Reasons and Control of Offset Printing Dots Expanding (I)

Dots are the basic unit of ink adhesion and play a role in transferring and organizing colors. Dot enlargement refers to the dot gain on dots printed on the substrate relative to the dots on the color separation sheet.

First, the formation of outlet expansion

1. According to the printing outlets change classification:

1 Normal expansion. This kind of printed dot enlargement is allowed, which means that the dots are expanding normally, with high density in the center of the dots and low density of edge locations. 2 ghosting outlets. This kind of printing dot enlargement is not allowed. If there are dot ghosts during printing, the dot color effect will be seriously affected. 3 There are no regular network expansions. When there are no regular dot gains, there is no rule in the printed dots, and the dots will expand around. At this time, stencils and dirty prints will appear, which will affect the quality of printed products. The network expansion scope does not exceed 15%, 12%, and 10%.

2. According to the status of the expanded network:

1 Linear enlargement means that the edge of the printing dot has a direction of expansion. Originally, the printing dot is a square, a circle, a prism, and a chain. The linear expansion means that the dot expands linearly toward a certain aspect. 2 The printed dots are expanded in all directions. The enlargement of the dot area is generally proportional to the length of the edge of the dot. In other words, the dot area is large, and the length of the edge of the dot also increases at this time. 3 The increase in density is proportional to the area of ​​the printing dot, the density increases, the ink color is large, the ink color is large, and the dot area is large.

Second, there are two kinds of mechanical and optical production

1. Mechanical dot enlargement: The ink attached to the metal plate is deformed and expanded by the extrusion pressure.

2. Optical dot enlargement: Due to light reflection, when the light reaches the surface of the dot, surface reflection occurs, and the ink penetrates into the edge of the dot in the paper, forming a diffusion vignette. This kind of reflection vignette forms a tone. The effect is the same as the mechanical dot enlargement during printing.

These two types of dot-extensions have a detrimental effect on printed materials to varying degrees and destroy the balance of the picture. In order to make a true reduction of the outlets in the printed matter, it is necessary to control the delivery process of the outlets (ie, printing plates—blankets—substrates).

Third, the factors that affect the expansion of outlets

1. Ink transfer affects the network expansion

The transfer of ink is done by ink rollers. The ink is transferred from the ink fountain through the ink transfer roller to the uniform ink roller. Under the shearing action of the uniform ink roller, the thixotropic phenomenon occurs, the structure is fully destroyed, and the film is extended into a uniform film, which is transferred to the surface of the printing plate through the ink roller. The distribution of the ink film on the ink roller has a strict rule, and only the ink system can ensure its high-precision ink supply, so that the ink can be continuously and evenly transferred to the printing plate. The ink is squeezed between the printing plate and the blanket, and then squeezed between the blanket and the paper. The ink layer formed after two extrusions is usually very thin. If the ink layer is thicker, the problem of dot gain will occur. At the same time, the hardness of the rubber roller must meet the requirements, the surface of the rubber roller must not have the vitrification phenomenon, and it must have good ink transmission performance, so that the dots of the printing plate have enough ink absorption.

2. The relationship between blankets and dot gain

In offset printing, blankets are used to transfer ink dots, so it is also called offset printing. Under the influence of printing pressure, the ink will spread around the dot. At the same time, due to the elastic deformation of the blanket, relative slippage between the printing plate and the blanket and between the blanket and the paper is caused. As a result of the above-described expansion and slippage, it is inevitable that a dot expansion phenomenon occurs. 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. Blanket is divided into air blanket and ordinary blanket. The air-cushional blanket has excellent printability on the structure, and it has a large printing capacity and a small amount of deformation. When the same compression amount is produced, the pressure applied by the air-cushion blanket is smaller than that of the ordinary blanket roller, so the dot gain is also small. If a common blanket is used, a rigid lining is used because of the large elastic modulus of the hard lining, the smaller amount of compression deformation and the width of the embossed area, and the better dot reproducibility. With good printing blankets and correct liners, care must be taken to keep the blanket surface clean. Preventing the aging of the surface of the blanket by the aging of the conjunctiva and the oxidized conjunctiva to smoothen the surface of the blanket and affect the lipophilic and hydrophobic properties of the blanket surface. In the printing process, the surface of the blanket is covered by paper fluff, paper powder, residual particles in the ink, adherence of fountain solution, dusting, and friction during high-speed operation, greatly reducing the spread of the blanket. The performance of ink, resulting in blurring of dots and floating on the ground, directly affects the quality of the printed matter. Therefore, it must be cleaned frequently to keep the surface of the blanket clean and to exert its best effect.

3. The relationship between printing pressure and dot gain

The reduction of outlets needs to use the effect of printing pressure. Printing pressure directly affects the transfer of ink and it is very important to properly adjust the working pressure of the printing press. When the printing pressure is too small, the printing surfaces cannot fully contact each other. The molecular force between the ink and the paper is very small. Only a small amount of ink may be transferred to the surface of the paper. The ink's conversion rate is very low, and the printed product is light in ink. , And there is a "emptiness phenomenon", and even graphic incomplete. If the printing pressure is too high, the ink will be squeezed into the blank space outside the graphic. On the one hand, the dots will enlarge and the image will be blurred; on the other hand, the ink transfer will also show a downward trend, causing the prints to appear shaded. Clear, part of the site and outlets have no grammatical reproduction of the original color. Unstable printing pressure, excessive ink transfer, and sometimes insufficient printing reproduction and color reproduction can not achieve the desired requirements, only in the appropriate range of printing pressure to get high-quality print. Choosing the right printing pressure becomes a very important part of the printing process. The determination of printing pressure, in addition to the structural performance of the machine itself, is also related to the ink, paper, dampening fluid, blanket, ink roller, printing speed and other factors used in the printing process, because these small changes in printing conditions will be Hue affects. Therefore, in the printing process, it must be adjusted according to the actual printing conditions to get the ideal printing pressure.

4. The relationship between paper and outlet expansion

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 from place to place. The quality problems that can be solved by this difference can only be achieved by adjusting the printing pressure.

5. Relationship between printing speed and dot gain

The change in printing speed directly affects the quality of the product. When the printing pressure is set at certain data, the printing speed is slowed down, the contact time between the printing surfaces becomes longer, the contact of the printing surface is sufficient, the transfer rate of the ink is high, the ink absorption of the dots is full, and the ink of the image is bright. When the printing speed increases, the contact time between the printing surfaces becomes shorter, the printing surface is not sufficiently contacted, the ink transfer rate is low, the ink absorption of the dots is insufficient, and the image is white. If the printing speed is not stable in a batch of printed products, it will cause inconsistencies in the ink before and after.

(to be continued)