Ink and ink balance control of lithographic offset printing (4)
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Fourth, do the "ink collection". When the commercial rotary machine adjusts the zero ink key, there is a thin layer of ink on the entire ink fountain roller. Because of the printing of different layout (wide) products, even if the ink cartridges receive zero position at both ends, there is a little ink. Ink transfer to the ink roller, I think the speed of the rotation is very fast, the ink transfer is also very fast (single machine is not prominent), the ink on both sides of the printed image is not easy to be taken away, it will accumulate more and even pass By the end of the water-proof glue, because there is a lot of water on the water gel, the water-in-oil type emulsification is easy to occur at both ends of the water gel, and the formed oil-in-water emulsion ink will splash ink at the high speed of the machine. On the printing plate, the blanks on both ends of the printed product are dirty. At this time, the amount of water is increased, and the problem cannot be solved. According to this, we can learn from the practice of some manufacturers, that is, at the two ends of the printing plate (can be printed on the two blank sides of the paper, and outside the cutting line) to make 70 lines, 50% of the dots, 2 to 4 cm width In this way, the excess ink on both sides of the printing plate will allow the "ink strip" (both ends of the paper) to be taken away, so that the ink does not accumulate, and it is easier to achieve the ink balance of the entire layout.
Fifth, the ink balance tracking curve. Since the commercial rotary printing process often has a transition from low speed to high speed or high speed to low speed, it must compensate the water quantity or ink quantity according to the speed change, that is, the ink tracking curve is completed. For the general printing machine, the ink tracking curve is set after leaving the factory, but our operators should explore the ink and water balance law when adapting to the speed change of the machine according to the long-term exploration and actual situation. If the machine is overprinted from a low speed (80000 rev / h), after optimistic about the ink color, it will accelerate to 40,000 rpm for 2 hours at a time, then its ink and water balance must have a certain change, you need to adjust the percentage of ink and water balance, the place where the ink is big. Add more ink, less ink in the small place, instead of increasing the amount of ink on average, and only have a good grasp of these, in order to control the dynamic ink and water balance with ease.
Sixth, the naked eye observed. In general, the surface of the plate is slightly diffuse, but not too bright, and there is no water drop on the bumper between the two rollers. However, in actual printing, the water volume of the rotary machine should be slightly larger than the water volume of the flattening machine, because the rotation speed is fast, the water is too small, the ink is more likely to accumulate on the blanket, and the rotary machine is filled with thermosetting ink. The ink is not easily taken away by the paper, and soon it will pile up thicker and crush the blanket. In addition, when the printing plate is printed to about 200,000 impressions, some of the dots on the layout are worn out, and it is not easy to naturally apply ink. If the version does not eat ink, there will be large water and large ink. The ink will start to emulsifie and eject, or a fine ink strip similar to the ink stick will appear, resulting in waste. In this case, it is necessary to collect small water and then reduce some ink. Of course, this Will affect the color saturation. The best way is to replace the worn version. These should be based on actual conditions.
Seventh, the balance of ink and wash is also related to the dust of the paper used for printing, the strength of water absorption, the pH, etc. This requires the paper supplier to provide relevant data, we can refer to its value, and do a good job of dampening water. The proportion of the item is better to control the amount of water and achieve the best balance of ink and water balance.
Seventh, the ink and wash balance in the printing of color newspapers
In the process of color printing, the overall control of the ink balance of blue ink, red ink, yellow ink and black ink will directly affect the printing quality of the color newspaper. Newsprint is relatively thick and its water absorption is stronger than coated paper. In general, the ink-to-ink balance ratio in newspaper printing is greater than the ink-to-ink balance ratio in sheet-fed offset printing. In the printing process of newspapers, as the printing speed continues to increase, the contact time between the paper and the blanket cylinder is shorter and shorter, and the water absorption of the paper is less and less. At this time, the water supply is mainly used to balance the ink on the printing plate. Therefore, the overall water supply will continue to decrease as the speed increases, eventually reaching a critical value. In addition, since the color report is generally only a picture or a small title with color, most of the text and title are still black, so the water supply amount of the black ink printing unit is always larger than that of the color printing unit during the printing process, but the water volume should not be too large, should The paper is not subject to lateral deformation. Newspapers printed when the amount of water is too large have the feeling of wet hands. Therefore, it is necessary to strictly control the ink-and-wash balance in the printing process.
The following theoretical analysis of the impact of ink balance on the quality of newspaper printing.
The so-called ink-and-wash balance means that in order to ensure the quality of the printing of the newspaper and the normal production, the supply amount of the fountain solution is adjusted at a certain printing speed and printing pressure, so that the dampening liquid contained in the emulsified ink is less than 26%. A slight W/O type emulsion ink is formed to compete with the ink on the plate with a minimum amount of liquid supply.
In the production process, when the prepared PS plate is printed on the machine, its graphic part is inked, directly transferred to the blanket cylinder, and the image is transferred to the paper by two blanket rollers, and at the same time, the printing blank Part of the water is used to prevent the ink from adhering to the blank surface. It can be seen that there is a balance problem between water supply and ink supply to the printing surface. If the ink supply to the layout exceeds the balance (ink>water), the ink will expand to the blank portion after being strongly squeezed, invading the blank portion, causing the upper dirt; on the contrary, if the water supply to the layout exceeds the balance (water) Ink) will leave more water on the surface of the printing plate. After the strong extrusion of the water roller and the ink roller, the ink layer attached to the image will be deeply emulsified, resulting in a dull impression.
In a sense, only when the water film on the blank surface and the ink film on the surface of the image have a very strict boundary line, it is objectively observed that the water blank area and the inked image area do not invade each other and do not mix with each other. Only when the ink balance is reached, in fact, absolute mismatch is impossible, and the emulsification of the ink is inevitable. Therefore, ink and water balance is only a relative concept.
It is worth noting that when the printing press maintains high speed and stable operation for a period of time, the water supply amount is not as low as possible. In actual production, due to the high speed of the imported rotary printing press, a large amount of heat is generated in the high-speed printing process, and water is required to dissipate heat and cool down. Therefore, the effect of the dampening solution is not only the ink balance, but also the reduction of the printing plate and The temperature of the blanket also plays an important role.
In addition, ink emulsification is not entirely a bad thing. In fact, a slight W/O type emulsification is advantageous for printing, which reduces the viscosity of the ink, improves the ink transfer performance, and prevents the ink from being excessively viscous to pull excess paper powder onto the rubber roll. In view of the above reasons, the water supply in actual production is slightly higher than the critical state.