Lithographic Offsets Struggling in Ink Balance (3)

Sixth, ink balance of commercial rotary machines

As a commercial rotary machine, ink and water balance should mainly be controlled from the following aspects:

First, what is ink? The purpose of using the 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 expansion of the ink on the graphic image to the blank part to prevent the dirty plate; to supplement the hydrophilic layer damaged in the printing process , Maintain the hydrophilicity of the blank part of the printing plate; Reduce the surface temperature of the printing plate. Rotary water is made up of pure water, fountain solution and alcohol. The use of pure water can avoid algae formation on the dampening water pipe wall and cause obstruction. Alcohol consumption is 8 to 11%. The fountain solution has a conductivity of 9-11 and a pH of 4.8-5.2. The state of these values ​​is the best. It depends on the operator's long-term practice and summary. The ink is dispersed in the binder by very fine pigments and fillers, and after grinding, it becomes a more stable fluid colloidal suspension. The commercial rotary ink is generally a thermosetting type (ie, a heat-dried ink), and its drying performance, oil content, and water-soluble latitude are greatly different from the oxidation-drying ink and the adsorption-drying ink. With large latitude, low water content, low latitude, and difficulty in grasping the degree of ink emulsification, ink manufacturers are generally required to provide matching (adaptive) fountain solution, because it has obtained the best formula through several professional experiments and production. And we need long-term exploration to master it.

Second, the operator's mastery of the printing plate and his ability to understand the proofs. Check whether the printing plate outlets and field density meet the requirements and whether it meets the requirements for rotary printing. Due to the high speed of rotary printing (generally up to 30,000 rev/hr or more) and the B-B type, if the print body is not enough ink, and the black layer of the proof is thicker, the operator needs to zoom in to get closer to the sample ink. Ink volume, in this way, excess ink accumulates more in the ink roller, which can result in ink ejection or ink emulsification. This can cause the ink to become dirty and damage the balance of ink and ink. After that, increasing the amount of water is no help. In addition, products printed by rotation are not required to be inked. As long as they are saturated in the field, if they do not analyze the difference between the proofs and the actual printing, they will increase the amount of ink blindly, which in turn will cause print quality problems.

Third, the adjustment of water glue. The adjustment of water glue is more critical, especially the adjustment of the water transfer roller and the printing plate glue requires moderate adjustment of the water glue (with printing plate printing width of 8 ~ 9mm, and water transfer roller printing width of 5 ~ 6mm), Being too big or too small is not conducive to water transfer. What's more important is that both ends must be leveled. If the two ends are not even, then at the time of printing, one end of the water will be large and one end of the water will be small, so that when the ink balance at the small end of the water is good, then the water at the other end must be large. If you reduce the water at the same time, the other end will damage the ink balance due to water, resulting in a dirty version. In short, if water glue is not leveled, then water or water will be small, which will result in serious emulsification and fail to achieve the best state of ink balance. The same applies to the adjustment of ink.

Fourth, do a "collection of ink." When the commercial rotary machine adjusts the zero position ink key, a thin layer of ink can be formed on the entire ink fountain roller, because when printing the products of different layouts (wide), even if the two ends of the ink fountain receive zero, there is a little ink. Ink transfer to the ink roller, Imagine that the speed of the rotation is very fast, and the ink transfer is very fast (the single-sheet machine is not prominent). The ink on both sides of the printed graphic is not easy to be taken away, and it will accumulate more and more, and even spread it. On both ends of the water repellent glue, due to the large amount of water on the water glue, the water-oil emulsification is likely to occur at both ends of the water glue, and the formed oil-in-water emulsion ink will splash ink under the high-speed operation of the machine. On the printing plate, the blanks at the two ends of the printed matter are dirty, and then the amount of water is increased, which does not solve the problem. Based on this, it is possible to learn from the practices of some manufacturers, namely to make 70 lines, 50% dots, and 2 to 4 cm widths at both ends of the printing plate (which can be printed on the two blank sides of the paper and outside the cutting line). In this way, excess ink on both sides of the plate and text will allow the ink strips (both ends of the paper) to be carried away. This will not cause the ink to accumulate, and it is easier to realize the ink balance of the entire plate.

Fifth, the ink balance tracking curve. Since the commercial web press often has a transition from low speed to high speed or from high speed to low speed, it must compensate for the amount of water or ink according to the speed change, that is, to perform the ink tracking curve. For the general printing press, the ink tracking curve is set after leaving the factory. However, according to the long-term exploration and actual conditions, our operators should explore the ink-water balance rule that suits the speed change of the machine. If the machine is overprinted at a low speed (80000 rpm), after accelerating the ink color, it will accelerate to 40,000 rpm for 2 hours. Then it must have a certain change in the ink balance, and it is necessary to adjust the percentage value of the ink balance. To add more ink, less ink in small inked areas, rather than the average increase in ink volume, and only a good grasp of these, can easily control the dynamic balance of ink and wash.

Sixth, the naked eye. In general, the surface of the printing plate is slightly diffuse but not too bright, and there is no water droplet on the bumper between the two rollers. However, in actual printing, the amount of water in the rotary press should be a few percentage points more than the amount of water in the flat-sheet machine. Because the rotation speed is fast and the water is too small, the ink is more likely to accumulate on the blanket while the thermosetting ink for the rotary press piles up. The ink is not easy to be carried away by the paper, and soon it will get thicker and crushed. In addition, when the printing plate is printed to about 200,000 impressions, some of the dots on the layout are worn out, and natural ink is not easy. If the version is not inked, there will be large water and large ink. The ink will begin to emulsify and flicker, or fine ink bars similar to ink bars will appear, resulting in waste products. In this case, we need to collect small amounts of water and then reduce the amount of ink. Of course, this Affects color saturation. The best way is to replace the worn plate. These should be based on the actual situation.

Seventh, the balance of ink and water is also related to the amount of dust on the paper used in printing, the strength of water absorption, pH, etc. This requires the paper supplier to provide relevant data, so we can refer to its value and do a good job The proportion of items to better control the amount of water to achieve the best value of ink and water balance.

Seven, ink and ink balance in color printing

In the printing process of color newspapers, the overall control of ink-ink balance between blue ink, red ink, yellow ink, and black ink will directly affect the print quality of color newspapers. Newsprint is thicker and more absorbent than coated paper. Under normal circumstances, the ink-water balance ratio in newspaper printing is greater than the ink-water balance in sheet-fed offset printing. In the newspaper printing process, as the printing speed continues to increase, the contact time between the paper and the blanket cylinder is getting shorter and shorter, and the amount of water absorbed by the paper is getting smaller and smaller. 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 newspaper is generally only a picture or a little title with color, most of the text and the title is still black, so in the printing process black ink printing unit water supply is always larger than the color printing unit, but the water volume can not be too large, should No lateral deformation of the paper will prevail. Newspapers printed when water is too heavy have wet hands. Therefore, we must strictly control the ink balance in the printing process.

The following theoretical analysis of the impact of ink balance on newspaper printing quality.

The so-called ink-water balance means that in order to ensure the normal printing quality and production of newspapers, under a certain printing speed and printing pressure, adjust the amount of fountain solution so that the emulsified ink contains less than 26% fountain solution. The formation of a slight W/O emulsified ink is counterbalanced by the minimum amount of liquid supplied and the ink on the printing plate.

In the production process, when the prepared PS plate is printed on the machine, the graphic part of the ink is transferred directly to the blanket cylinder, and the graphic is transferred to the paper by the two blanket rollers. At the same time, the blank of printing is printed. Part of the water to prevent the ink from adhering to the blank surface. It can be seen that there is a balance between water supply and ink supply on the printing surface. If the ink supply to the layout exceeds the balance (ink > water), the ink will expand toward the blank after being strongly squeezed, invade the blank part, and cause dirtiness; otherwise, if the water supply to the layout exceeds balance (water) Ink), will leave more water on the surface of the printing plate graphic, through the strong force of the water roller and the ink roller, so that the ink layer attached to the graphic part of the deep emulsification, resulting in pale and dull imprint.

In a certain sense, only when the water film on the blank surface and the ink film on the surface of the picture have a very strict demarcation line, that is, it is objectively seen that the water blank area and the inked graphic area do not invade each other and do not mix with each other. It is considered that the ink and water balance is achieved, but in fact, absolute mixing is impossible, and the emulsification of the ink is inevitable. Therefore, ink-water balance is only a relative concept.

It is worth noting that when the printing press maintains high-speed stable operation for a period of time, the lower the supply of water, the better. In actual production, due to the high speed of imported rotary presses, a large amount of heat will be generated in the high-speed printing process, and water will be used for cooling and cooling. Therefore, the effect of the fountain solution is not only on the ink and ink balance, but also on the reduction of printing plates and The blanket temperature plays an important role.

In addition, ink emulsification is not entirely a bad thing. In fact, a slight W/O emulsification is advantageous for printing, which lowers the viscosity of the ink, improves the ink transfer performance, and at the same time prevents the ink from sticking and pulling excess paper powder back onto the roller. In view of the above reasons, the actual supply of water is slightly higher than the critical level.