Gradually check the sample to find the cause of the ink stick
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In the offset printing process, the collision impact occurs when the drum enters the rolling surface from the neutral position, the inertia of the reciprocating and rotating motion mechanisms of the cam, the connecting rod, the drum, etc., the manufacturing or assembly error of the components, and the like, alone or in combination. It is possible that the offset printing machine cannot maintain the ideal working state, which causes the thickness of the ink layer in a local area of the printing to change, and finally the horizontal streaks of different ink colors, that is, the ink stick, appear on the surface of the printed product. The ink bar is a common printing defect in the offset printing process and is difficult to control. Therefore, equipment manufacturers and users are constantly exploring the causes and solutions. Below, the author takes the actual problem sample as an example, focusing on the causes and solutions of the ink bar, and hopes to provide help to the industry.
Verification fault and cause analysis
There are 3 ink sticks on the sample of the problem brought back from the customer, which are located at 70-110mm, 580mm and 660mm of the proof sheet (the circumference is from front to back). We retrieved the sample records of the relevant offset printing factory test, and did not find the "significant ink bar" record, and the three ink bars in the problem sample are not easy to detect if not carefully observed.
1. Verify the fault
Why is there no fault record in the factory test of the offset press, and there will be problems during the use of the customer? This also needs to further confirm the cause of the failure by increasing the difficulty of the test on the same batch of offset presses: First, 70% of the dots are used for the full-page printing test, and the sample is taken for observation; then, the full-scale printing test is further carried out in the field. Take the sample for observation.
The above test results show that three ink bars similar to the problem sample appeared in the test sample. The reason why this problem was not found in the factory test is because the new offset printing machine has higher precision and better debugging status. Even if there is a hidden danger of the ink stick, it cannot be immediately reflected.
2. Analysis of the reasons
There are many factors that cause the offset ink on the offset press, and the reasons are more complicated. Based on the experience accumulated over the years, the author concludes that the three ink bars in the problem sample can eliminate the cause of the offset gear, and focus on the impact of the impact, vibration and ink system stability of the printing machine. Then, each ink bar in the problem sample was tested.
Troubleshooting test and results
1.580mm ink bar
The ink bar at 580mm is more obvious and stable in position, and its length spans almost the entire sample (axial direction). From the position and appearance analysis of the ink stick, the author preliminarily speculated that there was a significant impact at the position during the printing process, and the specific impact source and mechanism of action still need to be further determined.
Therefore, we designed a test method: the wear-resistant printing plate was installed on the drum, and then a series of combined pressure, glue, and dry running tests were carried out. It was found that a polished bar was formed on the surface of the tested wear-resistant printing plate, and its position and width coincided with the ink bar at 580 mm. Through this test, the author concluded that the ink bar at 580 mm was caused by the impact of the plate at this position. This position corresponds exactly to the position where the impression cylinder and the blanket cylinder enter the rolling surface from neutral.
To eliminate the 580mm ink bar, it is necessary to eliminate or reduce the mutual impact of related parts, and can improve the notch structure, control the size of the shoulder iron, optimize the size and hardness of the lining, and improve the elasticity of the blanket. .
2.70～110mm ink bar
The ink bar is lighter at 70 to 110 mm, and the position is variable, and the length spans the entire sample (axial direction). From the position and appearance analysis of the ink stick, the author preliminarily speculated that this is also the ink stick generated by the impact force, and the impact force is small. The impact source in the offset printing machine work is smaller than the impact force caused by the roller entering the rolling surface from the neutral position. It should be the impact caused by the collision of the water roller and the ink roller into and out of the rolling surface. Therefore, the author infers 70~110mm. The "culprit" of the ink bar is here. In this regard, the author conducted relevant tests to verify the inference results.
After a period of normal printing, the drum is pressed and printed, the water roller and the ink roller are separated, and the printing is performed by using the residual ink on the printing plate and the blanket cylinder, and the sample is taken for observation. The results showed that the ink bar was missing at 70-110 mm, and this result confirmed the original speculation. Therefore, the author conducted further tests: respectively, unwinding the ink roller and the water roller for printing, observing the test sample, and finally confirming that the cause was on the water roller. The water rollers were removed one by one for investigation, and the water roller that produced the impact was finally found.
In order to eliminate the impact of this water roller, it can be improved from the aspects of improving the water roller support and controlling the water pressure.
3.660mm ink bar
The 660mm ink bar looks more complicated, the ink bar looks more than one, and one of them is the most obvious, and the ink bar
The length is not long, and the longest is no more than 1/3 of the sample layout. According to the previous analysis, the production of the ink bar is mainly due to the impact of shock or vibration, and combined with the performance of the ink bar on the problem sample, the author speculates that the ink bar in this area is caused by the instability of the water roller or the inking roller. In this regard, the author conducted a verification test.
First, after the offset printing machine was normally printed for a period of time, the printing was carried out by means of water and ink, and the sample was observed, and the ink stick on the test sample gradually disappeared. Then, the water roller was dropped for printing, and no similar ink stick was found. From this, it can be concluded that the ink bar at 660 mm is caused by the unstable movement of the inking roller group working close to the plate cylinder.
For the 660mm ink bar, it should be removed from the aspects of controlling the elasticity of the ink roller, reducing the pressure of the plate, and improving and adjusting the stability of the ink roller.
Through careful investigation, find out the "lesion", and the drug is removed, the ink bar removal effect is very significant, the customer starts printing again, the ink stick problem is basically eliminated. In general, to eliminate the ink bar, it is necessary to analyze the mechanism of its production, check one by one, and find the source of the fault so that the problem of the ink stick can be completely eliminated.