When evaluating the quality of color prints, the copy quality of outlets is the most important indicator. Among them, many standards related to the quality of print reproduction, such as uniform ink, no deformation of dots, no ghosting on prints, ink sticks, etc. are almost all associated with the control of outlets.
Changes in the dot can cause sharpness and subtle changes, color changes and contrast, ink hue, ink density, and ink overprint. Although electronic point methods have been used to predict and prevent some dot changes, the changes in control points are still necessary to ensure print quality.
The size of the dots and their position relative to each other determine the color tone of the print. For example, a densely printed dot will form a dark tone region, and a small dot with a large pitch will form a highlight. Therefore, dot gain can greatly affect the reproduction of these regional meshes and color photographs, and has a great influence on midtones.
It is precisely for the reasons mentioned above that we have proposed the concept of network control, that is, the quality of control points in all aspects of the entire workflow of printing and copying. Due to the close connection between various processes, we must also pay attention to the coordination of quality control.
Although the term of digital printing has been proposed for many years, in the relatively widely used traditional printing methods, the conversion of network point information between devices still uses analog information transmission, precisely because this transfer is achieved using analog information. , it will inevitably cause the network itself to appear in the equipment between the changes we do not want to see (such as the increase in network points, network outlets, network distortion and loss of network points or level).
Now we have more common dot-printing (the printing methods mentioned in this article are all offset printing). The workflow for product replication is this:
Manuscript → Separation (mainly used now is scanning or electronic color separation) → Image processing (including adjustment and compression of gradation, graphic mixing, color separation, screening, etc. This process is mostly done by computer. )→Imprinted version→Out of sheet (exposure, development, fixing)→Stencil printing (exposure, development, and baking for large prints)→Printing on the machine→End of printing.
The above is a relatively common work flow. The dots are formed from the image processing stage through to the printing process, through the light, film, plate, printer (mainly plate and blanket), ink, and substrate. To complete, but due to many inherent reasons such as equipment and technology, the nature of the transmission itself determines that changes in the network are 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 transmission means that no matter how we do it, we can't completely avoid changes in the network during these transmissions (the above-mentioned network distortions, network enlargement, and shrinking). In actual production, what we need to do is to control the change of outlets within a reasonable and predictable range, and at the same time standardize the production process and reduce the fluctuation of the changing parameters in the production process.
In the process of production, we can encounter the following changes in the process of network copying:
1. The narrowing of network points means that the area of ​​the dots obtained by copying is reduced relative to the upper layer (upper step) carrier of the dot.
2. Dot enlargement: With respect to the upper layer carrier of the dot, the dot area obtained after copying increases, and the resulting replica becomes denser.
The dot gain can be further divided into mechanical dot gain and optical dot gain.
"Machine dot gain" (also known as physical dot gain) is the physical increase in the dot size. The most common form of mechanical dot gain is non-directional, and if it is excessively large, it may cause stencil. This increase is caused by the ink film thickness, the type of substrate, the number of screens, or the transfer of ink under pressure between the blanket and the plate, and between the blanket and the substrate.
"Optical dot gain" is a visual phenomenon formed by the light absorption characteristics of the ink and the light scattering properties of the substrate. When light is irradiated on a non-image area, that is, a "blank area", the light is diffused, and a part of the light in the vicinity of the halftone dot is suppressed. This light is considered "absorbed" because it cannot be reflected back into the viewer's eyes. The dots appear to be larger than the actual density and size, and it seems as though dot gains have occurred.
3. Dot distortion: The shape of the dots obtained after copying has changed. For example, the original circular dot may become elliptical after copying, or there may be no fixed dots.
The three problems mentioned above have a great influence on the entire copying stage of the printed matter. However, when we can more accurately predict the transmission of the changes made by the outlets between devices, we can more effectively compensate for the changes that will occur before the outlets change. For example, if an outlet of a print product changes during the copying process, the copy of the positive image is reduced by 2%; the ban is reduced by 4%; the printing network is increased by 15%; and the optical magnification of the dot is 12%. Then after a simple addition and subtraction of mathematics, we will find that as long as the network points are pre-compensated by 21% before the network starts to transmit, we will just get the effect we want after delivery. Although the actual dot compensation settings are not as simple as those mentioned above, this compensation and dot change is not just a linear relationship, and different percentages of dot changes in the replication process are not the same, but we only need Knowing the above compensation relationship is sufficient to tell the software what changes will occur (typically, the rate of change of the network at 50%), and the rest can be given to the computer.
The above reasoning seems to be very simple, but on actual operation, we will find that due to the numerous types of work involved in the transmission of outlets, it is not easy to truly standardize management and standardize operations. However, this is an indispensable and important aspect of the copying of prints on the printed matter. Only by strictly implementing the standardization of management and the standardization of operations can it be possible to achieve a definite purpose in pre-press image processing and network point compensation. This is precisely how many Printing companies can't do it. The actual situation is often that the platemaking and printing do each one, which is very unfavorable for the correct reproduction of the outlets.
In the following, we will discuss some issues that need to be addressed in actual production and the factors that are likely to cause abnormal deformation of the dots.
The choice of printing materials: These include paper, inks, blankets, printing plates, wetting fluids, and more. The choice of these raw materials will often directly affect the reproduction quality of the outlets in the printing industry. A successful online presence is not only a better quality of our printed products, but also a minimum cost within the allowable quality range.
The choice of platemaking process: This is a very broad problem, but it involves the reproduction of outlets, and the most important one is nothing more than the shape of the outlets and the number of screening lines. The correct choice of dot shape can have a great influence on the surface effect of prints, and it is more important to note that the nature of each dot shape is different. For example, a square dot has a large tempo jump at 50%, and the total dot gain of a square dot is also larger than that of other dot dots, so the color screening process of high-grade prints rich in midtones is performed. You cannot use squares, but consider diamonds or elliptical dots first. The higher the number of lines in outlets, the richer the level of display at the outlets of the outlets will be, and the screen will appear more delicate, but at the same time, the length of the side of the entire layout will increase, according to the theoretical conditions for the increase of outlets. We know that the trend of outlets is also increasing. In many cases, blindly increasing the number of outlets leads to a decrease in the quality of the outlets. Therefore, in actual production, we must weigh the number of outlets according to the comprehensive conditions of many factors such as printed paper, ink, and machinery. . With the same percentage of outlets, the increase in outlets with different number of outlets: When the percentage of outlets is the same, and the increase in outlets is the same, the higher the outlets, the greater the increase rate of outlets; the same percentage of outlets Next, and the dot gain value is the same, the higher the cable number is, the larger the percentage area of ​​the dot is.
Printing: 1. Exposure. In the process of exposure of the PS plate, due to light penetration and other reasons, the dots on the plate will actually be smaller than the dots on the film. Generally, this amount of shrinkage should be controlled within 3% to 5%. However, this figure is not without change, due to the vacuum of the printing equipment, the transparency of the printing plate glass, the control of the printing time, the quality of the plate itself has a lot of different situations, so we must always use the printing plate Test the dot changes in the print, and then feed the data back to the pre-press image processing unit. 2. Develop: Regularly test the concentration of the developer. Determine the development time by the results of the test. If the development time is still used after the developer is fatigued, there will be no significant net edge, resulting in an increase in dot. During the printing process, the regular inspection of the working conditions of the printing machine and the development conditions are necessary. This process uses the printing plate special test strips, and the results obtained are promptly fed back to the prepress. Advance the reaction.
The effect of the fountain solution: a. The pH of the fountain solution: The pH of the fountain solution during the printing process should not be too low, preferably about 5 to 6, because the pH of the fountain solution is too low to affect the transfer of the offset ink. Large, at the same time will cause corrosion of the printing plate, reduce the printing plate printing rate, resulting in the loss of high-profile dots, or other printing problems; b water on the plate: the water thickness of the blank part of the plate is generally 0.5 to 1 Micron, ink film thickness of 2 to 3 microns. If the water on the printing plate is too large, it will erode the ink film in the graphic part, resulting in empty dots; if the ink film thickness is large, the ink will spread to the blank part under the pressure, resulting in an increase in dot .
Effect of ink: 1. Moderate fluidity. The cohesive force of the ink with too small fluidity is too strong to be evenly milled, thereby reducing the transferability of the ink; if the fluidity is too large, the adhesion of the ink may be poor, and the dots printed out may be flat, dim and even appear to spread around. Affect the quality of the product. Also pay attention to the printing ink products when the ink flow is smaller than the text, so that the printed outlets are clear and full. 2. Select the appropriate ink. The texture of the ink is required to be fine, smooth, good in flowability, proper in viscosity, without the disadvantage of retardation, vivid in color, and the ink is preferably translucent.
Reasonable emulsification of ink: This is an interesting phenomenon. Comparing offset printing with other printing methods, it is often due to ink emulsification that the color saturation is not enough. Therefore, everyone in the actual production is trying to reduce the emulsification of the ink in order to be able to To achieve a more vivid ink effect. However, the actual situation is that there is no reasonable ink emulsification (by experiments we know that when the ink emulsification reaches a desired emulsification state of 26%), the ink cannot be between the ink roller, the ink roller and the plate, the printing plate and the eraser A good transfer between cloths (because water forms a water film on the surface of the ink and hinders the transfer of ink), the direct consequence can lead to the deformation or emptiness of the dots. In addition, when the emulsification value of the ink exceeds 30%, a clear boundary between the water film and the ink film will be lost, and edges, blurs, and the like will occur at the edges of the dots. The more serious consequence is that the pigment particles in the ink are precipitated from the resin. Accumulation on the printing blanket can also affect the ink transfer.
Pressure: 1. Water roller pressure as long as the water roller can smoothly and evenly transfer water to the printing plate
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