In order to truly reproduce the colors of the original and improve the printing quality, we must first understand the changes in outlets. In the following, we will understand how the printing outlets change in several ways:

Dots are the basic unit of ink adhesion and play a role in transferring tunes 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.

There are two kinds of mechanical and optical conditions for dot gain.

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

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

These two kinds of dot-extension situations have, to varying degrees, damage to the printed matter and destroy the balance of the picture. In order to make a true reduction of the outlets on the printed matter, it is necessary to control the delivery process of the outlets (that is, a printing plate, a blanket, and a substrate).

1, ink transfer affects the expansion of outlets

The transfer of the ink is done by the ink roller. 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 strict rules, and only the ink system ensures 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.

For the ink to be smoothly transferred from the ink roller system to the plate, the ink is also required to have good fluidity. When the offset ink is stirred or under mechanical action, the ink will become soft and the fluidity will increase. Stop stirring, rest for a period of time, and return to the original solid state, this phenomenon is called thixotropic ink. Therefore, to ensure that the ink can be transmitted well, we must constantly stir the ink fountain. Use the thixotropy of offset printing inks to increase the flow of the ink and ensure that the dots can achieve the desired ideal color.

2. The relationship between blankets and outlet expansion

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 network expansion

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 small, the printing surfaces cannot fully contact each other. The molecular force of 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 ink color of the printed product is light. , 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, the parts of the field and outlets cannot reproduce the colors of the originals.

The printing pressure is not stable, the ink transfer is excessive and sometimes insufficient. The tone reproduction and color reproduction of the print cannot meet the desired requirements, and high-quality prints can be obtained only within an appropriate printing pressure range. 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 outlets

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 if the same coated paper, because of the different origin, the smoothness of the paper is also very different. Solving the quality problems brought about by this difference can be achieved only through the adjustment of the printing pressure.

There are many types of coated paper for printing, such as glossy coated paper and matt coated paper. Even if the same glossy coated paper or matte coated paper, because of the different origin, the choice of materials used in the paper, the ratio, processing and manufacturing processes are different, the paper has the printing performance is also different. The ideal printing pressure is determined according to the actual printing conditions.

5, the relationship between printing speed and network expansion

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.

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