Offset printing is based on dots as the basic element of the image, and the change of dots will inevitably affect the gradation and color change of the image on the print. The change of the dot image will inevitably lead to the decrease of the image clarity.
It can also be said that controlling the quality of printed matter is actually controlling the print quality of printed matter at the Internet site.
The so-called dot enlargement is the effective area of ​​the dot printed on the substrate is greater than that of the dot on the screen negative. This is a normal process phenomenon, and it is inevitable both in the process of plate making and printing.
However, this increase should be controlled within a certain limit. Once it exceeds a certain range, the quality of the printed matter will be seriously reduced (level loss, density, color change), and this increase is abnormal. (GB7705-87 stipulates that 50% outlet expansion value requires that fine products should not exceed 12%, and general products should not exceed 18%)
In the prepress production and printing process, we generally compensate for the normal dot gain (such as the double reflection effect caused by the laser photocopying, paper, and the printing dots under reasonable printing pressure), but they are not normal. The increase in the number of outlets is often not regular, so that reasonable compensation can not be made during the prepress production process. We can only control it during the printing process.

Next we analyze some of the causes of these abnormal dot gains.

The production of dot gains in printing mainly has the following aspects:

First, caused by the printing (sun drying) process

1. The exposure of the printing plate is too small or the exposure time is too short. (Incomplete exposure)

2. The development fluid is fatigued during development or the pH of the developer fluid is too low. (Especially in manual development, it is most likely to occur because the developing liquid is exposed to the air and it is easily reflected by the carbon dioxide in the air)

3. The development time is short (the unexposed part of the plate is not completely rinsed) or the development temperature is too low (to make the development slow, the experiment shows that for every five degrees lower temperature, the development speed will be reduced by 1-2 times).
• Using a reasonable amount of exposure, developing temperature, and development time during the printing process, these dot gains can be well controlled. In addition, when the development temperature is too low, it is appropriate to lengthen the development time. < It is recommended to use step size printing after each batch of PS version into the factory to experiment with the ideal exposure and exposure time, to facilitate the standardized management of printing>

Second, caused by the printing process

1. The supply of printing plates in printing is so large that the excess ink spreads out of the dots.

Strictly control the amount of ink according to the print sample.

2. Under the effect of printing pressure, the ink will spread everywhere, and when the printing pressure is too high, we will see dots that we do not want to see.

·Strictly control the printing pressure in printing, the surface smoothness is high (such as coating paper), the paper control rubber compression amount is 0.10mm～0.15mm, the paper with low surface smoothness (such as offset paper) controls rubber compression amount is 0.2 Mm or so. The principle of controlling the printing pressure is to use as small a printing pressure as possible on the basis of ensuring good ink transfer.

3. Due to the elastic deformation of the blanket, relative slippage occurs between the plate and the blanket, the blanket, and the substrate. As a result of the slippage, the dot gain increases.

Adjust the printing pressure and change the center distance of the roller as much as possible to ensure that the surface line speed of the roller is the same (complete assurance is impossible, because the linear velocity of the rubber surface is inconsistent due to the compression).

4. The tightness of the blanket is not enough, or the compressibility of the blanket is not good.

• After the first blanket tightening on the new blanket, it will be tightened again after 5,000 or 6,000 sheets are printed, and then it will be tightened once again when printing 20,000 sheets.

5. Excessive pressure on the *print roller will cause the ink to spread on the plate as it passes from the roller to the plate.

· Adjust the pressure of the * ink.

6. Due to the wear and tear of machinery and equipment, such as gears, it will lead to the expansion of local dots, such dots usually appear as "dark bars", and sometimes even "ghosting."

• It is possible to reduce the interdental distance properly without the need for top teeth.

7. The high emulsification value of the ink in the printing makes it impossible for the ink film on the dots to maintain a relatively clear boundary with the paper in the blank space, resulting in the enlargement of dots.

· Appropriately reduce the amount of liquid or wetting powder in this situation. This situation is usually caused by the low cohesion of the ink, and sometimes we have to deal with 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 * water roller so that it can transfer water evenly and stably.

· Appropriately increase the amount of wetting fluid, or increase the amount of wetting fluid.

From the above two points, it can be seen that the excessive use of the fountain solution may be a factor for dot gain. Therefore, we must reasonably control the amount of fountain solution in the printing process and ensure that the fountain solution has a PH value. A reasonable range (under normal conditions we require the fountain solution PH value between 5 to 6, in actual printing sometimes according to the actual conditions, such as the use of desiccant in the ink, can reduce the dampening properly The pH of the fluid > make the appropriate adjustments.)

Third, the ink cohesion (stickiness) is too low, the weakness of the ink rebound, will cause the printing product outlets to increase the spreading phenomenon.

In the ink transfer process, when the ink film is drawn and broken, the tensile stress disappears, and the viscosity recovery and elasticity return to the state before the stretching. In the viscous recovery process, the film layer has no internal stress and recovers due to gravity. As a result, the film layer is uneven, the edges are spread, and the mesh points are not smooth and increase. This is a very unsatisfactory recovery; the elastic recovery is based on * The inner layer of the film can function to recover. Due to the strong internal stress accumulated inside the film, the ink is pulled back by the elastic force to shrink.

When the cohesion of the ink is too low, the elastic recovery will be reduced, and more dependent on the viscosity recovery will lead to an increase in the dot.

· It is recommended to use as few ink additives as possible during printing, such as release agent, No. 6 varnish, thinner, and desiccant, to reduce the cohesion of the ink. If the ink itself can be properly added some ink improver or No. 0 varnish.