Plate paste problem and solution
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Offset printing is printed using the principle of water and oil repelling. The dampening solution wets the blank portion of the printing plate, and the repelling ink adheres to the non-image portion, so that the printing can be performed normally. In addition to the dampening solution, the dampening solution is also mixed with a certain proportion of isopropyl alcohol (IPA). On the one hand, IPA can reduce the surface tension of the dampening solution, so that it can spread better in the blank part. It also increases the viscosity of the dampening solution, allowing the dampening solution to pass through the water roller better. The amount of the dampening solution itself will affect the surface tension of the dampening solution. In actual printing production, if the dampening solution is not properly added, it will cause printing failure. The author takes an example of a customer's failure during the printing process due to improper addition of dampening solution.
Fault phenomenon
The customer uses three kinds of inks for printing. Among them, when the problem ink is turned on and printed to about 300 sheets, there is a phenomenon that the blank portion is dirty and part of the 50% dot paste is formed, and the method of increasing the amount of water cannot solve the problem.
Failure analysis
The author tested the viscosity and emulsification rate of the two inks (sample 1 and sample 2) and the problem ink (sample 3) reflected by the customer in the laboratory, and found that the viscosity of sample 3 was slightly higher than that of sample 1 and sample 2. It is low, and after emulsifying 2% of the dampening solution in these three kinds of inks, the emulsifying rate is substantially the same.
In order to analyze whether the fault generated by sample 3 is related to the amount of dampening solution added, the author analyzed and compared the three inks under the condition of adding different dampening solution, each ink was selected 30g, and the constant rate was 3000 rpm. After stirring for 1 minute, respectively, 30 g of dampening solution was uniformly added in 10 minutes. By measuring the torque, the curve of the viscosity of the ink with the increase of the amount of dampening solution is obtained, so as to analyze the ink balance point of different inks under different dampening solutions. In the experiment, the dampening solution selected by the author is a dampening solution containing 2% dampening solution, a dampening solution containing 5% dampening solution, a 5% dampening solution and a 10% IPA dampening solution, containing 5 % dampening solution and 20% IPA dampening solution, customer-supplied dampening solution.
1. Contains 2% dampening solution
The viscosity curves of the three inks in the condition of adding the dampening solution containing 2% dampening solution, the viscosity curves of the three inks are basically the same, and the viscosity of the three inks gradually increases as the amount of dampening solution increases slowly. .
2. 5% dampening solution
The viscosity curves of the three inks under the conditions of adding a 5% dampening solution to the dampening solution, the viscosity of the three inks was reduced under the condition of adding a dampening solution containing 2% dampening solution. The viscosity of the sample 3 began to stabilize from the dropwise addition amount of 7.5 g, and the amount of the dropwise addition fluctuated drastically at 25.5 g, whereby it was found that the ink balance point was at a drop amount of 7.5 g. Samples 1 and 2 had the same tendency as the conditions of the dampening solution containing 2% dampening solution, and the viscosity increased slowly. Note that sample 3 adds less dampening solution to achieve ink balance.
3. Contains 5% dampening solution and 10% IPA
The viscosity curves of the three inks under the condition of adding 5% dampening solution and 10% IPA dampening solution, it can be seen that the sample 3 has a decreasing trend when the dosage is about 13g. Here is the ink balance point; while the sample 1 and sample 2 have a decreasing tendency when the dosage is about 19g and 18g, which indicates that the ink balance points of the two inks are here. In addition, it can be seen from this experiment that the addition of IPA allows the ink to be added with less dampening solution to achieve the ink balance, and the dampening solution required for the sample 3 to reach the ink balance is less than the other two inks.
4. Contains 5% dampening solution and 20% IPA
The viscosity curves of the three inks in the condition of adding a 5% dampening solution and 20% IPA in the dampening solution, it can be seen that as the IPA increases, the viscosity of the ink further decreases, and the viscosity curve suddenly drops. Figure 3 is offset to the left, indicating that as the IPA increases, adding less dampening solution can bring the ink to ink balance.
5. Customer's own dampening solution
The viscosity curves of the three inks in the condition of adding the customer's own dampening solution, the viscosity curves of the three inks are similar to the viscosity curves of the inks, and the three kinds of inks require less dampening solution to achieve the ink balance. It can be inferred that the customer A higher proportion of dampening solution and IPA is added to the self-contained dampening solution.
Cause of issue
The author learned from the customer that the operator is more casual about the management of dampening solution. When the printing is easy to get dirty or the dampening solution is not spread well, add dampening solution or IPA to the sink. Although this can temporarily solve the problem of dirt, but because of the addition of more dampening solution and IPA, the surface tension of the dampening solution is greatly reduced, and the cohesive force of the ink is reduced after the dampening solution and the ink are mixed. Therefore, Sample 3 is prone to excessive emulsification under the printing conditions of the customer, resulting in a change in the state of the ink, which is prone to dirty and paste. Although Sample 1 and Sample 2 did not show major problems in printing, due to excessive addition of dampening solution and IPA, paste-type problems may occur after prolonged production.
Secondly, the customer has a four-color offset press and a four-color four-open press. The chance of a paste problem on the opposite press is small, and the four-print press is prone to paste problems. It is related to the ink consumption of the printing press. The four-open printing machine consumes less ink, and the contact time between the ink on the printing machine and the dampening solution is long, which tends to cause the viscosity of the ink to decrease and the degree of emulsification to be intensified.
Solution
In order to improve the printing quality and reduce the occurrence of printing failures, it is necessary to properly manage the amount of fountain solution added. The pH value of the dampening solution, the dampening solution and the amount of IPA must be controlled within a reasonable range. The pH should be controlled at 4.5~5.5, and the amount of dampening solution should be 1%~2%. IPA The amount added should be 8% to 15%.