1. The formation of network expansion
1. According to the change of printed outlets:
Normal expansion. This expansion of printed dots is allowed, that is to say, the dots are normally expanded, the density of dots in the center is high, and the density of edges is low.
Ghosting outlets. This expansion of printed dots is not allowed. If dot ghosting occurs during printing, it will seriously affect the color rendering of the dots.
There are no regular outlets to expand. When there is no rule to expand the dots, the printed dots do not have a rule, and the dots expand to the surroundings. At this time, it is prone to paste and dirty plates, which will affect the quality of the printed products. The expansion of outlets does not exceed 15%, 12%, 10%.
2. Classified according to the expanded status of outlets:
Linear expansion means that the edges of the printed dots expand in one direction. Originally, the printed dots were square, round, prismatic, and chain-shaped. Linear expansion means that the dots expanded linearly in a certain direction.
The printed dots expand to the surroundings, and the enlargement of the dot area is generally proportional to the length of the dot edge. In other words, the dot area is large, and the length of the dot edge also increases.
The increase in density is proportional to the area of ​​the printed dots. The density increases and the ink color is larger; the larger the ink color, the larger the dot area.
Second, the expansion of the network has two mechanical and optical situations
1. Mechanical dot expansion: refers to the deformation and expansion of the ink attached to the metal dots after being squeezed by the printing pressure.
2. Optical dot expansion: due to the effect of light reflection, when light reaches the dot surface, surface reflection occurs, and the ink penetrates the edge of the dot in the paper to form a diffuse halo, which forms a hue , The effect is the same as the mechanical dot expansion during printing.
The expansion of these two types of dots will damage the printed matter to varying degrees and disrupt the balance of the picture. To make the dots truly restore on the printed matter, it is necessary to control the transfer process of the dots (ie, printing plate-blanket-substrate).
3. Factors affecting the expansion of outlets
1. Ink transfer affects network expansion
The transfer of ink is done through the ink roller. Ink is transferred from the ink fountain to the ink leveling roller through the ink transfer roller. Under the shearing action of the ink leveling roller, thixotropy occurs, the structure is fully destroyed, spreads into a uniform film, and passes 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 when the inking system guarantees its high-precision ink supply can the ink 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 squeezes is usually very thin. If the ink layer is thick, the problem of dot expansion will occur. At the same time, the hardness of the rubber roller must meet the requirements. The surface of the rubber roller must not be vitrified. It must have good ink transfer performance so that the dots of the printing plate have sufficient ink absorption.
2. The relationship between blankets and network expansion
In lithography, the blanket is used to transfer ink dots, so it is also called offset printing. Under the effect of printing pressure, the ink will spread around the dots. At the same time, due to the elastic deformation of the blanket, relative slippage occurs between the printing plate and the blanket, and between the blanket and the paper. As a result of the above-mentioned expansion and slippage, the expansion of the dots inevitably occurs. This is an important factor that causes dot expansion in lithography. In order to control the expansion value of the dots in the minimum range, make the density of the dots uniform, and smooth the periphery, it is necessary to adjust the correct printing pressure and choose a rubber cloth with good printing suitability. The blanket is divided into air cushion blanket and ordinary blanket. The air cushion blanket has excellent printability in structure, and it has large printing capacity and small deformation. When the same amount of compression is generated, using an air-cushion blanket is less stressful than using an ordinary blanket roller, so the dot expansion value is also small. If you use ordinary blankets, you must use a hard liner, because the hard liner has a large modulus of elasticity, the compression deformation and the width of the embossed area are relatively small, and the dot reproducibility is good. With a blanket with good printing performance and the correct liner, we must always pay attention to keeping the surface of the blanket clean. Prevent the aging conjunctiva and oxidative conjunctiva on the surface of the blanket, smooth its surface, and affect the lipophilic and hydrophobic properties of the rubber layer on the surface of the blanket. During the printing process, the surface of the blanket is covered by paper residues, paper powder, particles in ink, adhesion of fountain solution, powder spraying, etc., and friction during high-speed operation, which greatly reduces the transmission of the blanket Ink performance, resulting in blurred dots and floating on the ground, directly affects the quality of the printed matter, so it must be cleaned frequently to keep the surface of the blanket clean and exert its best results.
3. The relationship between printing pressure and network expansion
The reduction of the dots needs to be effected by the printing pressure, which directly affects the transfer of ink, and it is very important to correctly adjust the working pressure of the printing press. When the printing pressure is too small, the printing surfaces cannot be fully contacted, the molecular force between the ink and the paper is very small, only a small amount of ink may be transferred to the paper surface, the ink transfer rate is very low, and the ink color of the printed product is light , And "emptiness phenomenon" appears, and even the pictures are incomplete. If the printing pressure is too high, the ink will be squeezed into the blank space beyond the picture and text, on the one hand, it will cause the dots to expand and the image will be blurred; on the other hand, the transfer of ink will also show a downward trend, making the printed matter appear shade Qing, the field and the dots of the site have no grammar to reproduce the color of the original. The printing pressure is unstable, the ink transfer is sometimes excessive, and sometimes insufficient. The transfer and reproduction of the printed matter and the color reproduction can not meet the expected requirements. Only in the appropriate printing pressure range can high-quality printed matter be obtained. Choosing the right printing pressure has become 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, wetting fluid, blanket, ink roller, printing speed and other factors used in the printing process, because small changes in these printing conditions will Hue has an effect. Therefore, in the printing process, it must be adjusted according to the actual printing conditions to obtain the ideal printing pressure.
4. The relationship between paper and network expansion
The printability of the paper depends on the surface characteristics of the paper, such as the amount of ink absorption and acceptance. The faster the absorption rate, the greater the degree of expansion of the printed matter network. The speed of paper absorption of ink plays a key role in the expansion of dots. The higher the paper smoothness, the smaller the dot expansion value; the lower the paper smoothness, the larger the dot expansion value. In printing paper, coated paper has lower absorption than non-coated paper. Uncoated paper has high absorption performance. As a result of high absorption, the percentage of dot expansion is very high. Even if it is the same as coated paper, the smoothness of the paper is very different due to the different origin. Solving the quality problems caused by this difference can only be achieved by adjusting the printing pressure.
5. The relationship between printing speed and network expansion
Changes in printing speed directly affect the quality of the product. When the printing pressure is set at a certain value, the printing speed is slowed, the contact time between the printing surfaces becomes longer, the contact of the printing surfaces is sufficient, the transfer rate of the ink is high, the dots absorb the ink, and the image color is bright. When the printing speed is increased, the contact time between the printing faces becomes shorter, the printing faces are not sufficiently contacted, the transfer rate of the ink is low, the dot ink absorption is insufficient, and the image is white. If the printing speed is unstable in a batch of printed products, it will cause inconsistent ink colors before and after.
Fourth, the relationship between the number of dots and the length of the edge
Since the dot changes occur around the printed dots, the more dots per unit area, the more the dots around the dots, and the greater the amount of dot changes, so it can be seen that the dots of the fine mesh line expand Larger, and the expansion of the corresponding thick network line is smaller.
Printed dots are divided into coarse dots or fine dots according to different uses. Fine printed products that are viewed at close range must use fine dots, such as periodicals, pictorials, trademarks, and packaging filled with printed products. Printed products viewed from a distance usually use thick dots, such as large advertisement posters and posters. The dot is the unit area in inches, that is, how many lines (horizontal and horizontal lines) are in an inch area. Common network cables are 60l / in, 90l / in, 133l / in, 150l / in, 175l / in.
V. Expansion of the control network of signal bars
1.GATF digital signal bar
The GATF digital signal strip is made of 27l / cm thick dots as the substrate, and the middle is inlaid with 80l / cm flat net numbers 0-9. The density of the digital 2 in the original signal strip is the same as the background color density. The specific use is as follows:
The GATF digital signal bar assumes that the coarse screen remains unchanged and there is no expansion of the dots. Observe the density difference between the digital and the background color quality inspection with the naked eye.
In the original version, the area of ​​digital dots from 1 to 7 is decreased by 3% to 5%, and the decrease from 7 to 9 is more than 5%. The area of ​​dot 0 is the largest, and the area of ​​dot 9 is the smallest.
Since it is assumed that the thick screen line does not expand at all, the expansion amount of the screen point is determined according to the number and the background color density.
2. Star control of outlets
36 wedge-shaped lines of equal black and white width are radiated from the center of the circle, and there is a small white dot in the center of the circle.
The star mark is based on the size of the center white dot to determine the size of the dot expansion. A large white dot in the center of the star indicates that the dots are enlarged; a small white dot in the center of the star indicates that the dots are enlarged.
As a testing tool to test the expansion of the network, the star mark can distinguish the type of network expansion.
The small white dots in the center of the star mark are oval, indicating that the dots have a directional expansion. When the small white dot in the center of the star shows an ellipse, it means that the dots expand left and right, which is called horizontal expansion; when the dots show the dots that expand up and down, it is called vertical expansion.
If there is a ghost image in the center of the star, the small white dot in the middle not only becomes smaller, but also appears in the shape of "8" or "∞", indicating that the dots are ghosted and pasted.
6. Measures to expand the control network
1. Master the exposure time and development time in offset printing
Control of offset printing mastering the ratio of exposure time, development time, developer humidity, and concentration is one of the ways to reduce the expansion and shrinkage of the dot. The four-color film separated by color transfers the graphic dots to the printing plate through the printing process, requiring the expansion and loss of dots as little as possible, 3% to 5% dots are not lost, 5% dots are not expanded, 95 % ~ 98% of the dots are not paste version, the dots are strong, the particles are round and neat, the dots have no white dots, burrs, and the levels are rich, complete and clear. This requires accurate preparation of the PS version of the developer, according to the formula ratio of the developer's instructions. Do not add more or less. The exposure time is digitally controlled by the gray ladder rule and the printing signal bar. The printing plate is used after it is exposed. 5-10 times magnifying glass to check the loss of ladder scale and signal bar level and the pasting and network expansion.
2. Reasonably select the offset printing liner material to adjust the printing pressure
Offset printing has three forms of hard, neutral and soft liner, which one is the best? It can be said that each has its own characteristics. Offset printing is an indirect printing method. It requires an elastic body in the middle of the rubber cylinder to transfer the graphic dots of the printing plate to the surface of the paper. The rubber patch and its lining material are the middle of the graphic transfer in offset printing. There is no printing pressure for the media, and there is no indirect printing and no offset printing. By adjusting the thickness of the liner of the offset printing machine, you can get a reasonable and optimal printing pressure, enhance the uniformity of the ink density of the printed product and the clarity of the dots. In addition, the reasonable installation of the liner reduces the dot expansion and improves The durability of the printing plate is very beneficial.
The printing pressure is obtained by two methods of adjusting the thickness of the lining and the center distance of the cylinder. 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 dots, clear graphics, bright colors and shades, and the smaller the applied, the better. It is critical to control the expansion of the dots and adjust the accurate printing pressure. The printing pressure is large, and the dots are expand. Accurately calculate and use a micrometer to measure the thickness of the rubber roller lining in the work, adjust the center distance between the two rollers, and use the minimum printing pressure evenly on the basis that the footprint is sufficiently strong within the allowable range of the dot expansion.
3. Offset printing controls the balance of water and ink, reducing the expansion of the network
Offset printing must be controlled to control the water and ink balance to improve the quality of the printed product. It is recommended to use as little water as possible on the premise of not dirty plates and paste plates, so as to compare with the emulsification of ink and the expansion and contraction of paper. The operator's technical level is high, only when the ink and water balance is reached, the paper has small expansion and deformation, high overprint accuracy, bright colors, and colors and hues can meet the proofs. A batch of printed inks have the same color depth, and the printed dot particles are round and strong. The network cable is uniform, and it also reduces the expansion of the network. When offset printing, avoid the large dots of water and ink, the dots of the ink layer spreading outwards are too thick, control the small dots of 3% to 5% of the bright tone area, do not lose, the 5% of the intermediate tone dots do not expand, do not stretch, dark tone area 95% to 97% of the large dots are not confused. The control standard for the amount of ink is: the graphic dots are strong and clear, and the color and hue of the printed ink are accurate. Use as little ink as possible.
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