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
Figure 1 shows the viscosity curves of three kinds of inks in the condition of adding dampening solution containing 2% dampening solution. The viscosity curves of the three inks are basically the same. As the amount of dampening solution is slowly increased, the three kinds of inks are The viscosity gradually increases.
2. 5% dampening solution
Figure 2 shows the viscosity curves of three inks in the presence of a dampening solution containing 5% dampening solution. The viscosity of the three inks is lower than that of the 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
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