Water and ink balance control in rotary printing

Commercial rotary presses are high-speed, high-quality presses imported from abroad in recent years. It has the advantages of high printing speed (some machine speeds up to 60,000 sheets/hour), multiple printing layouts, stable quality, and strong timeliness. It has many common places with sheetfed machines, but there are many differences, such as : Paper feeding mechanism, printing speed, paper for printing, ink drying, folding, quality standards, paper loss, etc. Therefore, we need a process to master it. Everyone knows that there are many factors that are closely related to the quality of printing, but the most difficult thing to control may be ink and water balance in printing.

Offset ink balance refers to adjusting the amount of fountain solution under a certain printing speed and printing pressure, so that the emulsified ink contains less than 26% of fountain solution, forming a slight W/O emulsion ink. The minimum amount of liquid is commensurate with the ink on the plate. Offset printing is the use of natural laws in which oil and water are incompatible with each other, forming graphic and blank parts on the printing plate. It supplies both ink and water to the printing plate. It absorbs water and resists water while the blank part absorbs water and resists oil. print. If the ink is in an ideal state, the blank part of the printing plate is affixed with a 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 the ink remain relatively balanced at the interface, do not infiltrate each other, in this case the printing effect is ideal. However, in actual printing, the balance of ink and water is a dynamic balance and it is changing all the time. If the ink supply of the layout exceeds the balance, the ink will expand toward the blank after being strongly squeezed, and invade the blank surface to cause stickiness. On the other hand, if the water supply to the layout exceeds the equilibrium value, when the water roller passes over the surface of the graphic, it will leave more water on the surface of the graphic, and then it will be strongly squeezed by the water roller and the ink roller and attached to the graphic part. Deep emulsification of the ink layer causes the imprint to be dull. When the amount of water supplied is too large, the water enters the ink fountain along the ink fountain roller and the ink fountain roller, causing a large range of ink emulsification, and eventually the printing cannot be continued.

In the actual printing operation, many operators do not pay attention to the ink and water balance: when the water is too large, it is not to consider the collection of water, but to increase the amount of ink, printing for a period of time, the ink is emulsified seriously; after the dirty, the water is added, and the ink is naturally dull. After the lightlessness was added, the vicious circle was formed. We cannot always reach a better balance between ink and water, and naturally we cannot print good products.

As a commercial rotary printing press, the ink balance should mainly be controlled from the following aspects:

First, the main purpose of using the fountain solution in offset printing is 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 to the blank part to prevent the dirty plate; to add to the pro destroyed during the printing process. The water layer maintains the hydrophilicity of the blank part of the printing plate and lowers the surface temperature of the printing plate. Rotary printer water is made 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 printing ink is generally a thermosetting type (ie, a heat-dried ink), and its drying performance, oil content, and water-soluble latitude are quite different from those of the oxidatively-dried ink and the adsorptive-dried ink. If the latitude is large, the amount of water is not easy to grasp; the latitude is small and the degree of ink emulsification is not easy to grasp. Therefore, ink manufacturers are generally required to provide matching (adjustment) fountain solution, because it has been a number of professional experiments and production to obtain the best formula, and we need a long-term exploration to master.

Second, the operator's mastery of the printing plate and his ability to understand the proofs. Check whether the printing plate outlets and the field density have reached or adapted to the rotary printing requirements. Due to the high speed of rotary printing (generally up to 30,000 sheets/hour or more) and the type of B-B, if the print body is not enough ink, and the black layer of the proof is thicker, the operator should further enlarge the color of the proof, so only enlarge 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, the products printed by the rotary printing press do not require ink, as long as the field saturation, if you do not analyze the difference between the proof and actual printing, blindly increase the amount of ink, but cause print quality problems.

Third, the adjustment of water glue is more critical, especially the adjustment of the water transfer roller and the printing plate glue, and the water glue adjustment force is moderate (the width of the printing plate is 8~9mm, and the width of the ink transfer roller is 5 ~ 6mm), too big, too small are not conducive to water transfer, the more important thing is to balance both ends. If the two ends are not even, then when printing, there will be one end of the water, one end of the water is small. In this way, 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 press adjusts the zero ink key, there is a thin layer of ink on the entire ink fountain roller. Because when printing products of different layouts (wide width), even if the two ends of the ink fountain receive zero, there is a little ink transfer to the ink roller. Imagine that the rotation speed is fast and the ink transfer is fast. (The single sheet machine is not prominent.) ) The ink on both sides of the printed graphic is not easy to be taken away. It will accumulate even more, and it will even spread to both ends of the plate. Because there is a lot of water on the glue, the two ends of the glue are prone to happen. Oil-in-water emulsification, the resulting oil-in-water emulsified ink will splash on the printing plate at a high speed of the machine, resulting in dirt on the blanks at both ends of the printing product. At this time, the amount of water is increased, and the problem cannot be solved. . 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 commercial rotary presses often have a transition from low speed to high speed or from high speed to low speed in the printing process, it is necessary to compensate 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 from a low speed and looks good on ink, it will accelerate to normal speed all of a sudden. Then there must be a certain change in its ink-moisture balance. It is necessary to adjust the percentage of ink-moisture balance. Ink-rich places are more ink-filled, and ink is smaller. Local ink consumption is less, rather than the average increase in the amount of ink, only a good grasp of these, in order to control the dynamic ink and water balance.

Sixth, the naked eye. In general, the surface of the plate is slightly diffuse but not too bright; there is no water droplets on the bumper between the rollers. However, in actual printing, the amount of water in the rotary press should be slightly more than the percentage of the amount of water in the flat-sheet machine, because the rotation speed is fast, the water is too small, the ink is more likely to accumulate on the blanket, and the thermosetting ink for the rotary machine, The accumulated ink is not easily carried away by the paper, and it will soon be 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 also has a lot to do with the amount of paper dust used in printing, the strength of water absorption, pH, etc., which requires the paper supplier to provide relevant data, 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.

Author/Chen Jiandong

Source: "China Packaging" 2005/5