Water control of lithographic offset printing
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In lithographic offset printing, the amount of water in the layout has a significant influence on the transfer printing, overprinting, ink balance, drying, plate life, paper collection and ink layer gloss. Therefore, how to control the amount of water in the optimal state becomes a problem that the printing operator is more concerned about. The consumption of ink in offset printing is relatively fixed and easy to control, but relatively speaking, the control of water volume is not so easy. The main reason is attributed to the large mobility of water and easy evaporation, and the consumption of many ways, coupled with shutdown wipes, machine idling, environmental conditions and so on. Then, whether there is a certain standard for the amount of water in the layout, we need to understand the decisive factors affecting the amount of water in the layout:
1. The area and distribution of the graphic on the printing plate. In general, the amount of water in the layout is proportional to the size of the graphic area.
2. The thickness of the imprinted ink layer and the nature of the paper. The amount of water in the layout increases as the thickness of the ink layer increases. In addition, the amount of water required for alkaline paper is greater than that of acid paper, and the amount of water required for coated paper is less than that of offset paper.
3. The nature of the ink and the size of the emulsified value. When printing with dark ink, the amount of water on the plate is slightly larger than that of light ink. When printing color, the water consumption of the four inks is M>K>C>Y. In addition, when the ink is formulated, a certain amount of dry oil is generally added as an auxiliary agent, and the amount of water should be increased as the amount of dry oil added and discharged increases.
4. Machine speed. The faster the machine, the smaller the amount of water used.
5. Temperature, humidity and ambient air flow. When the working environment humidity is less than 65%, the water consumption can be appropriately increased; when the working environment temperature is higher than 26 °C, the water consumption should be increased as appropriate.
6. The thickness of the sand particles on the surface of the printing plate. The plate with a coarser surface has a slightly larger water consumption than the fine sand plate.
7. Plate type and surface properties. When using a zinc oxide plate for printing operations, the water consumption is higher than the PS plate.
Only by knowing the above points well can we better control the amount of water in the layout, so as to ensure the smooth operation of the printing operation and produce high-quality printed products. In addition, since there are many variable factors affecting the amount of water in the layout, in the printing process, it is necessary to pay attention to the change of the amount of water in the layout and correctly determine the amount of water in the layout. Here are a few practical ways to identify the amount of water in the layout:
1. Observe the surface of the plate. If the plate is bright, the water volume on the plate is too large.
2. When the amount of water in the layout is large, a certain amount of water will be transferred to the ink roller. Therefore, when water droplets are visible in the ink fountain or on the ink roller, the water volume of the layout is too large.
3. When the amount of water remaining on the surface of the ink film is too much, there is water droplets when the ink squeegee is scraped, the ink is not easily uniform, and the transfer roller is slippery, etc., indicating that the water volume of the layout is too large.
4. There is a lot of ink on the ink roller, the ink is thick, and the sound is loud when the ink roller is separated, indicating that the water volume of the plate is too large.
5. The ink color is light, even if the amount of ink is added, it is not easy to make the ink color deeper in time, indicating that the water volume of the layout is too large.
6. Before and after the interruption of printing, the ink film is temporarily separated from the printing plate, so that the barrier effect of the water film on the ink transmission changes. If the water volume of the printing plate is too large, there will be a large difference before and after the ink color.
7. The printed sheet absorbs excess water, and the amount of water on both sides of the front and back is uneven, and the deformation of the paper is large, causing the paper to curl and causing difficulty in receiving the paper.
8. The blanket tip has water retention, and the excessive amount of water will form drops of water there.
9. After the shutdown, the layout is not dry, indicating that the water volume of the layout is too large.
10. Install a water film thickness gauge on the machine.
Under normal circumstances, while ensuring the print quality, the smaller the amount of water on the plate, the better. However, how to control the water quantity of the layout is just right, and it is still necessary to further explore in the future production practice to find a more scientific and effective way to solve the problem of difficult water volume control.