All commercial rotary presses are high-speed and high-quality printing machines imported from abroad in recent years. It has the outstanding advantages of fast printing speed (some machine speeds up to 60,000 rpm), many printing plates, stable quality and strong timeliness. Therefore, after the introduction in Shenzhen, Shanghai, Beijing and other places, the mainland has also introduced a lot in the past two or three years. As a brand-new printing machine (in the past, commercial printing was mainly based on sheet-fed machines), it has many common points with sheet-fed machines, but there are many differences, such as: paper feeding Institution, printing speed, paper used for printing, ink drying method, folding method, quality standard, paper loss, etc. Therefore, we need to master it still need a process, we should summarize, communicate and study in order to become familiar with it and use it well.
Everyone knows that as far as the current situation is concerned, it is closely related to the printing quality, but at the same time, the most difficult to control is the ink-water balance of printing.
Offset water-ink balance refers to: under a certain printing speed and printing pressure, adjust the supply of fountain solution to make the emulsified ink contain less than 26% fountain solution, forming a slight W / O emulsion ink 3. Use the minimum liquid supply to compete with the ink on the printing plate. Offset printing is to use the natural law of incompatibility of oil and water to form graphics and blank parts on the printing plate. It supplies both ink and water to the printing plate. It absorbs oil and resists water, and the blank part absorbs water and resists oil. print. If in the ideal water-ink balance state, the blank part of the printing plate is attached with fountain solution, and the graphic part is attached with ink, only when the surface tension of the fountain solution and the surface tension of the ink are equal, the diffusion pressure at the interface between the two is Zero, the fountain solution and ink maintain relative balance on the interface and do not wet each other. In this case, the printing effect is ideal. However, in actual printing, the water-ink balance is a dynamic balance, which is constantly changing. If the amount of ink supplied to the plate exceeds the balance, the ink will expand toward the blank portion after being strongly squeezed, invading the blank surface and causing stickiness. Conversely, if the amount of water supplied to the layout exceeds the equilibrium value, when the water roller passes through the surface of the graphic, more water will be left on the surface of the graphic, and then it will be strongly squeezed by the water roller and the ink roller and adhere to the graphic The ink layer is deeply emulsified, resulting in a dull imprint. And when the water supply volume is too large, the water will enter the ink fountain along the inking roller and the inking roller, causing a wide range of ink emulsification, and eventually printing will not be able to continue.
In actual printing operations, many operators do not pay attention to water and ink balance. When the water is too large, instead of considering water collection, they increase the amount of ink. After printing for a period of time, the ink is emulsified seriously, causing stickiness, and water is added. The ink color is naturally dim There is no light, and ink is added, which forms a vicious circle. If the ink-and-ink balance is not reached, a good product will not be printed.
The balance of water and ink as a commercial rotary machine should be controlled from the following aspects:
First, what is ink. The purpose of using fountain solution in offset printing is mainly to: form a water film that repels ink in the blank part of the printing plate, to resist the expansion of the ink on the graphic to the blank part, to prevent the dirty version; to supplement the hydrophilic layer destroyed during the printing , Maintain the hydrophilicity of the blank part of the printing plate; reduce the surface temperature of the printing plate. The water used in the rotary machine is composed of pure water, fountain solution and alcohol. The use of pure water can prevent algae from forming on the walls of dampening water pipes, causing blockage. The alcohol consumption is 8 ～ 11%. The conductivity of the fountain solution is 9 to 11, and the pH is 4.8 to 5.2. Under what conditions these values ​​are best, we must rely on the operator's long-term practice and summary to grasp. The ink is dispersed in the binder by very fine pigments and fillers, and becomes a more stable fluid colloidal suspension after grinding. The commercial rotary ink is generally a thermosetting type (that is, thermal drying ink), then its drying performance, oil content, and water tolerance are very different from oxidative drying ink and adsorption drying ink. The tolerance is large, the amount of water is not easy to master, the tolerance is small, and the degree of emulsification of ink is not easy to master. Generally, ink manufacturers are required to provide a matching (adapting) fountain solution because it has obtained the best formula after several professional experiments and production. And we need long-term exploration to master.
Second, the operator's ability to master the printing plate and understand the proofs. Confirm whether the dots and solid density of the printing plate meet the requirements and whether they meet the requirements of rotary printing. Because the rotary printing speed is fast (generally more than 30,000 rpm), and it is type B-B, if the printed version does not eat enough ink, and the black layer of the proof is thicker, the operator can only zoom in to get closer to the proof ink color. The amount of ink, so that the excess ink accumulates in the ink roller, the ink will be thrown away or the ink will be seriously emulsified, which will cause the printed product to become dirty and destroy the balance of water and ink. Afterwards, increasing the amount of water will not help. In addition, the products printed in rotation do not require large ink, as long as the field is saturated, if the difference between the proof and the actual printing is not analyzed, the amount of ink will be increased, which will cause print quality problems.
Third, the adjustment of water glue. The adjustment of the water glue is more critical, especially the adjustment of the water transfer roller and the plate-based water glue requires moderate adjustment of the water glue (with the printing plate ink width of 8-9mm, and the water transfer roller ink printing width of 5-6mm), Too large and too small are not conducive to water transfer, more importantly, the two ends must be leveled. If the two ends are not flat, then when printing, there will be a large water at one end and a small water at one end, so when the water and ink balance of the small water end is good, then the water at the other end must be large. If the water is reduced at the same time, the other end will destroy the water-ink balance due to the small water, resulting in a dirty version. In short, if the water glue is not leveled, then there will definitely be large or small water, resulting in serious emulsification and not reaching the optimal state of water-ink balance. The same is true for the adjustment of ink glue.
Fourth, do "ink sticks". When adjusting the zero ink key in a commercial rotary machine, there is only a thin layer of ink on the entire ink fountain roller, because when printing products of different layouts (wide format), there will be a little ink even if the zero position is received at both ends of the ink fountain Transfer ink to the ink roller. Imagine that the rotation speed is very fast and the ink transfer is also very fast (the single sheet machine does not protrude). The ink on both sides of the printed image is not easy to be taken away, and it will accumulate more To the two ends of the water glue, because there is a lot of water on the water glue, the oil-in-water emulsification is easy to occur at the two ends of the water glue, and the formed oil-in-water emulsified ink will splash ink under the high-speed operation of the machine. On the printing plate, the blanks at both ends of the printed product are dirty. At this time, increasing the amount of water will not solve the problem. According to this, we can learn from the practices of some manufacturers, that is, at both ends of the printing plate (which can be printed on the two blank edges of the paper, and beyond the cutting line), make 70 lines, 50% dots, 2 to 4 cm width In this way, the excess ink on both sides of the printing plate and text will be taken away by the "receiving ink strips" (both ends of the paper), and the ink will not accumulate, and it is easier to achieve the balance of water and ink on the entire layout.
Fifth, the ink-ink balance tracking curve. Since commercial rotary presses often have a transition from low speed to high speed or from high speed to low speed, then it must compensate for the amount of water or ink according to the speed change, that is, make a good ink and ink tracking curve. For the general printing machine, the ink-and-ink tracking curve is set after leaving the factory, but our operators should explore the ink-and-ink balance law that adapts to the speed change of the machine based on long-term exploration and actual conditions. If the machine prints overprint from a low speed (80,000 rpm), after seeing the ink color, it will speed up to 40,000 rpm for 2 hours, then its water and ink balance must have certain changes. You need to adjust the percentage of water and ink balance, where the ink is large More ink is added, and less ink is added where the ink is small, rather than increasing the amount of ink on average. Only by mastering these well can we control the dynamic water and ink balance with ease.
Sixth, visual observation. In general, the surface of the printing plate is slightly diffused, but not too bright, and there is no water drop on the bumper between the two rollers. However, in actual printing, the amount of water in the rotary machine should be a few percentage points larger than that of the flat sheet machine, because the speed of the rotation is too fast, the water is too small, and the ink is more likely to accumulate on the blanket. The ink is not easy to be taken away by the paper, and soon the pile will be thicker and crush the blanket. In addition, when the printing plate is printed to about 200,000 prints, some dots on the plate surface are worn out, and it is not easy to apply ink naturally. If you do n’t eat ink on the plate, there will be large water and large ink. The ink will start to emulsify and throw out, or a thin ink bar similar to the ink bar will appear, causing waste products. In this case, you need to collect a small amount of water and reduce the amount of ink. Of course this Will affect the color saturation. The best way is to replace the worn plate. These will depend on the actual situation.
Seventh, the water-ink balance also has a great relationship with the amount of dust on the paper used for printing, the strength of water absorption, and the pH, etc., which requires paper suppliers to provide relevant data. Item ratio, to better control the amount of water, to achieve the best value of water and ink balance.
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