Offset printing uses dots as the basic elements of an image, and changes in dots will inevitably affect the tone and color of the image on the printed matter. Changes in dot images will inevitably lead to a reduction in image clarity.
It can also be said that controlling the quality of the printed matter is actually controlling the printing quality of the printed matter's internet point.
The so-called dot increase means that the dots printed on the substrate are larger than the effective area of ​​the dots on the screen negative. This is a normal process phenomenon and is inevitable in both the plate-making and printing processes.
However, this increase should be controlled within a certain limit. Once it exceeds a range, there will be a serious decline in the quality of the printed matter (tone loss, density increase, color change). This increase is abnormal. (The 50% network expansion value specified in GB7705-87 requires that fine products should not exceed 12%, and general products should not exceed 18%)
In the pre-press production and printing process, we usually make certain compensation for the increase of normal dots (such as laser phototypesetting, double reflection effect of paper, and increase of dots under reasonable printing pressure during printing), but it is abnormal There is often no regularity in the increase of the dots, so that no reasonable compensation can be made in the pre-press production process, and we can only control it during the printing process.
Next, we do some analysis on the causes of these abnormal dot gains.
There are mainly the following aspects in the increase of dots in printing:
1. Caused by the printing process (sun exposure)
1. The exposure of the printing is too small or the exposure time is too short. (Incomplete exposure)
2. The developing solution is fatigued or the PH value of the developing solution is too low during development. (Especially in manual development, it is most likely to occur, because the developing solution is exposed to the air, it is easy to reflect with the carbon dioxide in the air)
3. The development time is short (the unexposed part of the printing plate is not rinsed completely) or the development temperature is too low (slow development, experiments show that every five degrees of temperature reduction, the development speed will be reduced by 1-2 times).
· Appropriate exposure, development temperature, and development time are used in the process of printing. This type of dot increase can be well controlled. In addition, when the development temperature is too low, the development time should be properly extended. ＜ It is recommended that every batch of PS plates should be printed with ladder scale after entering the factory, and the ideal exposure and exposure time should be experimented to facilitate the standardized management of printing plates ＞
Second, during the printing process
1. The supply of the printing plate to the printing plate is too large, so that the excess ink expands beyond the dot.
· Strictly control the amount of ink supply according to the print.
2. Under the effect of printing pressure, the ink will spread around. When the printing pressure is too large, the dots that we do not want to see will appear.
· The printing pressure should be strictly controlled during printing. The paper with high surface smoothness (such as coated paper) should control the rubber compression amount to 0.10? ~ 0.15 ?, and the paper with low surface smoothness (such as offset paper) should control the rubber compression amount to 0.2. ?about. The principle of controlling printing pressure is to use as little printing pressure as possible on the basis of ensuring good transfer of ink.
3. Due to the elastic deformation of the blanket, there will be relative slippage between the printing plate and the blanket, and between the blanket and the substrate. The result of the slippage will cause the dot to increase.
· When adjusting the printing pressure and changing the center distance of the cylinder, try to ensure that the surface linear speed of the cylinder is consistent (it is impossible to guarantee completely, because the linear speed of the rubber surface is inconsistent due to compression).
4. The tension of the blanket is not enough, or the compressibility of the blanket is not good.
· After the first tightening on the new blanket, it will be tightened again after printing 5,000 or 6,000 sheets, and then it will be tightened once when printing 10,000 or 20,000 sheets.
5. The excessive pressure of the plate against the plate ink roller will cause the ink to spread on the plate when it is transferred from the ink roller to the plate.
· Adjust the pressure of plate ink properly.
6. Due to the wear of machinery and equipment such as gears, it will cause the expansion of local dots. This type of dot increase is generally manifested as "ink bars" and sometimes even "ghosting".
· Tooth pitch can be reduced properly without top teeth.
7. The emulsification value of the ink in printing is too high, so that the ink film on the dots cannot maintain a relatively clear boundary line with the paper in the blank space, resulting in the expansion of the dots.
· If this happens, you should reduce the amount of liquid medicine or wetting powder appropriately. This situation is usually also caused by the low cohesion of the ink. Sometimes we have to deal with this aspect, and we will discuss it in detail later.
8. The amount of water in the printing plate is small, so that the water film in the blank part cannot effectively resist the expansion of the ink layer on the dot.
· Adjust the pressure of the plate water roller so that it can transfer water evenly and stably.
· Appropriately increase the amount of wetting fluid or the amount of the original wetting fluid.
From the above two points, it can be seen that the amount of fountain solution is too large or too small may become a factor of dot increase, so we must reasonably control the amount of fountain solution in the printing process and ensure that the PH value of the fountain solution is Within a reasonable range (under general conditions, we require the pH value of the fountain solution to be between 5 and 6, in actual printing, sometimes according to the actual conditions <such as using a desiccant in the ink, you can appropriately reduce the dampening PH value of the liquid> Make proper adjustment.)
3. The cohesion (adhesion) of the ink is too low, and the rebound of the ink filament is weak, which will cause the expansion of the printed dots.
During the ink transfer process, when the ink film is drawn into a wire and broken, the tensile stress disappears, mainly relying on the recovery of viscosity and elasticity to the state before stretching. In the process of viscous recovery, there is no internal stress in the film layer, and recovery is generated by gravity. The result is that the film layer is uneven, the edges are diffused, and the dots are not smooth and enlarged. The energy within the layer is recovered. Due to the accumulation of strong internal stress inside the film layer, the ink filament is pulled back by the elastic force and contracted.
When the cohesion of the ink is too low, the elastic recovery will be weakened, and more dependence on the recovery of viscosity will lead to the increase of the dot.
· It is recommended to use various additives of ink as little as possible in printing, such as debonding agent, No. 6 ink, thinner and desiccant, etc., in order to reduce the cohesion of the ink. If there is a problem with the ink itself, you can add some ink modifier or No. 0 ink.
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