only by the electrostatic ink removal device. Claim. â‘¡ Add static electricity to the paper through the electrostatic pressure roller, which will cause a strong static area to be formed within the range of about 300mm from the gravure, so a static elimination device must be installed in this part, otherwise the ink tank may catch fire due to the high electrostatic strength . Although the fire will not cause great losses, it will bury potential safety hazards. â‘¢ At present, the electrostatic ink transfer device still has a large deficiency in the printing of gold and coated paper, and the printing quality has not been significantly improved.
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