Increased dot in offset printing
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Offset printing uses the dot as the basic element of the image, and the change of the dot will inevitably affect the tone and color change of the image on the printed matter. The change of the dot image will inevitably lead to the decrease of the image sharpness.
It can also be said that controlling the quality of printed matter is actually controlling the printing quality of the printed matter on the Internet.
The so-called dot gain is that the dot printed on the substrate is larger than the effective area of the dot on the screen. This is a normal process phenomenon, which is inevitable in the process of plate making or printing.
However, this increase should be controlled within a certain limit. Once the range exceeds one, the quality of the printed matter will be seriously degraded (gradation loss, density increase, color change), and this increase is abnormal. (GB7705-87 stipulates that the 50% dot gain value requires fine products to be no more than 12%, and general products should be no more than 18%)
In the pre-press production and printing process, we generally compensate for the normal dot gain (such as laser photorangement, double reflection effect of paper, and dot gain under reasonable printing pressure in printing), but abnormal. There is often no regular increase in the dot size, so that reasonable compensation cannot be made in the pre-press production process. We can only control it during the printing process.
Next, we will do some analysis on the causes of these abnormal dot gains.
The increase in dot size in printing mainly includes the following aspects:
First, caused by the process of printing (yang)
1. The exposure of the printing plate is too small or the exposure time is too short. (exposure is not complete)
2. The developing solution is fatigued during development or the pH of the developing solution is too low. (especially the most likely to occur in manual development, because the developing solution is exposed to the air, it is easy to reflect with carbon dioxide in the air)
3. The development time is short (the unexposed portion of the plate is not completely rinsed) or the development temperature is too low (the development is slow, and the experiment proves that the temperature is reduced by 1-2 times for every five degrees of temperature decrease).
· Use reasonable exposure, development temperature, and development time during the printing process. Such dot gain can be well controlled. In addition, when the development temperature is too low, the development time should be appropriately extended. <It is recommended to apply the scales to the printing plate after each batch of PS plates, and experiment with the ideal exposure and exposure time to facilitate the standardized management of the printing plate>
Second, leading to the printing process
1. The supply of the printing plate is too large during printing, so that the excess ink is expanded outside the dot.
· Control the amount of ink in strict accordance with the print.
2. Under the effect of printing pressure, the ink will spread around. When the printing pressure is too high, there will be an increase in the dot size that we do not want to see.
· Strictly control the printing pressure in printing. The surface smoothness is high (such as coated paper). The paper control rubber compression is 0.10mm~0.15mm. The paper with low surface smoothness (such as offset paper) controls the rubber compression at 0.2. Mm or so. The principle of controlling the printing pressure is to use as little printing pressure as possible on the basis of ensuring a good transfer of the ink.
3. Due to the elastic deformation of the blanket, relative slippage occurs between the printing plate and the blanket, between the blanket and the substrate, and the result of the slip causes the dot to increase.
• When adjusting the printing pressure and changing the center distance of the drum, try to ensure that the surface speed of the drum is consistent (completely guaranteed is impossible, because the line speed of the rubber surface is inconsistent due to compression).
4. The tightness of the blanket is not enough, or the compressibility of the blanket is not good.
· After the first time the new blanket is tightened, it will be tightened again after printing five or six thousand sheets, and then it will be tightened once when printing one or two thousand sheets.
5. The pressure of the plate by the plate roller is too large, which causes the ink to spread on the plate as it passes from the ink roller to the plate.
· Adjust the pressure of the ink on the plate.
6. Due to the wear of machinery and equipment such as gears, local dot enlargement may occur, and such dot gains generally appear as "ink bars" and sometimes even "ghost images."
· It is possible to appropriately reduce the tooth pitch without the top teeth.
7. The emulsified value of the ink in the printing is too high, so that the ink film on the dot cannot maintain a relatively clear boundary with the paper at the blank, resulting in an enlargement of the dot.
· In this case, it is necessary to reduce the amount of the liquid stock or the wetting powder. This situation is usually caused by the low cohesion of the ink. Sometimes we have to deal with it from this aspect, we will discuss it in detail later.
8. The printing plate has a small amount of water, so that the water film in the blank portion 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 dampening solution, or increase the amount of dampening solution.
It can be seen from the above two points that the amount of dampening solution is too large or too small may become a factor of dot gain, so we must control the amount of dampening solution in the printing process and ensure that the pH of the dampening solution is at Within a reasonable range (under normal conditions, we require the pH of the dampening solution to be between 5 and 6. In actual printing, sometimes according to the actual conditions, such as the use of desiccant in the ink, the dampening can be appropriately reduced. The pH of the liquid > make appropriate adjustments.)
Third, the ink cohesion (adhesion) is too low, the rebound of the ink is weak, which will cause the printing dot to increase and spread.
During the ink transfer process, when the ink film is pulled into a wire and broken, the tensile stress disappears, mainly relying on the recovery of elasticity and the recovery of elasticity to the state before stretching. During the viscous recovery process, the film has no internal stress and relies on gravity to recover. As a result, the film is uneven, the edges are diffused, the dots are not clean and enlarged, and it is a very unsatisfactory recovery; the elastic recovery depends on the film. The inner layer of the layer acts to recover, and the ink is pulled back by the elastic force to shrink due to the accumulation of strong internal stress inside the film layer.
When the cohesion of the ink is too low, the elastic recovery will be weakened, and more dependence on the viscous recovery will lead to an increase in the dot size.
· It is recommended to use as few ink additives as possible in printing, such as debonding agent, No. 6 varnish, thinner and desiccant to reduce the cohesion of the ink. If the problem of the ink itself can be added to some ink improver or No. 0 varnish.