Among the four printing methods of flat concave and soft hole, only one method is obviously different from the other three printing methods on the principle of ink transfer. This is lithography, which is what we usually call offset printing.
The most basic printing principle in offset printing is to use the two rules of incompatibility of oil and water, and selective adsorption of the printing plate to keep the ink and water balanced on the printing plate to achieve the transfer of the dots, and to achieve clear images and full colors Effect. This is where the term ink balance is derived.
1. Understanding the balance of ink and wash
The ink and water on the printing plate must exist and maintain balance at the same time.The purpose is to maintain the maximum ink load of the graphic printing area, make the ink color bright, saturated, and the dots clear and smooth, but also keep the non-printing area highly clean and tidy. The balance between water supply and ink supply is called water-ink balance.
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.
The balance of water and ink is the basis of lithography. In the offset printing process, whether the water-ink balance is just right has a very close relationship with the normal transfer of the blot, the shade of the ink color, the accuracy of the overprint, and the dry state of the printed matter. Whether or not the ink-ink balance can be properly controlled and controlled is the key to ensuring stable print quality.
Second, the conditions for generating ink and ink balance
During the printing process, even if the water conveying device is adjusted and operated correctly, due to the impact of the shutdown, the temperature and humidity changes in the printing workshop, the operating speed of the printing machine, the type of ink, paper, and the blank space of the layout increase with the number of printed copies Restrictions such as weakening the hydrophilic property will also directly affect the water-ink balance.
1 The printing plate must have a solid graphic and blank foundation to keep the lipophilic and hydrophilic relative stable. If a certain condition is not met, the ink and wash will be out of balance.
2 The thickness of the sand should be controlled well. The grit on the surface of the printing plate must be kept even and fine, with a certain physical strength, so that the graphics and blank parts have a solid attachment foundation and good water storage conditions.
3 The temperature and humidity of the workshop should be constant. In principle, the workshop is required to have conditions of constant temperature and constant humidity, because changes in temperature will cause changes in the viscosity and fluidity of the ink, and the increase in temperature will evaporate water, which will make the water and ink lose balance. Especially resin ink is very sensitive to temperature. It is best to install an air conditioner to control the temperature within the range of 20 ℃ ～ 25 ℃.
4 The machine should run at a constant speed. In production, the machine must maintain normal speed and uniform speed. The machine will quickly and slowly, and it is easy to lose control of the water and ink balance.
5 To grasp the characteristics of different types of paper, ink and other raw materials. The performance of ink and paper used in printing differs greatly. The use of different papers and inks has different requirements for water-ink balance. For example, paper has a close texture and loose texture, and a moisture content; ink has viscosity, fluidity, and pigment particle size. Under the same conditions of printing plate, temperature, humidity, etc., we must grasp the changes of their properties.
6 The amount of drying oil should be moderate. Putting dry oil in the ink promotes the drying of the ink, but the dry oil promotes the stickiness of the ink. Therefore, the unsuitable amount of dry oil is the main factor that makes the balance of the water and ink out of control and the dirty version.
7 The pH value of dampening dilution should not fluctuate. The PH value of fountain solution is also an important condition that affects the balance of water and ink. It must be made constant to avoid large-scale fluctuations.
8 Adjust the pressure correctly. The printing press must have precise cylinder pressure, ink roller pressure, and water roller pressure. These three pressures directly determine the water-ink balance.
Third, the principle of ensuring ink-water balance
To maintain the balance of water and ink, we should first manage the water. A deep understanding of the nature and function of water is the foundation for good water management. Water is widely distributed in nature, he is a colorless and odorless transparent liquid. Water is a dipolar substance and can be dissolved with many other substances. But there are also substances that are incompatible with it. For example, it is incompatible with oils, and it is also incompatible with substances that are compatible. It can be seen from the order of the hydrophilic properties of metals.
The following arrangement is the order of the hydrophilic properties from strong to weak: potassium, calcium, sodium, magnesium, aluminum, zinc, chromium, iron, nickel, tin, lead, etc. The plate machine we use is the one with better hydrophilic properties Made of aluminum metal.
Offset printing inks generally use inks with good water resistance, bright colors, transparency, saturation, and purity. Oil is a non-polar substance and is immiscible with water under normal conditions. Nothing in the world is a pure single-element substance, so is the water repellency of ink. From the molecular structure of oil, all kinds of fats in oil molecules are mainly composed of two parts: one part is a very long hydrocarbon chain, which has water-repellency and lipophilicity, and is called a water-repellent group; Oil hydrophilicity, called hydrophilic group. These two groups are interconnected and contradict each other. Groups with completely opposite properties exist in a molecule. The former is a hydrophobic and lipophilic water-repellent group, which is non-polar, and the latter is an oil-repellent hydrophilic group, which is polar. .
In other words, oil has duality, both non-polar and polar components, there is both oil-water immiscibility, and the possibility of oil-water miscibility. However, since the polar carboxyl groups of the hydrophilic oil repellent are very few in the entire oil molecule, they are only subordinated in the oil molecule, while the non-polar water-repellent lipophilic groups account for the absolute majority in the entire oil molecule. Because of its dominant position, oil is immiscible in oil and water at normal temperature and pressure. Through the above analysis and discussion, we can see that it is impossible for the offset printing process to make the ink not emulsified at all. The key is to master properly and achieve ink-and-ink balance.
4. Measures and control methods to ensure the balance of water and ink
1 During the printing process, the printing plate must have a solid graphic and blank foundation to maintain lipophilic and hydrophilic stability.
2 On the premise of ensuring that the printing plate is not stained, the water supply volume is controlled to the smallest possible range (the water supply of the plate should be 26%), and the water supply volume and the ink volume are in a relatively stable state, so as to ensure the printed matter The ink color is consistent before and after and the printing operation is stable.
3 Master the principle of less water and thick ink. The less water here refers to the premise that the blank part of the layout is not stained. The so-called thick ink is also based on less water. Ink is emulsified due to large water, and the ink layer cannot be thick. From the transfer process of offset ink, it can be seen that in a water supply and ink supply, a total of three times the mixing and emulsification of ink and water, it is impossible to maintain a strict boundary between water and oil. . Therefore, the ink-water balance in offset printing can only be a relative concept, and a perfectly ideal ink-water balance does not exist. As long as the optimal balance is reached, the ideal print can be printed.
4 Select the size of the ink according to the material type of the printing plate. The water volume of the PS version can be appropriately smaller, the water volume of the PVA version can be slightly larger; the water volume of the smooth paper can be smaller, the rough paper can be larger, and the machine runs fast , The water volume can be slightly smaller, and it can be larger at low speeds.
5 Environmental conditions, temperature and humidity can not be ignored. Since the moisture on the layout is distributed in two forms, direct and indirect, while the moisture on the layout meets the needs of water-ink balance during printing, most of it is distributed to the space.
6 The PH value of fountain solution must be controlled (generally controlled at about 4.5 to 5.5). In addition, since the PH value of the offset printing paper surface has a great influence on the PH value of the fountain solution, it is best to measure the PH value of the paper. If the PH value of the paper is too low, the PH value of the fountain solution should be increased slightly; on the contrary, if the PH value of the paper is too high, the PH value should be appropriately lowered so that it can neutralize the OH- of the paper, thereby buffering Excessive PH value of fountain solution. According to the conclusions stated in relevant materials and practice, when the paper PH value is 9, the fountain solution PH value is 4; when the paper PH value is 8, the fountain solution PH value is 5 is better.
7 Use the method of scientific instrument detection to control the water-ink balance. Because the ink obtained in the state of water-ink balance must have thick ink color and consistent density, the change of water-ink balance in the printing process can be detected by continuously measuring the density. When the density value reaches the standard value range, it can be concluded that the water-ink balance is normal.
In addition, in the work, in addition to the offset printing process of water and ink, the operator should also consider the different types of raw and auxiliary materials used (paper, ink, plates, blankets, etc.) and the differences in working environment and other related factors , So that the ink can reach or basically reach the balance suitable for the printing process.
V. Analysis of the phenomenon of imbalance of ink and wash
How to achieve ink-and-ink balance, the operator must not only understand and study the organic relationship between ink and ink, the constraints between them and the rules of subtle changes between them, but also analyze and discuss the undesirable consequences caused by the imbalance of ink and ink.
1 Analysis of the phenomenon that there is little water and little ink: In the printing process, there is a certain water film in the blank part of the printing plate. When the water film and the oil can contend, it will not be stained by the ink on the ink roller. If the amount of the water layer is too small to resist the adsorption of the ink on the blank part, the blank part will adhere to the ink, resulting in hanging dirt, and the amount of ink supply will make the printed matter dull, light gray, and the imprint is not true. The blots are covered with snowflake-like white dots. In this state, although the "water-ink balance" is also reached, this "water-ink balance" is not the "water-ink balance" we want.
2 Analysis of the phenomenon of less water and more ink: At this time, the ink color of the printed product is most likely to be uneven, hanging dirty, a certain part or a large layout is caused by water shortage, resulting in paste and paste. At the same time, the printed ink is darker. Makes the printed matter dark, the dots are not clear, especially for the reproduction of fine dots, the image is not clear.
3 Analysis of the phenomenon of less water and ink: If the water content of the plate is too large, it will gradually spread to all the ink roller surfaces to form a water layer of a certain thickness, which hinders the normal transfer of ink. The emulsification speed of the ink is accelerated, and the color of the printed ink is gradually unsaturated. The picture becomes lighter, the handwriting is dull, gray, hairy, hairy, and dull. There is a halo around the imprint, the image is not clear, and there are no layers.
4 Analysis of the phenomenon of large amount of water and ink: When the water content of the plate is excessive, the ink color will become lighter, and it will often blindly think that the amount of ink supply is small, so the amount of ink is continuously increased, and the long-term circulation back and forth, the ink emulsification loses stability, causing The vicious cycle of water-ink imbalance leads to serious emulsification of the ink, which accumulates on the surface of the ink roller, making printing impossible.
6. Ink and ink balance of commercial rotary machines
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.
7. Water and ink balance in color newspaper printing
In the process of color newspaper printing, the overall control of the blue ink, red ink, yellow ink and black ink water and ink balance will directly affect the printing quality of the color newspaper. Newsprint is coarser and more absorbent than coated paper. Normally, the water-ink balance ratio in the newspaper printing process is larger than that in the sheet-fed offset printing. In the newspaper printing process, as the printing speed continues to increase, the contact time between the paper and the rubber cylinder becomes shorter and shorter, and the paper absorbs less and less water. 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, and eventually reach a critical value. In addition, since the color newsletter is generally only pictures or a few titles with colors, most of the text and titles are still black, so the water supply 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. Subject to no lateral deformation of the paper. Newspapers printed when the amount of water is too large have a wet hand feel. Therefore, we must strictly control the water-ink balance in the printing process.
The following theoretically analyzes the effect of water-ink balance on newspaper printing quality.
The so-called water-ink balance refers to adjusting the supply of fountain solution under a certain printing speed and printing pressure to ensure the newspaper printing quality and normal production, so that the fountain solution contained in the emulsified ink is less than 26%. A slight W / O type emulsified ink is formed, and the minimum liquid supply is used to compete with the ink on the printing plate.
In the production process, when the prepared PS plate is printed on the machine, its graphics and text are partially inked and transferred directly to the rubber cylinder, and the graphics and text are transferred to the paper through the two rubber cylinders, and at the same time, the printed blank Partially touch the water to prevent the ink from adhering to the blank surface. This shows that there is a balance between the water supply and ink supply to the printing surface. If the amount of ink supplied to the layout exceeds the balance (ink> water), the ink will expand to the blank part after being strongly squeezed, invading the blank part, causing dirty, otherwise, if the water supply to the layout exceeds the balance (water> Ink), it will leave more water on the surface of the printing plate and text, and the strong squeeze of the water roller and the ink roller will deeply emulsify the ink layer attached to the graphic and text, causing the blot to be dull.
In a sense, only when the water film on the blank surface and the ink film on the graphic surface have a very strict boundary line, that is, the water blank area and the inked graphic area are objectively seen not to invade and mix with each other. Only when the water-ink balance is reached, in fact, absolute non-mixing is impossible, and the emulsification of ink is also inevitable. Therefore, the balance of water and ink is just 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 is not as low as possible. In the actual production, due to the high speed of the imported rotary printing press, a large amount of heat will be generated during the high-speed printing process, and it will also rely on water to dissipate the heat and cool down. Therefore, the role of the fountain solution is not only to balance the water and ink, but also to reduce 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, slight W / O emulsification is beneficial to printing. It reduces the viscosity of the ink and improves the ink transfer performance. It also prevents the ink from sticking too much and pulling too much paper powder onto the rubber roller. In view of the above reasons, the water supply in actual production is slightly higher than the critical state.
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