Offset Quality Control - Use of Fountain Solution (2)

Third, fountain solution additives formula principles and requirements

The formula of the PS version of the wetting fluid must follow the good wetting properties, the emulsification of the ink can not be too large, the pH of the solution is appropriate. In particular, the relationship between the wetting property and the emulsification value must be solved.

1, to fully wet the layout, you need to reduce the surface tension, to add emulsifier in the fountain solution, wetting agent, surfactant.
2, with the ability to clean, can wash away dirty spots. Add acid to clean the plate.
3. Stabilize the pH of the wetting fluid and add the buffer. Reduce ink emulsification.
4. It has good non-lipid capacity, ie it is not lipophilic, such as adding gum arabic. Avoid parts of the graphic text without ink.
5, can adapt to different climate changes, add moisture absorbent. Prevent dirt on the plate.
6, can inhibit the growth of bacteria and mold to prevent the wetting fluid corruption.

Fourth, the application of conductivity and PH value in printing

The correct ink balance largely depends on accurately controlling the pH and concentration of the fountain solution. When many printers use the fountain solution, they only pay attention to the pH of the fountain solution to be controlled within the required range (pH 4.5 to 5.4), ignoring the concentration of the fountain solution.

At present, the majority of fountain solutions sold on the market are buffered fountain solutions. When the concentration of the fountain solution changes greatly, the pH value does not change significantly. However, changes in the concentration of the fountain solution will result in the occurrence of various components in the fountain solution. Change, resulting in changes in surface tension, ink emulsification properties, etc., and ultimately affect the dampening effect, resulting in decreased quality of printed products. Therefore, controlling only the pH value will lead to the occurrence of a printing failure.

Through experiments, there is a linear relationship between the concentration change of the fountain solution and its conductivity change, ie, the greater the concentration, the greater the conductivity. In this way, we can control the concentration of the fountain solution by measuring the conductivity.

V. Measurement of Electrical Conductivity and PH

The pH meter measures the pH of the fountain solution on the press. The electrode of the pH meter is enclosed in a glass tube and 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 in the pH meter to determine its value. 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 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 replacing test heads, detecting corrosive chemical samples, and when precise measurements are required, be sure to re-calibrate them. Even if they are used properly, they must be calibrated at least once a month.

The pH meter immerses the test head in a calibration solution of known concentration, and then adjusts the 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 it is better not to check if the calibration fluid used does not match the calibration value given by the pH meter. For example, the set points are 4, 7 and 10 in some pH meters. If you use the buffer values ​​4, 7, and 9.2 to verify the pH meter, an error occurs at the last control point (10 here). Because the fountain solution is usually acidic, its value is close to 4.5 to 5.5, so only 4 or 7 points can be corrected.

The calibration of the conductivity meter is similar to that of the PH meter, but the matching calibration fluid (eg 1413us/cm, 1382us/cm, etc.) must be selected according to the model, inserted into the calibration fluid, and the displayed value is adjusted to match the actual value of the calibration fluid.

Six, determine the fountain solution conductivity

For the same fountain solution, using different places of water to prepare, even with the same concentration of fountain solution, their conductivity is not the same, because it is related to the hardness and hardness of water (Ca2+, Mg2+ in the water) . If the same kind of water and different brands of fountain solution mixed to make the same concentration of fountain solution, the conductivity is not the same, the conductivity is large, indicating that the solution of more ions, some ions have an impact on the printing, If the concentration of Ca2+ and Mg2+ in the fountain solution is too high, the ink will be emulsified and the imprinting will be hindered from drying out. Therefore, the water needs to be softened before the fountain solution is prepared.

Based on the above factors, before formulating the dampening 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. The range, and then within this range, a certain conductivity value was found by the quality of the printed matter as a standard value for future use of this fountain solution to formulate the fountain solution. Different water quality, different dampening potions, and different conductivity values ​​to achieve the best wetting effect are not the same. Therefore, it can be tested according to the above method to achieve the best wetting effect.

(to be continued)