Offset quality control - use of dampening solution (one)
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Offset printing is a printing method that requires water. The principle of incompatibility between oil and water is used, first water supply, and then ink supply, so that the image part has ink and water, and the blank part has water and no ink. During printing, the ink on the image on the plate is transferred to the surface of the elastic blanket cylinder by pressure, and then the image is transferred by the blanket cylinder onto the paper on the surface of the impression cylinder.
The printing process is the process of ink and water balance, and 75% of the defects in printing are caused by chemical factors. Among them, water is the most prone to problems in the production process. Therefore, proper control and use of dampening solution is the quality control of printing. One of the important links.
First, wetting phenomenon
Wetting refers to an interfacial phenomenon that occurs when a liquid comes into contact with a solid. When the liquid is in contact with the solid, the free energy of the system is called wetting, otherwise it is called non-wetting.
When the liquid and the solid come into contact with each other, the liquid molecules in the adhesion layer are on the one hand subjected to the internal force of the liquid - cohesive force, and on the other hand by the force of the solid molecules - adhesion, therefore, whether the liquid can wet the solid , depending on the cohesive force and the amount of adhesion.
The interfacial tension between liquid and solid is a physical quantity that cannot be directly and accurately determined. Therefore, the wetting phenomenon is judged, and the degree of wetting is usually measured by the contact angle θ.
Relationship between contact angle and wetting
1 When the contact angle θ is 0°, sufficient wetting occurs.
2 When 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 beads can be spread out, the surface tension is low, and the wetting property is good.
Second, the two ways to improve the wetting performance
1. Increase the adsorption capacity of the PS plate on the dampening solution.
Selecting materials with good hydrophilic properties, strong wear resistance and stable chemical properties as the basis of the blank part, starting from the hydrophilic and oil-repellent ability of the blank part of the printing plate, using advanced processing technology to form a dense surface on the printing plate. The sand mesh, thereby increasing the surface area of the PS plate, forms the basis of the blank portion of the printing plate, and thereby expands the adsorption capacity, and uses the pores of the sand to store moisture, thereby creating conditions for resisting the expansion of the ink.
The printing plate with better sand treatment is selected, the tolerance of ink adjustment during printing is large, and the printing machine is easy to operate.
2, reduce the surface tension of the fountain solution
To improve the wetting property of the dampening solution on the PS plate, it is necessary to reduce the contact angle. To reduce the contact angle, it is necessary to add a surfactant, an interface active substance, a hydrophilic colloid, etc., thereby reducing the surface tension of the solution and achieving sufficient wetting performance. .
The dampening solution additive with good wetting performance is not easy to be affected by the uneven change of related materials such as ink and dampening solution, which can effectively reduce the water consumption, reduce the emulsification, accelerate the drying, and improve the gloss of the printed product, thereby effectively reducing the printing failure rate. .
Third, the formulation principles and requirements of dampening solution additives
The formulation of the PS version of the dampening solution must follow the wettability, not too much for the ink emulsification, and the pH of the solution is suitable. In particular, it is necessary to solve the relationship between the wetting property and the emulsification value.
1, to fully wet the plate, it is necessary to reduce the surface tension. Add emulsifier, wetting agent and surfactant in the water bucket solution.
2, with cleaning ability, can wash away dirty spots. Add acid and clean the plate.
3, tabilize the pH of the dampening solution and add buffer. Reduce ink emulsification.
4, has a good ability to not feel the fat, that is, not lipophilic properties, such as the addition of gum arabic. Avoid the graphics part not inked.
5, can adapt to changes in different climates, add moisture absorbent. Prevent the plate from getting dirty.
6, can inhibit the growth of bacteria and mold, prevent the corrosion of the wetting fluid.
Fourth, the application of conductivity and PH value in printing
The correct ink-and-water balance is highly dependent on accurately controlling the pH and concentration of the dampening solution. In many printing plants, when using dampening solution, only the pH of the dampening solution should be controlled within the required range (pH 4.5-5.4), and the concentration of dampening solution is neglected.
Most of the dampening liquids currently on the market are buffer-type dampening solutions. When the concentration changes greatly, the pH change is not obvious. However, the change of the dampening solution concentration will cause the content of each component of the dampening solution to occur. Change, which causes changes in surface tension, ink emulsification performance, etc., and finally affects the dampening effect, resulting in a decline in the quality of printed products. Therefore, controlling only the pH will result in a printing failure.
Through experiments, there is a linear relationship between the concentration change of the 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 the fountain solution by measuring the conductivity.
Fifth, measurement of conductivity and PH value
The pH meter is used to measure the pH of the fountain solution on the press. The electrode of the pH meter is enclosed in a glass tube, and the ionic charge in the fountain solution induces the electrode in the glass tube as it is immersed in the fountain solution. The charge induced in the electrode produces a small current in the circuit in the pH meter, which is amplified and digitized and then compared to the value already stored in the electronic memory in the pH meter to determine its value, and finally The result is displayed on the LCD panel.
Conductivity is defined as the ability of a conducting electron to pass through a solution, and actually measures the amount of dissolved salt in the solution. The higher the concentration of the fountain solution, the higher the conductivity.
While pH meters and conductivity meters are easy to use, they need to be calibrated frequently to maintain their proper function. Especially when replacing test heads, testing corrosive chemical samples, and requiring accurate measurements, be sure to recalibrate, even if it is used normally, it must be calibrated at least once a month.
The pH meter calibrates the test head by immersing it in a calibration solution of known concentration and then adjusting its value to display the desired value. The commonly used calibration solutions have pH values of 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 check. For example, in some pH meters the set points are 4, 7, and 10. If the pH values are verified using buffer values 4, 7, and 9.2, an error will occur at the last control point (here 10). Since the dampening solution is usually acidic, its value is close to 4.5 to 5.5, so it is only necessary to perform 4 and 7 two-point calibration.
The calibration of the conductivity meter is similar to that of the PH meter, but the matching calibration solution (eg, 1413us/cm, 1382us/cm, etc.) must be selected according to its model, inserted into the calibration solution, and the adjusted display value is consistent with the actual value of the calibration solution.
Sixth, determine the conductivity of the fountain solution
For the same dampening solution, it is prepared from water in different places. Even if the same concentration of dampening solution is dispensed, their conductivity is different, because it is related to the softness and hardness of water (the amount of Ca2+ and Mg2+ in water). . If the same water is mixed with different brands of dampening syrup to make the same concentration of dampening solution, the conductivity is different, and the conductivity is large, indicating that there are many ions in the solution, and some ions have an effect on printing. If the concentration of Ca2+ or Mg2+ in the dampening solution is too high, the ink will be emulsified and the blotting will be hindered. Therefore, it is necessary to first soften the water and then prepare the dampening solution.
Based on the above factors, before formulating the dampening solution, we should ensure that its pH value is between 4.8 and 5.3 according to the water quality of the water used. Through the experimental method, the conductivity at different concentrations is measured, and the conductivity is drawn. The range, and then within this range, find a certain conductivity value by the quality of the printed matter, as a standard value for the future use of this dampening solution to prepare dampening solution. Different water quality, different dampening syrup, the conductivity value to achieve the best wetting effect is different, therefore, can be tested according to the above method to achieve the best wetting effect.
Seventh, the basic relationship between the amount of fountain solution added and conductivity and pH value
1, ideal water
Before the dampening solution is added, the conductivity of the water should be 300 us/cm-500 us/cm, the hardness is 5 ゚-8 ゚, soft water, and the PH value is between 6.5 and 7.
2, the relationship between the change of PH value of ideal dampening solution and conductivity
The dampening solution is diluted, and when the pH value is changed between 4.5 and 5.5, the electrical conductivity should be changed in a proportional manner between 1200 and 600 us/cm.
When the conductivity exceeds the initial configuration value of 500 us / cm, the dampening solution should be replaced. Namely: according to the local water quality, through the printing test, assuming a conductivity of 900us / cm, PH value of 4.9, the dampening effect is good, then when the conductivity reaches 1400us / cm, the water in the tank is contaminated, not suitable for printing The tank should be cleaned and the dampening solution replaced.
Eighth, the printing machine fountain solution automatic addition system PH value management requirements
The dampening solution can be divided into two types: “buffer type” (such as liquid B) and “unbuffered” (such as liquid A). When the concentration of “buffered” dampening solution changes, the pH value changes nonlinearly.
At present, most printing presses control the amount of dampening solution by controlling the pH of the dampening solution. The concentration of the dampening solution and the pH value need to be linearly changed, that is, the concentration of the dampening solution changes slightly. The pH value should be changed. When the concentration of the "buffered" dampening solution changes slightly, the pH value may not change, and the concentration of the dampening solution cannot be controlled. Therefore, the "buffering type" dampening solution is not suitable for automatic machine addition.
The "non-buffered" dampening solution has a small change in concentration, and the pH value changes greatly, making printing difficult to control. Therefore, the market is basically a "buffer type" fountain solution.
In order to reduce the occurrence of printing failure, the "buffer type" dampening solution needs to be manually configured and added to the water tank, and equipped with the corresponding measuring cup to control the accurate addition amount.
Ninth, testing of two fountain solutions
Since the conductivity of tap water is high, and the pH of pure water is low, the conductivity is 0. Therefore, water prepared in a ratio of 3:1 is selected in this experiment.
The tap water has a pH of 7.4 and a conductivity of 890 us/cm; pure water has a pH of 5.6 and a conductivity of zero. After being prepared in a ratio of pure water to tap water of 3:1, the water has a pH of 7.02 and a conductivity of 270 us/cm.
Tenth, test analysis
As can be seen from the test chart, both A and B dampening solutions are "buffered" dampening solutions. Not suitable for automatic machine addition, manual concentration of dampening solution should be used.
A fountain solution PH value: 4.5-5.5; concentration: 4% -0.6%; conductivity: 1500-400us / cm.
B fountain solution PH value: 4.5-5.5; concentration: 5.2% -0.5%; conductivity: 2500-500us / cm.
The two fountain solution additives have different conductivity at the same water quality and the same ratio.
The recommended ratio of the fountain solution supplier is: the standard concentration of A dampening solution is 2%-2.5%, the pH value is 4.5-5.5; the standard concentration of B dampening solution is 2% (soft water), 3% (hard water), The pH is 4.9-5.2.
According to the test results, the ratio of the A fountain solution is 2%, the pH is 4.7, and the conductivity is 820 us/cm. B dampening solution has a concentration of 2%, a pH of 4.85, and a conductivity of 1382 us/cm.
Since it is necessary to add about 10% of isopropanol, when the ratio is actually added, the amount of dampening solution added should be calculated by adding the total volume of isopropanol. Otherwise, the concentration of dampening solution will be insufficient and the pH will increase. , affecting the wetting effect.
Through the actual use verification in production, from automatic addition to manual addition, dampening solution concentration control is accurate, wettability is good, and the printing failure caused by dampening solution is effectively reduced.