The reason of offset printing dot increase and its solution

In offset printing, the dots are the basic elements that make up an image. Changes in dots will inevitably affect the tone and color of the image, and thus affect the sharpness of the image. Therefore, the key to the quality control of printed matter is to control the printing quality of printed matter on the Internet.

Dot enlargement refers to the fact that the effective area of ​​the web access point of the substrate is greater than the effective area of ​​the web access point of the screen. This is a normal process phenomenon. Both the prepress production and the printing process are inevitable. However, it must be controlled within a certain range. Once it exceeds a certain range, the quality of the printed matter will be severely degraded (level loss, density, color distortion, etc.), and this network point increase is abnormal. GB7705-D7 stipulates that, for 50% of the outlets, the value of the outlets for fine products should not exceed 12%, and the value of outlets for general products should not exceed 18:6. In the prepress production and printing process, it is generally possible to compensate for the normal dot gains (such as the double reflection effect of the laser photolithography, the paper, and the dot gain generated under reasonable printing pressure), but abnormal dots There is often no regularity to increase, and reasonable compensation cannot be made in the prepress process, and can only be controlled during the printing process.

This article will analyze some of the abnormal causes of the abnormal increase in the printing process in different processes and solutions.

Stencil factor

The cause of the dot gain during the printing process.

1. The exposure is too small or the exposure time is too short, making the exposure incomplete.

2. Developer develops developer fatigue or developer pH is too low. Especially in manual development, it is most likely to occur because the developer is exposed to air and reacts easily with carbon dioxide in the air.

3. The development time is short (the area on the plate is not exposed; the medium is washed completely), or the development temperature is too low, making the development process slow. It has been proved that the development speed is reduced by 1 to 2 times for each decrease of 5.c in the development temperature.

A reasonable amount of exposure, development temperature, and development time during the printing process can effectively control the dot gain. In addition, when the development temperature is too low, it is appropriate to lengthen the development time. It is recommended that after each batch of Ps plates enter the factory, they must be printed with an ash step to test the ideal exposure and exposure time so as to facilitate the standardized management of printing plates.

Printing factors

1. During the printing process, if the ink supply of the printing plate is too large, excess ink will spread out to the outside of the dots, so the amount of ink must be controlled strictly according to the printing sample.

2. Under the effect of printing pressure, the ink will spread outwards. If the printing pressure is too large, dot gain will occur. Therefore, the printing pressure should be strictly controlled. For papers with high surface smoothness (coating paper), the compression should be controlled between 0.10-0.15μm, while the paper with low surface smoothness (offset paper) should be controlled to have a compression of about 0.2μm. The principle of control is to use as little printing pressure as possible on the basis of good ink transfer.

3. Due to the elastic deformation of the blanket, relative slip occurs between the printing plate and the blanket, between the blanket and the substrate, causing dot gain. Therefore, when adjusting the printing pressure and changing the center distance of the roller, the surface line speed of the roller should be as uniform as possible. But complete agreement is impossible because the surface speed of the blanket cylinder itself is not uniform due to the compression of the blanket.

4. The tightness of the blanket is not enough or the compressibility is not good. After the new blanket had been printed on the machine for the first time for five or six thousand sheets, the blanket was once stretched. After printing one or twenty thousand sheets, the blanket was again stretched.

5. The stencil pressure on the printing roller is too large, which causes the ink to spread on the printing plate when it is transferred from the ink roller to the printing plate. Solution: Adjust the pressure on the plate roller.

6 equipment wear caused by the increase of network points, such as gear wear caused by the increase in dot, the general performance of the ink bar, ghosting. Solution: Generally, the tooth spacing can be properly reduced without tipping.

7. The high emulsification value of the ink makes it impossible for the ink film on the dots to maintain a relatively clear boundary with the blank portion of the paper, resulting in an increase in the dots. Solution: Reduce the amount of fountain solution or wetting powder.

8. The small amount of water in the printing plate makes it impossible for the water film in the blank portion to effectively resist the ink layer of the dot. Solution: Adjust the pressure of the water roller, so that it can transfer water evenly and stably, or increase the amount of fountain solution or fountain solution. However, if the amount of the fountain solution is too large or too small, the dot gain may be increased. Therefore, the amount of the fountain solution must be reasonably controlled in the printing process, and the pH of the fountain solution should be within a reasonable range. Between 5 and 6 In the actual printing, it is necessary to properly adjust the actual conditions, such as the use of desiccant in the ink, can properly reduce the pH of the fountain solution.

Ink factor

When the ink cohesion (viscosity) is too low, the ink will rebound and it will be weak, causing the phenomenon of the printed dot network to increase and the ink spreading. In the ink transfer process, when the ink film is pulled into a wire and broken, and the tensile stress disappears, it mainly relies on the stickiness and elasticity to return to the state before stretching. In the viscous recovery process, because the film has no internal stress, it can only rely on gravity. As a result, the film layer is uneven, the edges are spread, the dots are not smooth, and there are dot gains. This is a very undesirable phenomenon. Recovery status. The recovery of elasticity depends on the recovery of the internal energy of the film. Because the internal stress of the film builds up strong internal stress, the ink is pulled back by the elastic force and shrinks.

When the cohesion of the ink is too low, the elastic recovery ability will be weakened, and can only rely more on viscous recovery, which will cause the dot gain. Therefore, various additives should be used as little as possible during printing to avoid reducing the cohesion of the ink. If the ink itself is a problem, some ink improver or No. 0 varnish can be added as appropriate.