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. The pressure of the water roller can be as long as the water roller can smoothly transfer water to the printing plate. The pressure is too large or too small to be inconvenient for printing. When the pressure is too high, it will cause the roller to jump at the mouth and cause friction. Under the pressure of the water, the mesh will be squeezed smaller or deformed. Of course, the consequences of too little pressure are obvious and the water volume cannot be transmitted normally. Of course, The ink on the layout will spread to the blank. 2. The pressure of the ink roller should be light and heavy, the pressure is too heavy, and the net points are easy to press and paste, which affects the product quality. 3. Keep the roller pressure moderate and uniform. The pressure is too large to cause the ink on the printing plate dot to produce the spreading deformation under the big pressure; The pressure is small, will cause the ink transfer rate to drop directly, the ink that transfers to the printing material is few and thin. However, if substrates with relatively smooth surfaces (such as textured paper, embossed paper, and newsprint) are used, it is necessary to increase the printing pressure appropriately to compensate for the unevenness of the surface of the substrate and improve the ink transfer efficiency so as to ensure the transfer of dots. 4. Ensure that the linear speed of the drum is consistent (as far as possible). If the linear speed difference between the two adjacent drums is large, then the transmission of the web between the drums will be deformed.
Avoid "ghosting": refers to the double outline of the same color dots, lines, or text appearing on the print. It is a directional dot gain that can cause ghosting between the plate and the blanket, or the pressure between the blanket and the impression cylinder. Excessive pressure plastically deforms the rubber surface of the blanket and changes the shape of the printed dot.
Ghosting can also occur with different diameters of the plates and blanket cylinders. If the blanket on a sheetfed press is overtight on the cylinder and the plate is loose on the cylinder, the surface speed of the blanket cylinder will exceed the surface speed of the plate cylinder, and the resulting friction will make the dots Increase. Dots can be doubled from one side to the other in the crosswise direction of the drum, and can also appear in the printing direction around the drum.
Printing plate protection: 1. The printing plate coating glue should be thin and even. If the glue is too thick, it will not be easy to clean after drying, and it is easy to damage the dot. 2. When the printing plate is not in use, try not to place it in the sun. If you shut down for a long time, pay attention to the protection of the coating glue; be careful not to scratch the printing plate when cleaning the printing plate. 3. Take a long time to shut down. Be careful not to place the water roller directly on the plate, and to prevent the plate from being damaged due to prolonged contact with the fountain solution. The failure of these tasks will have a great impact on the printing sites.
The normal working condition of the printing machine: to ensure the normal working state of the printing machine is the key to printing and copying outlets. In normal printing, dots will appear approximately 15% dot gain due to normal embossing and other reasons. This is a normal phenomenon and compensation can already be made during prepress processing. However, when the printing machine is abnormally vibrated, for example, due to improper meshing of the printing roller gear, excessive clearance between the roller bearing and the bearing pad, excessive printing pressure, and bouncing of the water roller and ink roller, etc., this can cause dots in the printing process. Transmission occurs when the dot deformation (this deformation may be a dot increase, or simply the shape of the dot changes), resulting in overlapping changes in the dot, an abnormal jump in density. Changes in these outlets will directly lead to failures such as bar, ghosting, merging, and loss of outlets.
Sanqin: In the operation of the machine, it must be ensured that Sanqin, that is, stirring ink fountains (guarantee certain fluidity of the ink), diligently compares printing proofs (according to the amount of ink used on proofs, to proofread the ink on prints), Diligently observe the amount of water in the layout (which is what we mentioned earlier?
Source: Printing Technology
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