Offset dot gain control
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The dot is the basic unit of ink adhesion, which plays the role of transfer and tissue color. Dot enlargement refers to the dot gain of a dot printed on a substrate relative to a color separation patch.
First, the formation of outlets
1. Classified according to printed outlet changes:
1.1 Normal expansion. This printing dot gain is allowed, that is to say, the dot is normally enlarged, the dot density of the dot is high, and the edge density is low. 1.2 ghosting outlets. This kind of printing dot enlargement is not allowed. If there is dot ghosting during printing, it will seriously affect the coloring effect of the dot. 1.3 There is no rule to expand the outlets. When there is no rule-wide dot enlargement, there is no rule for the printed dot, and the dot is enlarged to the periphery. At this time, the paste and the dirty plate are prone to be affected, which may affect the quality of the printed product. The expansion of outlets does not exceed 15%, 12%, and 10%.
2. Classified according to the state of network expansion:
2.1.Linear expansion means that the edge of the printed dot has a direction expansion. Originally, the printed dot is square, round, prismatic, and chain-shaped. The linear expansion means that the dot expands linearly in a certain aspect. 2.2 The printed dots are enlarged to the periphery, and the enlargement of the dot area is generally proportional to the length of the edge of the dot. That is to say, the area of the dot is large, and the length of the edge of the dot is also increased. 2.3 The increase in density is proportional to the area of the printed dot, the density is increased, the ink color is large; the ink color is large, and the dot area is large.
Second, the expansion of outlets has both mechanical and optical conditions.
1. Mechanical dot enlargement: The ink attached to the metal plate dot is deformed and expanded by the pressing of the printing 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 to form a diffuse vignette, which forms a hue. The effect is the same as the mechanical dot enlargement during printing.
The expansion of these two outlets has damaged prints to varying degrees, destroying the balance of the picture. In order to make the dots actually restore on the printed matter, it is necessary to control the transfer process of the dots (ie, printing plate - blanket - substrate).
Third, factors affecting the expansion of outlets
1. Ink transfer affects the expansion of outlets
The transfer of ink is done by an ink roller. The ink is transferred from the ink fountain to the leveling roller through the transfer roller. Under the shearing action of the ink roller, the thixotropic phenomenon occurs, the structure is fully destroyed, and the film is extended into a uniform film, which is transmitted 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. Only the inking system ensures high-precision ink supply, and the ink can be continuously and uniformly transmitted 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 thick, the problem of dot enlargement occurs. At the same time, the hardness of the rubber roller should meet the requirements. The surface of the rubber roller should not be vitrified. It should have good ink transfer performance, so that the dot of the printing plate has sufficient ink absorption.
2. The relationship between blanket and dot enlargement
In lithography, a blanket is used to transfer ink dots, so it is also called offset printing. Under the influence of printing pressure, the ink will expand around the dots. At the same time, due to the elastic deformation of the blanket, a relative slippage occurs between the printing plate and the blanket, between the blanket and the paper. Due to the above expansion and slippage, the enlargement of the dots is inevitably caused. This is an important factor in the expansion of outlets in lithography. In order to control the expansion value of the dot to a minimum range, the density of the dots is uniform, and the periphery is smooth, it is necessary to debug the correct printing pressure, and select a blanket with good printability. The blanket is divided into an air cushion blanket and a general blanket. The air cushion blanket has excellent printability in structure, and has a large printing capacity and a small amount of deformation. When the same amount of compression is produced, the air cushion blanket is used to withstand less pressure than the ordinary blanket cylinder, so the dot gain value is also small. If a plain blanket is used, a rigid lining is used because the elastic lining has a large elastic modulus, a small amount of compression deformation and a nip area, and good dot reproducibility. With a blanket with good printing properties and the correct lining, always pay attention to keeping the surface of the blanket clean. Prevent the surface of the blanket from aging the conjunctiva and oxidize the conjunctiva, smoothing the surface and affecting the lipophilic hydrophobic properties in the rubber layer on the surface of the blanket. During the printing process, the surface of the blanket is covered by paper wool, paper powder, particles in the ink, adhesion of dampening solution, dusting, etc., and friction during high-speed operation, which greatly reduces the transmission of the blanket. The performance of the ink, causing blurring of the dots, and the phenomenon of floating on the ground, directly affect the quality of the printed matter, so it is necessary to clean 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 the dot needs to be through the printing pressure, the printing pressure directly affects the transfer of the 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 be fully contacted, and the molecular force between the ink and the paper is small. Only a small amount of ink may be transferred to the paper surface, the ink conversion rate is very low, and the printed product is light in color. And there is a "empty phenomenon", and even the picture is incomplete. If the printing pressure is too large, the ink will be squeezed into the blank space outside the graphic. On the one hand, the dots will be enlarged and the image will be blurred; on the other hand, the ink transfer will also show a downward trend, which will make the printed matter appear darker. Qing, the part of the field and the outlets have no grammar to reproduce the color of the original. The printing pressure is unstable, the ink is transferred excessively, and sometimes it is insufficient. The reproduction and color reproduction of the printed matter cannot meet the expected requirements, and high-quality printed matter can be obtained only under the appropriate printing pressure range. Choosing the right printing pressure is a very important part of the printing process. The determination of printing pressure, in addition to the structural properties of the machine itself, is related to the ink, paper, dampening solution, blanket, ink roller, printing speed and other factors used in the printing process, because slight changes in these printing conditions will be Hue has an effect. Therefore, during the printing process, it is necessary to adjust according to the actual printing conditions to obtain the desired printing pressure.
4. The relationship between paper and dot enlargement
The printability of the paper depends on the surface characteristics of the paper, such as the amount of ink absorbed and received. The faster the absorption, the greater the extent to which the printed dots are enlarged. The rate at which paper absorbs ink plays a key role in the extent of dot gain. The higher the smoothness of the paper, the smaller the dot gain value; the lower the smoothness of the paper, the larger the dot gain value. In printing paper, coated paper has lower absorbency than non-coated paper, non-coated paper has high absorption performance, and high absorbency results in a high percentage of dot enlargement. Even if it is the same as coated paper, the smoothness of the paper varies greatly depending on the place of production. The quality problem caused by solving this difference can only be achieved by adjusting the printing pressure.
5. Printing speed and dot gain
Changes in printing speed directly affect the quality of the product. When the printing pressure is set to a certain data, the printing speed is slowed down, the contact time between the printing surfaces becomes long, the contact of the printing surface is sufficient, the transfer rate of the ink is high, the ink dots are full, and the ink color is vivid. When the printing speed is increased, the contact time with respect to the printing surface becomes short, the printing surface is not sufficiently contacted, the ink transfer rate is low, the dot is insufficiently sucked, and the image is white. If the printing speed is unstable in a batch of printed products, the ink color will be inconsistent before and after.
Fourth, the number of lines of the dot and the length of the edge
Since the change of the dot occurs in the surrounding part of the printing dot, the more the number of dots per unit area, the more the surrounding part of the dot, and the larger the variation of the dot, the more the dot of the fine mesh is expanded. Larger, and the corresponding coarse mesh line expansion is smaller.
Printed outlets are divided into coarse dots or fine dots according to different purposes. Fine-cut products that are viewed at close range must use fine mesh dots, such as periodicals, art magazines, trademarks, and packaging to fill printed products. Printed products viewed from a distance should generally use coarse mesh points, such as large-scale advertising stickers and posters. The dot is in inches, that is, how many lines (longitudinal and horizontal lines) are in an inch area. Common network wires are 60l/in, 90l/in, 133l/in, 150l/in, and 175l/in.
Fifth, the signal bar control network expansion
1.GATF digital signal strip
The GATF digital signal strip is made up of a coarse mesh point of 27 l/cm as the substrate, and the number of 0 to 9 paved with 80 l/cm flat net is embedded in the middle. The density of the digital 2 is the same as the density of the ground color in the signal strip on the original plate. The specific use is as follows:
The 1GATF digital signal bar is based on the assumption that the coarse mesh is unchanged, the dot does not expand, and the difference in density between the digital and the ground color is observed with the naked eye. 2 In the original version, the area of 1~7 digital dot is decremented by 3% to 5%, 7-9 is decremented by 5% or more, No. 0 is the largest, and No. 9 is the smallest. 3Because it is assumed that the thick mesh line does not expand at all, the dot gain is determined according to the digital and background density.
2. Star point control
36 wedge-shaped lines of equal black and white width are radiated from the center of the circle, with a small white point in the center of the circle.
The star is based on the size of the center white point to determine the size of the dot enlargement. The white point of the star center is large, indicating that the outlet is small; the white point of the center of the star is small, indicating that the outlet is large.
As a testing tool for testing outlets, the star is used to identify the type of dot gain.
The small white point at the center of the star is oval, indicating that the dot has a directional expansion. When the center of the star shows a small white point, an ellipse indicates that the dot expands left and right, which is called lateral expansion; when the dot exhibits a dot that expands up and down or is called a longitudinal expansion. If there is a ghost in the center of the 2 stars, the small white dots in the middle will not only become smaller, but also appear in the shape of "8" or "∞", indicating the ghosting of the dots and the paste.
Sixth, measures to control the expansion of outlets
1. The offset printing plate masters the exposure time and development time.
Offset printing control mastering the ratio of exposure time, development time, developer humidity and concentration is one of the ways to reduce the expansion and reduction of outlets. The four-color film separated by color separation will transfer the graphic and text points to the printing plate through the process of printing, requiring the expansion and loss of the network to be as small as possible, 3% to 5% of the network points are not lost, and 5% of the network points are not enlarged. %~98% of the outlets are not paste-printed, the outlets are strong, the dots are round and neat, the outlets have no white spots, the burrs are edged, and the tone levels are rich, complete and clear. This requires accurate preparation of the PS version of the developer, according to the formulation ratio of the developer instructions, do not have more, less, exposure time with gray ladder, print plate signal to data control, printing plate after drying out 5-10 times magnifying glass checks the loss of the ladder and signal bar level and the expansion of the paste and dot.
2. Reasonably choose the offset lining material to adjust the printing pressure
Offset printing has three forms of hard, neutral and soft lining. Which kind of lining is best? It can be said that each has its own characteristics. Offset printing is an indirect printing method. The rubber intermediate roller is required to transfer the printing plate to the surface of the paper. The rubber compound and its lining material are in the middle of the transfer of the graphic in the offset printing. Medium, there is no printing pressure without lining, there is no indirect printing, there is no offset printing. Through the adjustment of the thickness of the lining of the offset printing machine, reasonable and optimal printing pressure can be obtained, the uniformity of the ink color and the clarity of the dot of the printing product can be enhanced, and the lining of the plaque can be reasonably added to reduce the dot enlargement and improve The printing durability of the plates is very beneficial.
The printing pressure is obtained by adjusting the thickness of the lining and the center distance of the roller. In general, the appropriateness of the printing pressure is often expressed by the quality of the printed matter. The pressure should be based on the premise that the printed product has strong dot, clear image, bright color and light and dark, and the smaller the better, the smaller the control dot is, the more accurate the printing pressure is, the pressure of printing is large, and the dot is expand. Accurately calculate and measure the thickness of the blanket cylinder liner with the micrometer in the work, adjust the center distance between the two rollers, and within the allowable range of the dot enlargement, the print is sufficiently strong to uniformly use the minimum printing pressure.
3. Offset printing controls ink balance and reduces dot gain
Offset printing to control the balance of ink and water, in order to improve the quality of printed products, advocate the use of as little as possible on the premise of not dirty, paste, so that the emulsification of the ink and the deformation of the paper, the ink balance is also a measure of offset printing The technical standard of the operator is only to achieve the ink-and-balance balance paper. The telescopic deformation is small, the overprinting accuracy is high, the color is bright, the hue and the hue can match the depth of the printed samples of the sample, and the printed dots are round and firm. The network cable is flat, and it also reduces the expansion of the network. During offset printing, avoid large water and large ink, and the ink layer is too thick to spread outward. The small dots of the 3% to 5% bright adjustment area are not lost, and the 5% intermediate adjustment points are not enlarged. 95%~97% of the large mesh points are not ambiguous. The control standard of the ink quantity is: the graphic net dot is firm and clear, and the ink color hue is accurate, and the amount of ink is used as little as possible.