Dots in offset printing
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One: About the dot control in offset printing
The dot is the pixel of the printed image and is the smallest unit of ink that is absorbed in the offset. It is responsible for the task of passing the tone and tone of the original. According to the theory of color, the color has three properties of hue, brightness and saturation. The yellow, magenta, and cyan primary color dots are superimposed, and by changing the size or coverage of the dots, thousands of different colors can be formed. Therefore, no matter how rich the layer and how the color changes, the color and tone of the original can be simulated by the proper matching of the three primary colors.
When evaluating the quality of color prints, the quality of dot reproduction is the most important indicator. Many of the standards related to the quality of print reproduction, such as uniform ink color, no deformation of dots, no ghosting on printed matter, and ink sticks are almost always related to the control of dots.
Changes in dot can cause problems with sharpness and subtle levels of change, color changes, and contrast, ink tones, ink density, and ink overprinting. Although some predictions and prevention of dot changes have been made electronically, changes in control sites are still necessary to ensure print quality.
The size of the dots and the position between each other determine the hue of the print. For example, a large dot that is printed very tightly will form a dark tone area, and a small dot with a large pitch will form a highlight area. Therefore, the increase in dot size can greatly affect the reproduction of color images in these areas, and has a great influence on the midtones.
It is for the reasons mentioned above that we propose the concept of site control, which controls the quality of outlets throughout the entire workflow of the print copy. Due to the close connection between the various processes, it is also necessary to pay attention to the quality control that coordinates with each other.
Although the term digital printing has been proposed for many years, in the current widely used traditional printing methods, the conversion of dot information between devices is still more using analog information transmission, and it is precisely because this transmission is realized by using analog information. It will inevitably cause the information of the dot itself to appear in the device that we do not want to see (such as dot gain, dot gain, dot break and dot loss or level).
Now the workflow of our more common dot printing (the printing method mentioned in this article is offset printing) is like this:
Original → color separation (currently mainly used for scanning or electronic color separation) → image processing (including tone adjustment and compression, image mixing, color separation, screening, etc., this process is mostly done by computer ) → spelling large version → filming (exposure, development, fixing) → printing (exposure, development, for the large amount of printing also use the baking version) → machine printing → printing end.
The above is a relatively common workflow. The entire process of forming a dot from the image processing stage to the completion of printing is through light, film, plate, printing machine (mainly printing plate and blanket), ink, substrate. To complete, but due to many inherent reasons such as equipment and technology, the nature of the transmission itself determines the change of the network is inevitable, plus these materials and equipment itself have a lot of uncertainty, resulting in the network can not be done at every step Accurate copying and delivery, that is to say, no matter how we do it, it is impossible to completely avoid the changes of the dot in these transfer processes (the above-mentioned dot deformation, dot gain, shrinkage, etc.). In actual production, what we need to do is to control the changes of the outlets within a reasonable and predictable range, and at the same time regulate the production process and reduce the fluctuation of the variation parameters in the production process.
The changes in the dot-copying process that we can encounter in production are mainly as follows:
1. The dot size is reduced, that is, the area of the dot obtained by copying is reduced relative to the upper layer (upper process) carrier of the dot.
2. Increase in dot size: Compared with the upper carrier of the dot, the area of the dot obtained after copying is increased, and the density of the obtained replica becomes large.
The dot gain can be further divided into mechanical dot gain and optical dot gain.
"Mechanical dot gain" (also known as physical dot gain) is the actual physical increase in dot size. The most common form of mechanical dot gain is non-directional, which may result in a paste if excessively increased. This increase is caused by the thickness of the ink film, the type of substrate, the number of high screen lines, or the transfer of ink between the blanket and the printing plate, and between the blanket and the substrate.
"Optical dot gain" is a visual phenomenon formed by the light absorbing properties of the ink and the light dispersion characteristics of the substrate. When the light is irradiated in a non-image area, that is, a "blank area", the light is diffused, and a part of the light near the half point is suppressed. This light is considered "absorbed" because it cannot be reflected back into the viewer's eye. The dot appears to be larger than the actual density and size, and it seems that the dot gain has occurred.
3. Deformation of the dot: The shape of the dot obtained after copying has changed. For example, the original circular dot may become elliptical after copying, or a dot with no fixed shape.
The three issues mentioned above have a great impact on the entire reproduction phase of the printed matter. But when we can more accurately predict the changes made by the network between the devices, we can more effectively compensate for the changes that will occur in the network before the network changes. For example, if the dot of a certain print has the following changes during the copying process: the copy positive dot is reduced by 2%; the sunscreen dot is reduced by 4%; the printed dot is increased by 15%; and the dot gain is increased by 12%. Then, after a simple math addition and subtraction, we will find that 21% of the pre-compensation of the outlets is made before the transmission starts, and we will get the effect we want after delivery. Although the actual dot compensation setting is not as simple as the above, there is not only a linear relationship between this compensation and the dot change, and different percentages of the dots have different trends during the copying process, but we only need It is enough to understand the above compensation relationship. Tell the software about the changes that will occur at the site (typically the rate of change of the 50% dot), and the rest can be handed over to the computer.
The reason mentioned above seems to be very simple, but as soon as we get to the actual operation, we will find that it is not easy to standardize the management and standardize the operation because of the variety of work involved in the transmission of the network. However, this is an indispensable important aspect for dot reproduction on printed matter. Only by strict implementation of management standardization and standardization of operations, it is possible to achieve targeted image processing and dot compensation in prepress, which is precisely a lot. Printing companies can't do it. The actual situation is often that the plate making and printing are different, which is very unfavorable for the correct copying of the outlets.
Below we will discuss some of the issues that need to be addressed in actual production and the factors that tend to cause abnormal deformation of the dots.
Choice of printed materials: This includes paper, ink, blankets, plates, dampening fluids, and more. The choice of these raw materials will often directly affect the reproduction quality of the dots in the printing. A successful printing is not only to obtain a better quality of our printed materials, but also to achieve the minimum cost within the allowable quality range.
The choice of plate making process: This is a very large and wide problem, but it involves the copying of the dot, the most important thing is the shape of the dot and the number of screens. The correct choice of dot shape can have a great influence on the surface effect of the printed matter. More importantly, it should be noted that the properties of each dot shape are different. For example, the square dot has a large tone jump at 50%, and the total dot gain rate of the square dot is also larger than that of other dots, so the color separation process of high-grade prints with rich middle notes is performed. Squares cannot be used, and diamonds or elliptical dots are the primary consideration. The higher the number of dots, the richer the level of dot prints will be, and the more detailed the screen will be, but at the same time, the larger the side length of the entire site, the larger the theoretical condition according to the dot gain. We know that the trend of changes in outlets is also greater. In many cases, blindly increasing the number of dot lines will result in a decrease in the quality of the dots. However, in actual production, we must weigh the number of dots according to the comprehensive conditions of many factors such as printed paper, ink, and machine. . The percentage of dot points is the same, and the dot gain of different dot lines is increased: when the percentage of dot is the same, and the dot gain value is the same, the higher the dot line number, the larger the dot gain rate; the same percentage of dot dots Next, and the dot gain value is the same, and the higher the number of network lines, the larger the percentage of the dot gain.
Printing version:
1. Exposure. In the PS version exposure process, due to the light seepage and other reasons, the dots on the printing plate are actually narrower than the dots on the film. Generally, the reduction should be controlled within 3% to 5%. However, this figure is not unchanged. Due to the vacuum degree of the printing equipment, the transparency of the printing glass, the control of the printing time, and the quality of the printing plate itself, there are many different situations, so it is necessary to use the printing control bar frequently. Testing the change of the dot in the printing plate, and then feeding the data back to the pre-press image processing unit;
2. Development: The concentration of the developer is regularly tested, and the time of development is determined by the results of the test. If the constant development time is still used after the developer is fatigued, then the edge of the dot will not appear clean, causing the dot to increase. It is necessary to regularly check the working condition of the printing machine during the printing process, and the development conditions are necessary. This process can be carried out by using the special test strip for the printing plate, and the obtained result can be fed back to the prepress in time. The reaction is carried out in advance.
The effect of dampening solution:
a PH value of dampening solution: the pH value of the dampening solution should not be too low during the printing process, preferably about 5-6, because the pH value of dampening solution is too low, which has a great influence on the transfer of offset ink, and will cause The plate is corroded, reducing the printing durability of the printing plate, resulting in the loss of high-profile dots, or other printing problems;
b The amount of water on the plate: the thickness of the water film on the blank part of the plate is generally 0.5 to 1 μm, and the thickness of the ink film is 2 to 3 μm. If the moisture on the plate is too large, it will erode the ink film in the image part, causing the dot to be emptied. If the thickness of the ink film is large, the ink will spread to the blank under the action of pressure, resulting in an increase in the dot. .
The effect of the ink:
1. Moderate liquidity. If the fluidity is too small, the cohesive force of the ink is too strong, and it is difficult to be grounded, thereby reducing the transferability of the ink. If the fluidity is too large, the adhesion of the ink may be poor, and the printed dots may be flat, dull, or even spread around. Affect the quality of the product. Also note that the ink flow is smaller than the text when printing the dot product, so that the printed dots are clear and full.
2. Choose the right ink. The ink is required to be fine, smooth, with good flowability, proper viscosity, no hysteresis, bright and durable, and the ink is preferably translucent.
Reasonable emulsification of ink: This is an interesting phenomenon. Compared with other printing methods, offset printing often results in insufficient color saturation due to emulsification of ink. Therefore, in actual production, everyone is trying to reduce the emulsification of ink, in order to be able to A more vivid ink color effect. However, the actual situation is that there is no reasonable ink emulsification (through experiments we know that when the ink emulsification reaches 26%, it is an ideal emulsified state), the ink cannot be between the ink rollers, the ink roller and the printing plate, the printing plate and the rubber. Good transfer between the cloths (because water forms a water film on the surface of the ink and hinders ink transfer), and direct consequences can cause deformation or emptiness of the dots. In addition, when the emulsified value of the ink exceeds 30%, a clear boundary will be lost between the water film and the ink film, and the edges of the dots will be burred, blurred, etc., and the more serious consequence is that the pigment particles in the ink are precipitated from the resin. , stacked on the printing blanket, will also affect the transmission of ink.
Pressure:
1. The water roller pressure can make the water roller smoothly and evenly transfer water to the printing plate, and the pressure is too large or too small, which is unfavorable for printing. When the pressure is too high, it will cause the water roller to jump at the mouth to cause friction. Under the pressure of water, the mesh point will be squeezed smaller or deformed; of course, the consequences of too little pressure are obvious, the water volume cannot be transmitted normally, of course The ink on the layout will spread to the blanks.
2. The pressure of the ink roller should be flat and light, the pressure is too heavy, and the outlets are easy to pressure, which affects the quality of the product.
3. Keep the roller pressure moderate and uniform. Excessive pressure causes the ink on the printing plate to produce spreading deformation under the action of large pressure; the small pressure will directly lead to a decrease in the transfer rate of the ink, and the ink transferred to the substrate is less and thinner. However, if you encounter a substrate with a relatively smooth surface (such as fabric paper, embossed paper, newsprint), you need to increase the printing pressure to compensate for the uneven surface of the substrate and improve the ink transfer efficiency, thus ensuring the transfer of the dots.
4. Ensure that the linear speed of the drum is consistent (as much as possible). If the linear speeds between the adjacent rollers are large, the transmission of the dots between the rollers will be deformed.
Avoid "ghosting": Refers to the double outline of the same color dot, line or text on the printed matter. It is a directional dot gain that can cause ghosting between the plate and the blanket, or between excessive pressure between the blanket and the impression cylinder. Excessive pressure causes the rubber surface of the blanket to plastically deform and change the shape of the printed dots.
Ghosting can also occur with different plate and blanket cylinder diameters. If the blanket on the sheetfed offset press is over-tightened on the drum and the plate is too loose on the drum, the surface speed of the blanket cylinder will exceed the surface speed of the plate cylinder, and the resulting friction will cause the dots. Increase. The dots can be ghosted in the lateral direction of the drum from one side to the other, or can be ghosted around the drum in the printing direction.
Plate protection:
1. The plate coating glue should be thin and uniform. If the glue is too thick, it is not easy to clean after drying, and it is easy to damage the dots.
2. When the printing plate is not in use, try not to face the sun. If you stop the machine for a long time, pay attention to the coating glue protection; clean the plate and take care not to scratch the plate.
3. Long downtime should be careful not to place the water roller directly on the printing plate to avoid the plate being damaged due to the long-term contact of the printing plate with the dampening solution. If these work are not done well, it will have a great impact on the outlets on the plates.
The normal working state of the printing press: ensuring the normal working state of the printing press is the key to printing and copying the network. In normal printing, the dot will increase by about 15% due to normal imprinting, etc. This is a normal phenomenon and can be compensated during the prepress processing. However, when the printing machine exhibits abnormal vibration, for example, due to improper gear meshing of the printing machine, excessive gap between the roller bearing and the bearing bush, excessive printing pressure, and jumping of the water roller, it will lead to the dot in the printing process. The transmission has a deformation of the dot (this deformation may be an increase in the dot, or simply a change in the shape of the dot), so that the lap change of the dot occurs, and the density jumps abnormally. Changes in these outlets will directly lead to faults such as bars, ghosting, parallelization, and dot loss on the printed matter.
Sanqin: In the operation of the machine, it is necessary to ensure the three-dimension, that is, to stir the ink fountain (to ensure the certain fluidity of the ink), and to compare the printed proofs (to check the ink color on the printed matter according to the amount of ink on the sample), Diligently observe the amount of water in the layout (this is a regular monitoring of the water control we mentioned earlier). The three operations mentioned above must be done frequently in the operation of the printing press. These operations can better ensure the transfer and copying of the dots during the printing process.
The above is just a question that should be paid attention to in the replication process of the network, and some simple introductions to some of the main problems, and there is no more in-depth discussion. The problems that can be encountered in the actual work should be various. It can be said that there are too many enumerations. I hope this article can play a role in inspiring the jade, and it will lead to a more in-depth discussion on how to better control the dots in printing.