In order to reproduce the color of the original manuscript and improve the printing quality, the printed matter must understand the changes of the dots. The dots are the basic unit of ink adhesion, and play the role of transferring tone and organizing colors. In offset printing, halftone dots are the basic elements of an image. Changes in halftone dots inevitably affect the tone and color of the image, which in turn affects the clarity of the image. Therefore, the key to the control of printing quality is to control the printing quality of the printed matter on the Internet.
There are two types of network expansion: mechanical and optical.
1. Mechanical dot expansion: refers to the deformation 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.
These two kinds of network expansion have an impact on the quality of the printed matter to varying degrees, destroying the balance of the picture. In order to make the dots truly restore on the printed matter, it is necessary to control the transfer process of the dots (ie, printing plate-rubber-substrate).
Dot expansion refers to the fact that the effective area of ​​the network access point of the printed matter is larger than the effective area of ​​the network access point of the screen negative film. This is a normal process phenomenon, and it is inevitable whether it is pre-press production or the printing process. But it must be controlled within a certain range. Once it exceeds a certain range, the print quality will seriously decline (tone loss, density becomes larger, color distortion, etc.). This increase in dots is already abnormal. GB7705-7 stipulates that for 50% of the dots, the increase of the dots of fine products should not be greater than 12%, and the increase of the dots of general products should not be greater than 18%. In the pre-press production and printing process, it can generally compensate for the increase of normal dots (such as laser phototypesetting, the double reflection effect of paper and the increase of dots under reasonable printing pressure), but the abnormal dots There is often no regularity to increase, and reasonable compensation cannot be made during the prepress process, which can only be controlled during the printing process. [next]
1. Analysis of factors influencing network expansion
(1) Material factors
1. Plate factors
The photosensitivity of poor printing plates is poor. The PS version has some coarse meshes and some poor photosensitivity, and it is easy to form a halo around the dots after exposure. After the PS version is inked, the virtual halo part is also inked. The actual effect is to increase the dot area and cause the dot to increase. You need to replace the better quality PS version.
During the printing process, the reasons for increasing the dots are:
â‘ The exposure is too small or the exposure time is too short, making the exposure incomplete.
â‘¡ Developer fatigue or the pH value of the developer is too low during development. Especially in manual development, it is most likely to occur. Because the developer is exposed to the air, it easily reacts with carbon dioxide in the air.
â‘¢The development time is short (the unexposed parts on the printing plate are not rinsed completely), or the development temperature is too low, making the development process slow. Practice has proved that every time the development temperature is lowered by 5 degrees, the development speed will be reduced by 1 to 2 times.
Using reasonable exposure, development temperature, and development time in the process of printing, the increase of the dots can be effectively controlled. In addition, when the development temperature is too low, the development time should be appropriately extended. It is recommended that each batch of PS plates be printed with gray ladder rulers after entering the factory to test the ideal exposure and exposure time, which is convenient for the standardized management of printing plates.
2. Ink factor
â‘ The cohesion of the ink is too low. When the cohesion (viscosity) of the ink is too low, the ink will rebound weakly, causing the dots of the printed matter to increase and the ink to spread. In the ink transfer process, when the ink film is drawn into a wire and breaks, and the tensile stress disappears, it mainly depends on the viscosity and elasticity to return to the state before stretching. In the process of viscosity recovery, because the film has no internal stress, it can only rely on the action of gravity. The result is uneven film, edge diffusion, dots are not smooth, and there are dots increase phenomenon, which is very undesirable Recovery status. The elastic recovery depends on the recovery of the internal energy of the film layer. Due to the strong internal stress accumulated in the film layer, the ink filament is pulled back by the elastic force and contracted.
When the cohesion of the ink is too low, the elastic recovery ability will be weakened, and it can only rely more on the recovery of viscosity, which will cause the dots to increase. Therefore, various additives should be used as little as possible in printing, so as not to reduce the cohesion of the ink. If the ink itself has problems, you can add some ink modifiers or No. 0 ink.
â‘¡The viscosity of the ink is too small and the fluidity is too high. The hydrophilicity of the ink is too sensitive, the thixotropy and yield value are too small, the viscosity is too low, the ink filament is too long, the fluidity is too strong, and more fountain solution and Contend. The emulsified ink is easy to spread on the PS plate, which is easy to cause the increase of the dots. Transferring to the substrate is not conducive to the solidification of the ink, and a clear mesh tone image cannot be obtained. [next]
3. The relationship between paper and network expansion
The paper surface is not completely smooth. There are many pits and capillary holes. During printing, under the pressure of ink, the ink first fills the pits in the pressure direction, and the remaining ink spreads to the periphery of the dot. 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.
The effect of paper on the expansion of dots is also reflected in the double reflection effect of paper. After the dots are printed on the paper, most of the white light shining on the paper is reflected, but a considerable part of the white light is still absorbed by the ink layer, and the remaining light is reflected by the paper surface and reflected through the ink layer, becoming diffusely reflected light. Under the dual effect of paper reflection and diffuse reflection, a halo is formed at the edge of the dot, which causes the visual dot expansion phenomenon. In fact, these dots have not expanded, but the illusion of dot expansion. If the surface of the paper has high smoothness, more reflected light, and diffuse reflection is not serious, the double reflection effect is small.
4. The relationship between the blanket and the network expansion
The blanket, as a medium for the transmission of graphic ink, has the characteristics of strong elasticity and good lipophilicity. It deforms when subjected to pressure during printing. Under the action 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.
The blanket is divided into air cushion blanket and ordinary blanket. 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 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. The blanket is too thick or too soft to increase the pressure between the plate cylinder and the rubber cylinder, the rubber cylinder and the impression cylinder (the roller rubber cylinder is both the transfer cylinder and the impression cylinder), and the amount of compression deformation of the blanket And the width of the contact area are relatively large, the dot quality is not ideal, the dot reproducibility is poor, and the dot increase value is relatively high. [next]
(2) Printing factors and process factors
1. Determination of printing pressure
In the printing process, the transfer of ink and the transfer of imprints require appropriate printing pressure. The extrusion of the cylinder and the deformation of the blanket will inevitably cause the deformation and increase of the dot. Therefore, when printing the PS version, it is necessary to combine different models (different printing machines have different dot increase values) to select the appropriate exposure time, so that the dots on the PS version are slightly smaller than the proof, the printed matter can be consistent with the sample . Under the effect of printing pressure, the ink will spread outward. If the printing pressure is too large, dots will increase, so the size of the printing pressure should be strictly controlled. For paper with high surface smoothness (coated paper), the amount of compression should be controlled between 0.10 and 0.15 mm, while for paper with low surface smoothness (offset paper), the amount of compression should be controlled around 0.2 mm. The principle of control is to use as little printing pressure as possible on the basis of ensuring good ink transfer.
2. 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 The color of the original and the dots on the ground and dots cannot be reproduced.
The printing pressure is unstable, the ink transfer is sometimes excessive, and sometimes insufficient. The tone reproduction and color reproduction of the printed product can not meet the expected requirements. Only in the appropriate printing pressure range can high-quality printed products 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.
3. 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. [next]
4. 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 strict rules. 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.
For the ink to be transferred from the ink roller system to the printing plate smoothly, the ink must also have good fluidity. Under stirring or mechanical action, the offset printing ink will become soft and the fluidity will increase. Stop stirring, rest for a while, and then return to the original state of solid glue. This phenomenon is called the thixotropy of ink. Therefore, to ensure that the ink can be transported well, it is necessary to constantly stir the ink fountain. Use the thixotropy of offset printing ink to increase the fluidity of the ink to ensure that the dots can obtain the desired ideal color.
5. Poor adjustment of ink transfer device
The relative position between the ink rollers changes, and the shearing force between the rollers also changes, which destroys the ink transfer relationship. The platen roller is under heavy pressure, the squeeze deformation is large, the diameter of the rubber roller becomes thinner, the line speed of the platen roller and the plate cylinder are inconsistent, and the dots are elongated in the circumferential direction of the cylinder, resulting in tensile deformation. The pressure of the plate against the plate roller is too high, which causes the ink to spread on the plate when it is transferred from the ink roller to the plate. Therefore, it is necessary to re-adjust the plate-pressing pressure of the plate-ink roller. The increase in dots caused by equipment wear, such as the increase in dots caused by gear wear, is generally manifested as ink bars and ghosting. Solution: Generally, it is possible to reduce the tooth spacing properly without topping the teeth.
6. Too much ink
Under ideal conditions, the thickness of the transferred ink layer should be controlled within 25 μm. As the amount of ink supplied increases, the thickness of the ink film increases, and the separation force also increases rapidly. The excess ink cannot be transferred immediately by the substrate. There is more ink accumulation on the surface of the printing plate and the blanket, and the excess ink will diffuse out of the dot As a result, the dots increase, and even scumming occurs, so the amount of ink must be strictly controlled according to the print.
7. Control of the amount of fountain solution
The small amount of water in the printing plate makes the water film in the blank part unable to effectively resist the expansion of the ink layer on the dot. Solution: Adjust the pressure of the water roller, so that it can transfer water evenly and stably, or increase the amount of fountain solution or fountain solution. However, too much or too little fountain solution may cause dot gain, so the amount of fountain solution must be reasonably controlled during the printing process, and the pH of the fountain solution should be within a reasonable range, generally in Between 4.8 and 5.3. In actual printing, appropriate adjustments should be made according to actual conditions. For example, if a drying agent is used in the ink, the pH value of the fountain solution can be appropriately lowered. However, it is necessary to control the excessive emulsification of the ink to prevent the ink film on the dots from maintaining a relatively clear boundary with the blank part of the paper, resulting in the dots increasing. At this time, the amount of the fountain solution or the dampening powder can be appropriately reduced.
8. The deformation of the blanket
Due to the elastic deformation of the blanket, relative slippage occurs between the printing plate and the blanket, and between the blanket and the substrate, causing the dots to increase. Therefore, when adjusting the printing pressure and changing the center distance of the cylinder, the surface linear speed of the cylinder should be as consistent as possible. But it is impossible to be completely consistent, because the surface linear velocity of the rubber roller is inconsistent due to the compression of the rubber cloth. The blanket is not sufficiently tight or compressible. After the new blanket was printed on the machine for the first time, 5,000 or 6,000 sheets were printed, and then the blanket was tightened again. After printing 10,000 or 20,000 sheets, the blanket was tightened again. [next]
Second, the measurement of the expanded value of the outlet
There are two methods for measuring the dot expansion value: one is the reflection density meter measurement method, and the second is the Brunel printing measurement and control strip measurement method.
1. The reflection densitometer measurement method is to measure the density with a densitometer, and use the Murray-Davies formula to calculate the dot expansion value, which can be quantitatively analyzed to achieve printing data and standardization. The dot expansion value is equal to the dot area on the printed product minus the dot area of ​​the corresponding part on the original (film used for printing).
2. Brunel printing measurement and control strip measurement method. The Brunel measurement bar has a fine line segment of 60 lines / cm and a coarse line segment of 10 lines / cm. The 50% dot area ratio of the fine and thick nets is equal. The line number ratio of the fine mesh and the thick mesh is 6: 1, that is, the ratio of the edge length of the fine mesh to the edge length of the thick mesh is 6: 1. Since the dot expansion amount is proportional to the edge length of the dots, under the same conditions, the larger the dot expansion amount, the greater the density. The calculation is based on the coarse network segment, and the difference between the density of the fine network segment and the thick network segment is taken to obtain the 50% dot area expansion value. The calculation formula is:
= 50% fine network segment density-50% coarse network segment density According to Brunner's research, the calculation formula for the dot expansion value of about 75% is:
= (75% fine network segment density-75% coarse network segment density) ÷ 2
The calculation formula for the expansion value of the outlets around 25% is:
= (25% fine network segment densityï¼25% coarse network segment density) × 2 [next]
3. Control of the expansion of the offset printing network
Although the increase of outlets is inevitable, it can be controlled. Under normal circumstances, the increase value of fine printing dots of coated paper is less than 12%, the general printing is less than 18%, and newspaper printing is controlled within 35% (generally refers to 50% dots). In the production process, you can effectively control from the following aspects.
1. Select high-quality plates, reasonably control the exposure time and developer concentration
The printed printing plate with clear graphics and strong dots can ensure the transfer effect of the dots, and also ensure that the printing plate has a high printing resistance. Under normal circumstances, it is necessary to ensure that 5% of the outlets are not lost, and 95% of the outlets are not blurred.
2. Strictly control the balance of water and ink in printing
In the printing process, if the water supply is too large, it will increase the ink supply on the plate, the ink layer will become thicker, the ink emulsification will be serious, and the blot will be dull and dull. In the production process, in order to reduce the waste of starting up, the water level is usually increased to 150% of the normal value when starting up. After normal printing, the amount of water should be gradually reduced to the normal value to ensure that the graphics are clear, saturated and shiny.
3. Control various printing pressures within a reasonable range
Control the pressure between the plate cylinder and the rubber cylinder at 0.15 ～ 0.20mm, and ensure that the two plate-based ink rollers are parallel to the plate cylinder, and the platen roller and the plate cylinder with a diameter of less than 100mm have an embossing width of 3 ～ 4mm, the platen roller and plate cylinder with a diameter of 100mm or more have a width of 5 ～ 6mm.
4. Correctly control the concentration of fountain solution
The control of the fountain solution concentration plays a vital role in the balance of water and ink and the quality of the printing quality. The appropriate concentration of fountain solution can quickly wash away the "dirt" around the dots. If the concentration of the fountain solution is low, the ink around the dots cannot be removed, which will cause the dots to increase, and the paste will cause the heavy version; if the concentration of the fountain solution is too high, it will cause the PS printing resistance to decline and the fountain A lot of waste. Normally, the pH value of fountain solution is controlled at 4.0 ～ 6.0, and the conductivity value is controlled at 1400 ± 200μs.
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