Talking about Overprint Improper and Solving Method in Actual Production

Imprinting is not one of the most common failures in sheetfed offset printing. It directly affects the printing quality and production efficiency. It also has a certain degree of influence on the company’s image. Therefore, in practical work, how to effectively eliminate it becomes a The required technology of each machine worker. Here are some of the methods and experiences I have summarized in my work practice. I hope that my colleagues will criticize and correct me!

Prepress section

1. When the film is output, if overprinting of the four-color film is not allowed, it must be re-produced.

2. When making printing plates, it must be ensured that the perforation is correct; during printing, it should be carried out in accordance with normal operating procedures to prevent sun drying and sun drying; under the appropriate time and temperature conditions, the baking plates are produced according to the technological requirements.

3. When cutting paper, ensure that the paper is flat, straight, and clean.

Printing section

1. When using a monochrome machine for multi-color printing, the paper is brought into contact with the fountain solution before printing, which increases the water content inside the paper. When the color is printed, there is a certain time difference and the paper is deformed to a different degree. One-color images and overcolor images are not overprinted. Therefore, the printing factory usually controls the temperature and humidity of the paper storage warehouse and printing shop. In order to prevent overprinting, some large companies also perform temperature and humidity control on high-precision semi-finished products.

2. Since a multi-color printer performs printing of a plurality of colors instantaneously, there is basically no problem of deformation of the paper caused by the time difference of the monochrome machine. However, due to the action of the fountain solution and a certain printing pressure, the paper will also have a constant deformation. This deformation is usually overcome by increasing the 0.02mm increment from the first color to the last color when the film is produced. Therefore, once the printing color sequence is determined, do not change it at will.

Although multicolor printing presses have a lower probability of overprinting than monochrome printing presses, multicolor overprinting is generally used for high-end exquisite packaging products. Therefore, high precision is required for overprinting, and overprinting in multicolor printing machines is not The quasi-exclusion is also much more complex than monochromatic machines, which should be eliminated one by one according to different situations. In addition, there is still a problem of misalignment in the multicolor printing machine, which is generally caused by mechanical reasons. According to the requirements of the printing products, they should be thoroughly considered and solved one by one.

Operation section

1. If the amount of fountain solution is too much, it is very unfavorable for printing overprinting, so the amount of fountain solution should be as little as possible without affecting the smooth progress of printing.

2. The paper did not come into place, causing the needle to jump, causing overprinting and ghosting.

3. Aging blankets and loss of elasticity can cause overprinting.

4. Loose blankets or plates can cause misprints. Among them, when the plate is loose, there are also phenomena such as ink bars and water bars.

5. Improper misregistration of plate linings, blanket linings, or folds or defects in the lining.

6. When the printing plate is installed, if the plate is mounted too obliquely, the amplitude of the plate will be too large, which may cause overprinting. At this time, the printing plate needs to be replaced.

7. The printing pressure is the most critical factor in printing. If the printing pressure is not adjusted properly, the most important effect is the transfer of ink and the printing effect. At the same time, there is a certain influence on overprinting.

8. The difference in line speed between the surface of the printing plate and the surface of the blanket is too large, resulting in a certain slippage of the two surfaces within the embossing arc, thus affecting overprinting. By increasing or decreasing the blanket lining and the lining of the printing plate, the longitudinal overprint due to the deformation of the paper can be solved, but this method is not conducive to the transfer of the ink.

9. In the maintenance of the machine, always check the cleaning to prevent the paper powder and paper from entering, and plug the front gauge and side gauge. Correctly adjusting the height of the front gauge and the lateral gauge can prevent overprinting.

10. If there is too much oil and residual ink between the three pillows, overprinting will not be allowed. Therefore, the three-roller should be cleaned frequently.

Mechanical part

1. There is a close connection between the pressures of paper-passing, paper-handling, impression cylinder cavities, and tooth row. The pressure requirements of the cavities are uniform and even. If a single or a few cavities are loose, partial image overprinting will be inaccurate. If the entire tooth row is loose, it will cause the entire image to be overprinted. Therefore, it is necessary to prevent the cavities from springing. Aging screw and loosening.

2. Another mechanical reason that causes overprinting is that it is the plate cylinder gear, the impression cylinder gear, the transfer tooth row axis, the bearing, the transfer bar row axis, the bearing, the impression roller row axis, the bearing and others. Damaged paper-related mechanical parts, excessive oil or excessive clearance.

In short, there are many unstable factors in the offset printing process, such as equipment factors, environmental factors, and human factors. Therefore, when overprinting is found to be inaccurate, it should be based on the actual situation, specific analysis, and one by one.

Source: Printing Technology