Editor's note: The biggest distress for the operators of offset presses lies in the fact that a fault occurs during the printing process and the cause of the fault cannot be found. Often, the school bus has to be shut down for failures such as inaccurate overprinting, bumps, ghosting, paper crepe, inverted smooth hair, etc. It is common to spend two or three hours each time. Sometimes it takes several days to solve a failure, and the work hours are seriously lost . It is estimated that the time it takes for the offset printing operation to troubleshoot in daily production accounts for about 5 to 10% of the total work hours, which is more than double that of lead printing. This is detrimental to improving product quality, shortening the publishing cycle, and increasing economic benefits. For this reason, every operator of offset presses can make greater contributions to the factory and the country if they can strive to improve the technical level in troubleshooting.
The offset press has a complicated structure and high printing process requirements. In addition, the variable factors of paper and ink are large, so the operation technology should be improved. Generally, new workers learn to operate easily, but whenever a defect or malfunction occurs, it is more difficult to find out the cause and eliminate the malfunction in time.
There are many factors for offset printing faults, and sometimes there are dozens of possible causes for a fault. In order to avoid or avoid detours and avoid wasting unnecessary and effective working hours, the following methods can be used.
1. Analysis
After the offset printing fault occurs, there are many ways to find out the cause of the fault, and among the various methods, the analysis method is the fastest way to find the cause of the fault.
The so-called analysis method is based on the analysis of the time, location, condition and shape of the fault. List the subjective factors of these four aspects one by one, and then conduct a comprehensive analysis, and many failures can be solved. In actual production, some operators lack comprehensive analysis of these four aspects after encountering a fault, resulting in extended downtime and unnecessary loss of working hours.
An example is as follows: During the operation of an offset press, the operator found that the paper was bowed and deformed. The product was a magazine cover, with two colors on the reverse side and four colors on the front side. When printing on the reverse side, the paper is normal, and the reverse side is printed on the front side. When printing to the 25th order paper, it is found that the trailing edge of the paper has a 50mm long crepe phenomenon. Analysis, preliminary conclusions:
â‘ The fault was just generated, indicating that there are new factors leading to bowing.
â‘¡ The part is on the trailing edge of the paper, which may be a machine factor or a paper factor.
â‘¢ Judging from the length of the paper crepe is only 50mm, it is impossible to be caused by mechanical failure.
â‘£ From the analysis of the paper conditions that produce more than 50mm long bow crepe, it is generally possible under the condition of "lotus leaf edge", but intuitively the paper is very flat and it is impossible to produce bow crepe.
After this analysis, negating the machine factor and the paper factor, they focused on a "why has just occurred" problem, which made people immediately think of the possibility of the back imprint sticking on the impression cylinder. Because the two-color prints printed on the back are often accumulated on the surface of the impression cylinder because the imprint has not yet dried out, this ink accumulation is both irregular and thicker, which is equivalent to gradually increasing the rubber cylinder and the irregular The pressure between the impression cylinders causes the paper to be pressed on the uneven surface, which causes the trailing bow crepe failure. After checking the impression cylinder, it is determined that the ink is accumulated. This problem disappeared after printing after cleaning, and it took more than ten minutes before and after.
Of course, some faults are more complicated, and the analysis method alone cannot solve the problem, and the detection method and the test method must be used together.
Second, the detection method
The so-called detection method is to use tools and testing instruments such as magnifying glass, micro-card, iron absorption dial gauge, diameter gauge, vibration oscilloscope, density meter, etc. to observe the fault, inspect and measure the machine parts, and find out The cause of the failure comes.
The detection method is often used to find faults such as bars, ghosting, and empty dots.
The detection method requires much longer time to resolve the fault than the analysis method. Generally, before using the detection method, try to use the analysis method to analyze, and then focus on one or two points for the necessary detection to find the cause of the fault.
For example: the dots on the printed matter produce false hairs. In terms of the entire screen, the dots on the end of the transmission side are empty, but the dots on the control side are completely solid. How to solve the problem like this?
It can be known from the analysis that there are two main reasons for this type of failure: the first may be the ink transfer failure, and the layout image does not have enough ink; the second may be the pressure failure, the pressure regulator of a certain roller moves automatically , The pressure disappears.
The factors of ink transfer failure include: too much water at one end of the layout; the pressure of the inking roller at the layout is light; the pressure between the end of the inking roller and the inking roller is too small; the surface of the inking roller is aging and cracked.
Factors of pressure failure include: the pressure between the plate cylinder and the rubber cylinder disappears; the pressure between the rubber cylinder and the impression cylinder disappears, etc.
The detection method should be carried out first and then difficult, because the water content of the plate is too large, and it will be known under the light when the machine is running. This factor is first eliminated, and then the pressure between the ink roller and the plate and the ink roller is checked to make it Meet the requirement of consistent pressure at both ends. If the surface of the inking roller is aged or cracked, replace it with a new one. The ink roller part is normal and the fault is still not resolved. Then check the pressure between the rollers. You can use an electrician to use a fuse (No. 20 fuse) and a micrometer to measure the gap between the roller pillows. You do n’t need to remove the printing plate and blanket. If the clearance of the roller pillow exceeds the value specified in the manual, adjust the pressure regulator so that the gap at both ends of the roller pillow meets the requirements in the manual, and the fault can be eliminated.
3. Test method
The test method is commonly referred to as "try it out". Some inexperienced new workers and those who do not have comprehensive analytical skills often use this method. The test method takes a long time, is easy to take detours, and brings a certain loss to production. But once successful, the operator not only accumulated experience, but also improved the technology.
The test method is usually more effective for the use of new paper, ink, and plate materials. But sometimes the common faults that occur in the printing process are caused by the lack of experience of new workers, and they are also regarded as new problems for experimentation. In this way, simple problems are often complicated and a lot of man-hours are wasted.
For example, the overprint that is not in place at one end is not allowed, and it is not allowed to be around 2mm. The cause of this failure is either the paper is skewed and the end is not in place, or the hard paper with more than 80g / m2 is ejected from the front regulation. . However, it took a lot of time for some operators to correct the front gauge, teeth pressure, and handover relationship, and people were also very tired. As a result, the failure was still not ruled out. Therefore, it is recommended to use a negotiated method to find the cause of the failure after a failure occurs during the printing process. New workers must consult with middle-aged and elderly workers with higher skills in order to eliminate the failure quickly, which is conducive to improving product quality. I also learned the technique of troubleshooting and improved the operation level.
In short, for the various faults encountered in the production of offset printing, it is recommended to use analytical methods and detection methods to find out the cause of the fault. When encountering complex faults, it is necessary to comprehensively use the analysis method and the detection method, so as to eliminate various faults quickly and well, and achieve the purpose of improving product quality and increasing effective working hours.
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