Offset printing is a printing method that requires water. Using the principle of incompatibility of oil and water, water is supplied first, and then ink is supplied, so that there is ink and no water in the graphic part, and water and no ink in the blank part. During printing, the ink on the printing plate is transferred to the surface of the elastic rubber cylinder by pressure, and then the rubber cylinder transfers the image to the paper on the surface of the impression cylinder.
The printing process is the process of water-ink balance, and 75% of the faults during printing are caused by chemical factors. Among them, water is the most prone to problems in the production process. Therefore, the correct control and use of fountain solution is the printing quality control One of the important links.
1. Wetting phenomenon
Wetting refers to an interface phenomenon that occurs when a liquid and a solid are in contact. When the liquid and solid are in contact with each other, the free energy of the system decreases is called wetting, otherwise it is called non-wetting.
When the liquid and the solid are in contact with each other, the liquid molecules in the adhesion layer, on the one hand, are affected by the internal force of the liquid-cohesion, and on the other hand, by the force of the solid molecule-adhesion, so whether the liquid can wet the solid , It is determined according to the cohesion and adhesion.
The interfacial tension between liquid and solid is a physical quantity that cannot be measured directly and accurately. Therefore, to determine the wetting phenomenon, the contact angle θ is usually used as a measure of the degree of wetting.
Relationship between contact angle and wetting
①When the contact angle θ is 0 °, the phenomenon of sufficient wetting.
②When the contact angle θ <90 °, it is said to be wettable.
③ When the contact angle θ> 90 °, it is called non-wetting.
The contact angle of the liquid to the solid is small, the liquid water droplets can be spread out, the surface tension is low, and the wetting performance is good.
2. Two ways to improve wetting performance
1. Increase PS plate's adsorption capacity of fountain solution
Choose a material with good hydrophilic properties, strong wear resistance and stable chemical properties as the basis of the blank part. Starting from enhancing the hydrophilic and oil resistance of the blank part of the printing plate, the advanced processing technology is used to form a dense Sand mesh, thereby increasing the surface area of ​​the PS plate, forming the basis of the blank part of the printing plate, and expanding the adsorption capacity accordingly, using the capillary holes of the sand mesh to store moisture, creating conditions for resisting the expansion of the ink.
Choose a printing plate with better sand-mesh processing, the tolerance of water and ink adjustment during printing is large, and the printing machine is easy to operate.
2. Reduce the surface tension of fountain solution
To improve the wetting performance of the fountain solution to the PS plate, it is necessary to reduce the contact angle, and to reduce the contact angle, surface active substances, interface active substances, hydrophilic colloids, etc. are added to reduce the surface tension of the solution to achieve full wetting performance .
Choose a fountain solution additive with good wetting performance, which is not easily affected by the uneven changes of related materials such as ink, fountain solution, etc., which can effectively reduce water consumption, reduce emulsification, accelerate drying, and improve the gloss of the printed product, thereby effectively reducing the printing failure rate .
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3. The formulating principles and requirements of fountain solution additives
The formulation of the PS version of the wetting liquid must follow good wetting performance, not too much emulsification of the ink, and the pH value of the solution is appropriate. In particular, the relationship between wetting performance and emulsification value should be solved.
1. To be able to fully wet the plate surface, the surface tension needs to be reduced, and emulsifier, wetting agent, surfactant should be added to the water bucket solution.
2. With cleaning ability, it can wash away the dirty spots. Add acid to clean the plate.
3. Stabilize the PH value of the wetting fluid, add buffer. Reduce ink emulsification.
4. It has good non-lipophilic ability, that is, non-oleophilic performance, such as adding gum arabic. Avoid inking of graphics and text.
5. Can adapt to the change of different climates, add moisture absorbent. Prevent the printing plate from getting dirty.
6. It can inhibit the growth of bacteria and mold and prevent the decay of the wetting fluid.
4. Application of conductivity and PH value in printing
The correct water-ink balance depends largely on accurately controlling the pH and concentration of the fountain solution. When many printing plants use fountain solution, they only pay attention to the pH of the fountain solution within the required range (pH value of 4.5 to 5.4), and ignore the concentration of fountain solution.
At present, most of the fountain solutions on the market are buffered fountain solutions. When the concentration of the fountain solution changes greatly, the change in pH is not obvious. However, the change in the concentration of the fountain solution will cause the content of each component of the fountain solution. Changes, which cause changes in surface tension, ink emulsification properties, etc., and finally affect the dampening effect, resulting in a decline in print quality. Therefore, only controlling the pH value will cause printing failures.
It has been experimentally measured that there is a linear relationship between the concentration change of fountain solution and its conductivity change, that is, the greater the concentration, the greater the conductivity. In this way, we can control the concentration of fountain solution by measuring the conductivity.
V. Measurement of conductivity and PH value
The pH meter is used to measure the pH of the fountain solution on the printing press. The electrode of the pH meter is enclosed in a glass tube. When it is immersed in the fountain solution, the ionic charge in the fountain solution induces the electrode in the glass tube. The charge induced in the electrode generates a tiny current in the circuit in the pH meter. This current is amplified and digitized, and then compared with the value already stored in the electronic memory of the pH meter to determine its value. Finally The results are displayed on the LCD panel.
Conductivity is defined as the ability of conductive electrons in a solution to pass through, and actually measures the amount of dissolved salt in the solution. The higher the fountain solution concentration, the higher the conductivity.
Although pH meters and conductivity meters are easy to use, they need to be calibrated frequently to maintain their correct function. Especially when changing the test head, detecting corrosive chemical samples, and when accurate measurement is required, it must be re-calibrated. Even under normal use, it must be calibrated at least once a month.
The pH meter immerses the test head in a calibration solution of known concentration for calibration, and then adjusts its value so that it displays the desired value. The pH values ​​of commonly used calibration solutions are 4, 7, 9.2 and 10. However, in many cases, if the calibration solution used does not match the calibration value given by the pH meter, it is best not to perform calibration. For example, in some pH meters the set points are 4, 7 and 10. If buffer values ​​4, 7 and 9.2 are used to verify the pH meter, an error will occur at the last control point (here 10). Because the fountain solution is usually acidic, its value is close to 4.5 to 5.5, so only need to perform two-point calibration 4,7.
The calibration of the conductivity meter is similar to that of the PH meter, but the matching calibration fluid (such as: 1413us / cm, 1382us / cm, etc.) must be selected according to its model, inserted into the calibration fluid, and the adjusted display value matches the actual value of the calibration fluid.
6. Determine the conductivity of fountain solution
For the same fountain solution, it is prepared with water from different places. Even if the fountain solution of the same concentration is prepared, their conductivity is not the same, because this is related to the degree of water hardness (how much Ca2 + and Mg2 + are in the water) . If the same kind of water is mixed with different brands of fountain solution to make fountain solution of the same concentration, the conductivity is also different. The conductivity is greater, indicating that there are many ions in the solution, and some ions have an effect on printing. If the concentration of Ca2 + and Mg2 + in the fountain solution is too high, the ink will emulsify and hinder the drying of the blot. Therefore, it is necessary to soften the water before preparing the fountain solution.
Based on the above factors, before preparing the fountain solution, we should ensure that its pH is between 4.8 and 5.3 according to the quality of the water used. Through the experimental method, the conductivity at different concentrations is measured and the conductivity is plotted Range, and then in this range, find out a certain conductivity value by the quality of the printed matter, as the standard value of the fountain solution prepared by using this fountain solution in the future. Different water quality, different fountain solution, the conductivity value to achieve the best wetting effect is different, so you can test according to the above method to achieve the best wetting effect.
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Seven, the basic relationship between the amount of fountain solution added and conductivity, PH value
1. Ideal water
Before the fountain solution is added, the conductivity of the water should be 300 us / cm-500 us / cm, the hardness is 5? -8 ?, it is soft water, and the PH value is between 6.5-7.
2. The relationship between the change of ideal fountain solution PH value and conductivity
Dilute the fountain solution, when the PH value changes between 4.5-5.5, the conductivity should change in equal proportion between 1200-600us / cm.
When the conductivity exceeds the initial configuration value of 500us / cm, the fountain solution should be replaced. That is: according to the local water quality, through the printing test, assuming that the conductivity is 900us / cm, and the PH value is 4.9, the dampening effect is good. When the conductivity reaches 1400us / cm, it means that the water in the water tank has been contaminated and is not suitable for printing , You need to clean the water tank and replace the fountain solution.
8. PH value management requirements of the automatic adding system of the printing press fountain solution
The fountain solution can be divided into two types: "buffer type" (such as B liquid) and "non-buffer type" (such as A liquid). When the concentration of the "buffer type" fountain solution changes, the PH value changes nonlinearly.
At present, most printing presses control the amount of fountain solution added by controlling the pH value of fountain solution. The concentration of fountain solution and pH value need to change linearly, that is, the concentration of fountain solution changes slightly. The PH value should change accordingly, and when the concentration of the "buffer type" fountain solution changes slightly, the PH value may not change, resulting in the concentration of the fountain solution being uncontrollable. Therefore, the "buffer type" fountain solution is not suitable for automatic addition by the machine.
The "non-buffer type" fountain solution has a small change in concentration and a large change in PH value, making printing difficult to control. Therefore, what is on the market is basically a "buffer type" fountain solution.
In order to reduce the occurrence of printing failures, the "buffer type" fountain solution needs to be manually configured and added to the water tank, and a corresponding measuring cup is equipped to control the accurate addition amount.
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Nine, the test of two fountain solutions
Since the conductivity of tap water is high, and the PH value of pure water is low, the conductivity is 0, so this experiment chose to prepare the water in the ratio of 3: 1.
The PH value of tap water is 7.4 and the conductivity is 890us / cm; the PH value of purified water is 5.6 and the conductivity is 0. After being prepared according to the 3: 1 ratio of purified water and tap water, the PH value of the water is 7.02, and the conductivity is 270us / cm.
The test results are shown below
Ten, test analysis
As can be seen from the test chart, both fountain solutions A and B are "buffer type" fountain solutions. Not suitable for automatic addition of the machine, you should use the manual configuration of the fountain solution concentration.
PH value of fountain solution A: 4.5-5.5; concentration: 4% -0.6%; conductivity: 1500-400us / cm.
PH value of fountain solution B: 4.5-5.5; concentration: 5.2% -0.5%; conductivity: 2500-500us / cm.
The two fountain solution additives have different conductivity under the same water quality and the same ratio concentration.
The solution ratio recommended by the fountain solution supplier: A fountain solution standard concentration is 2% -2.5%, PH value is 4.5-5.5; B fountain solution standard concentration is 2% (soft water), 3% (hard water), The PH value is 4.9-5.2.
Choose according to the test results: the proportion concentration of A fountain solution is 2%, PH value is 4.7, and conductivity is 820us / cm. The proportioning concentration of fountain solution B is 2%, PH value is 4.85, and conductivity is 1382us / cm.
Due to the need to add about 10% of isopropanol, the actual volume of isopropanol should be used to calculate the amount of fountain solution added, otherwise, the concentration of fountain solution will be insufficient and the PH value will increase. , Affect the wetting effect.
Through the actual use verification in production, from automatic addition to manual addition, the fountain solution concentration is accurately controlled, the wetting performance is good, and the printing problems caused by the fountain solution are effectively reduced.
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11. Water tank management
Water tank temperature setting: winter temperature is set to 10 ℃, summer temperature is set to 8 ℃, the temperature is too low will lead to ink pulling, the temperature is too high and easy to paste. And often check whether the cooling system is working properly.
Water tank cleaning cycle: It is recommended to change the water once a week and clean the water tank.
Cleaning and replacement of the water tank filter: It is recommended to clean it once a week and replace it every month.
Cleaning the air filter of the cooling device: It is recommended to clean it once a week, otherwise it will affect the cooling effect.
12. Selection of alcohol and fountain solution
There are many factories that use edible alcohol or industrial alcohol in fountain solution. Its composition is: ethanol. The water-ink balance is slower during printing and the amount added is larger.
It is recommended to use isopropyl alcohol. The water and ink balance is fast when printing, and the amount of addition is small, generally: 8% -12%. The better the fountain solution, the smaller the amount of isopropyl alcohol.
When buying isopropyl alcohol, buy it from a large chemical plant as much as possible, otherwise it will lead to insufficient isopropyl alcohol content and affect the wetting effect.
At present, there are many brands of fountain solution additives on the market, and the prices are different. Because many factories often only consider their unit price to reduce printing costs, without considering their wetting properties, resulting in a large amount of water and poor print quality. A fountain solution with good wetting performance has a relatively high unit price, but its added amount is small, and the total amount is small. The total cost is not necessarily higher than that of a cheap fountain solution additive. If used correctly, it can reduce costs. Therefore, it is recommended that when using fountain solution additives, buy brand fountain solution as much as possible, such as Heidelberg superdot fs-12, etc., the brand fountain solution additive has stable quality, small addition amount, small addition amount of isopropyl alcohol, when printing The amount of water is small, the ink is not easy to emulsify, and the printing quality is stable.
13. Consequences of high water hardness
The higher the content of Ca2 + and Mg2 + ions, the higher the hardness of the water. In the fountain solution, the excessive concentration of Ca2 + and Mg2 + ions will cause the emulsification of the ink. In the high-speed printing process, due to the temperature, the fountain solution will Mg (HCO3) 2 and Ca (HCO3) 2 decompose to produce water-insoluble CaCO3 and MgCO3. Calcium adheres to the water roller, it is easy to prevent water filling, and there is a paste fault. Adhering to the ink roller will cause the ink roller to crystallize, affect the ink transmission, and cause the ink roller to deink.
14. Consequences of incorrect use of alcohol and fountain solution
Poor performance or excessive addition of fountain solution additives will cause excessive emulsification of the ink, resulting in ink roller deinking, ink return to the water tank, ink accumulation on the water supply roller, poor color balance, ink drying, and dull printing.
If the quality of alcohol is not good or the amount is too large, it may dissolve the ink, causing the ink to emulsify extremely, and cause the above failure.
If the fountain solution additive does not match the ink used, it will also cause the above failures.
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15. Control of water consumption and water supply of offset printing
The consumption of offset fountain solution is mainly in the following aspects:
1. The emulsification of ink consumes water. The slight emulsification of the ink contributes to rapid water-ink balance, and the larger the amount of ink, the greater the amount of water required to counterbalance.
2. Paper surface absorbs water consumption. Due to the different fiber characteristics and smoothness, uniformity, sizing degree, water content, expansion rate and other indicators of various papers, the water absorption of papers during printing is different. Generally speaking, non-coated paper absorbs more water than coated paper.
3. Evaporate water consumption into space.
4. Squeeze out the amount of water.
Offset printing determines the conditions of water consumption
1. The area and distribution of the graphic parts of the layout. The graphic area on the printing plate has a large area and large water consumption.
2. The thicker the printing ink layer, the greater the water consumption, the thinner the ink layer, the smaller the water consumption;
3. The paper fiber structure is loose, the surface is rough, and the paper has strong water absorption performance; the structure of coated paper and card paper is dense, the surface is smooth, and the water absorption is small.
4. The ink has good water resistance, small emulsification value, and low water consumption.
5. The higher the operating speed of the offset press, the more water is consumed. The high ambient temperature and low humidity, the air circulation is smooth, and the consumed water is large.
6. The better the quality of the printing plate, the better the sand processing, and the smaller the water supply.
7. The printing pressure of offset printing is large, the consumption rate of fountain solution is high.
8. The higher the hardness of the blanket, the greater the consumption of fountain solution.
Based on the above reasons, different amounts of water should be used according to different prints when printing. When changing the printing plate, the amount of water should be reduced to the paste version of the plate, and in the process of registering and coloring, the amount of water should be gradually increased to the printing. The printing plate is not pasted, so as to achieve the printing with the smallest amount of water, avoiding the large water and large ink, thereby controlling the printing quality.
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