The Influence of Network Control on Print Quality (I)

In the printing process, the colors and levels of the image are mainly represented by dots. The dots are the basis for print reproduction and color reproduction. It is the task of transferring the color and gradation of the original. It is the smallest unit of ink absorption in offset printing. A true reflection of the tone, hierarchy, and color reproduction of prints. The changes in outlets will cause changes in the clarity and subtle levels of printed matter, changes in color, and contrast, ink hue, ink density, and ink overprinting. For example, densely printed large dots will form dark areas, and large dots will form high light areas. Therefore, dot gain can greatly affect the reproduction of these regional meshes and color photographs, and has a great influence on midtones. Therefore, the quality of outlets will directly determine the quality of prints. When evaluating the quality of color prints, the copy quality of the dot is the most important indicator. For example, even ink, no dot distortion, no ghosting on the printed matter, and ink sticks are all related to the control of the dot.

It is because of the above reasons that the dot control in the printing process is extremely important. Next, we will discuss some of the factors affecting the outlets.

1 Network distortion analysis

The processing of the dots from the image processing stage to the printing process is completed through light, film, plate, printer (mainly plate and blanket), ink, and substrate, but due to equipment and technology, etc. For many intrinsic reasons, the nature of the transmission itself determines that changes in the network are inevitable. In addition to these materials and equipment, they have many uncertainties. This results in the network not being accurately copied and transmitted at each step, that is to say, It is impossible to completely avoid the changes of outlets during these transmissions (deformation of outlets, enlargement of outlets, shrinkage, etc.). In the process of copying color prints, what is needed is to control the change of outlets within a reasonable and predictable range, and at the same time standardize the production process and reduce the fluctuation of the changing parameters in the production process.

The changes in outlets during the process of copying are mainly in the following situations;

1) Reduction of outlets: Compared with outlets with pre-carriers, the area of ​​outlets obtained after copying will be reduced, which will result in the loss of details of the printed materials.

2) Dot enlargement: Compared with the dot of the carrier of the previous process, the dot area obtained after the copying is increased, and the resulting replica becomes larger in density. The enlargement of the dot area will have a certain influence on the balance of grayscale and various colors of the printed matter. If it is excessively increased, it may result in stencil printing and the printed product may not be clear.

The dot gain can be further divided into mechanical dot gain and optical dot gain.

1 "Machine dot gain" (also known as physical dot gain) is the physical increase in dot size, and the most common form of mechanical dot gain is non-directional.

2 "optical dot gain" is a visual phenomenon formed by the light absorption characteristics of the ink and the light scattering properties of the substrate.

3) Deformation of dots: The shape of the dots obtained after copying has changed. For example, the dots of the original circle may become elliptical after copying, or dots with no fixed shape.

2Influencing factors of network changes and their control

There are many factors that can cause abnormal dot distortion in the process of copying a color print. Including the shape of the dot, the number of screens and the choice of paper, ink, blanket, printing plate, wetting fluid and other raw materials. As well as the working status of the printing press, strengthening the control of printing dot changes is an important guarantee for improving the printing quality.

1) Influence of dot shape

The correct choice of dot shape has a great influence on the surface effect of the print, and the nature of each dot shape is different. For example, a square dot has a large tempo jump at 50%, and the total dot gain of a square dot is also larger than that of other dot dots, so the color screening process of high-grade prints rich in midtones is performed. You cannot use squares, but consider diamonds or elliptical dots.

2) Influence of screen number

The higher the number of lines added to the screen, the richer the level of expression that can be displayed by the dot-prints will be, and the picture will appear more delicate. However, it is not necessarily the case that the higher the number of screen lines is, the better. In many cases blindly increasing the number of screen points will lead to a decrease in the copy quality of the network points. However, we have to rely on the comprehensive conditions of many factors such as printing paper, ink, and machinery. To weigh the number of outlets.

3) The effect of film on outlets

In general, the film's on-site density is greater than 3.5, preferably around 4.2; at the same time, the dot type of the dot is better, that is, the edge around the dot is required to be clear, and the boundary with the blank is clear. If the density in the field is too low, the small dots of the image will easily shrink or even be lost when the printing is in progress. Fine text will have broken lines and other phenomena. If the edges of the dots on the film are blurred, the dot on the PS plate will randomly change as the exposure and development conditions change, and the dot may increase or decrease.

4) Influence of printing equipment

The degree of vacuum in the printing equipment, the transparency of the printing glass, and the control of the printing time will all cause changes in the outlets. In the process of exposure of the PS plate, due to light infiltration and the like, the dots on the printing plate tend to be smaller than the dots on the film. In general, the amount of reduction should be controlled within 3% to 5%. Therefore, you should often use the print control bar to test the dot change in the print, and then feed the data back to the image processing unit before printing.

5) Effect of developer:

When the developer is fatigued, if the constant development time is still used, there will be no clear net edge, resulting in an increase in dot. The developer concentration is too large or the development time is too long. Even under normal exposure, the developer will excessively corrode the PS plate, which will dissolve or even completely lose the small dots of the graphic, and the printing plate is also resistant to printing. Will fall. In general, 5% or less of the outlets are easy to lose, and 40% of the outlets have smaller outlets. If the PS version of the body is overexposed, the concentration of the developer is too large and the development time is too long, which will also cause the dot to shrink.

6) Effect of fountain solution

1 In order to prevent the blank part of the printing plate from absorbing ink when printing, it is required to moisten the printing plate with dampening solution prior to ink application. The pH of the dampening solution during the printing process is not too low, about 5 to 6 is appropriate. The low PH value of the printing solution has a great influence on the transfer of the offset printing ink. At the same time, it will cause corrosion of the printing plate, reduce the printing rate of the printing plate, cause the loss of high-profile dots, or other printing problems; 2 the amount of water on the printing plate. The water thickness of the blank portion of the printing plate is generally 0.5 um, and the ink film thickness is 2 to 3 um. If the water on the printing plate is too large, it will erode the ink film in the graphic part, resulting in empty dots; if the ink film thickness is large, the ink will spread to the blank part under the pressure, resulting in an increase in dot .

7) Influence of friction during printing

During the high-speed operation of the printing press, the surface of the printing plate is rubbed by the water roller and the rubber roller. Friction wears the sand around the dot, and the dampening solution immerses in it, forming a hydrophilic, oil-repellent film. For the high-profile part of the printing plate graphic, the area coverage of its own dot is smaller, and the hydrophilic area is larger than the lipophilic area, which is conducive to the expansion of the fountain solution, and the formation of a hydrophilic and oil-repellent film due to abrasive wear. Preventing the normal spread of ink will inevitably cause the dot to shrink. In addition, friction in the high-speed operation of the machine will greatly reduce the ink transfer performance of the blanket, resulting in blurring of dots and virtual floating in the field, which directly affects the quality of the printed matter.

8) Effect of Blanket

Due to the elastic deformation of the blanket, under the effect of printing pressure, relative slip between the blanket and the blanket, between the blanket and the paper, the ink will spread around the dot, which will result in the expansion of the dot Phenomenon, this is an important factor that causes dot gain in lithography. In order to make the expansion value of outlets within the minimum range, make the density of the outlets even, and the periphery smooth, it is necessary to debug the correct printing pressure and use a printing blanket with good printability.

9) Influence of paper

The printability of the paper depends on the surface characteristics of the paper, such as the amount of ink absorbed and accepted. The faster the speed of absorption, the greater the degree of expansion of the printed network. The rate of ink absorption by the paper plays a key role in the extent of dot gain. The higher the paper smoothness, the smaller the dot gain value; the lower the paper smoothness, the larger the dot gain value. In printing paper, coated paper has lower absorptivity than non-coated paper, non-coated paper has high absorption performance, and as a result of high absorbency, the percentage of dot gain is high. Even with the same coated paper, the smoothness of the paper varies greatly depending on the place of production. Solving the quality problems brought about by this difference can be achieved only through the adjustment of the printing pressure. The ideal printing pressure is determined according to the actual printing conditions.