Seven, color gravure color separation operations
Gravure printing is the main printing method for the color packaging and printing industry. Gravure printing has the characteristics of high gloss of ink, strong coloring, good color reproduction, large image contrast, high printing plate resistance, wide distribution of dots, and many types of substrates. The modern gravure platemaking basically adopts the plano-convex conversion technology, directly uses the electronic color separation and the flat-printing color separation picture proofing to inspect the gravure image quality and corrects it, and then the electronic color separation machine produces the electronic color separation screen gravure or is based on an electronic engraving machine. The color separation image of the electronic color separation machine makes a gravure plate roll, so that the electronic color separation machine becomes the key to the flat, convex and concave plate making technology.
1. Electronic color dot corrosion etching process
The electronic color dot corrosion and gravure process flow is shown in Figure 6.18.
The technical points of the electronic color separation in this process are: (1) Grasp the difference between gravure printing dot enlargement rule and plain printing. The dot enlargement rule of the gravure printing dot is shown in Figure 6.19. In the high school, some changes are linear and change. The rate is small, but the change in the dark tone region is convexly nonlinear, and the rate of change is large. In the figure, AB is the level curve of the gravure image, and AC is the image level curve after the etching of the printing plate. AD is the level curve after gravure printing, so when the gravure version is color-calibrated, its level range is controlled from 3% to 55% at least 5% or more lower than the offset standard. The difference between plain and concave is shown in Figure 6.20. It is not difficult to find that the difference between the gravure and the lithographic plate is mainly manifested in the difference in the rate of enlargement of the dark zone. (2) Select the gray balance curve of gravure replication. (3) Use a higher level of detail to emphasize. (4) Use the limit function to control the maximum value of gravure dots. (5) Other technical parameters can use the same data as offset printing.
2. Electronic engraving gravure electronic separation
Electronic engraving is the process of scanning the gravure roll directly by an electronic engraving machine using an electronic color separation screen printing net dot positive picture (separation signal) as a manuscript. The requirements for electronic color separation are as follows: (1) Study the rule of network dot deformation between flat and concave, as shown in Figure 6.21. The dashed line in the figure shows the relationship between the image density and the reflection density of the unprinted flattened halftone dot. The curve 2 shows the relationship between the printed dot size and the printing density. The relationship between the engraved gravure roller dots and the print density. It is not difficult to find that the level of the dark tone changes rapidly in the gravure printing process. (2) Make full use of the function of the electronic engraving machine. The electronic engraving machine is provided with multiple sets of level curves and gray balance curves. The appropriate curve should be selected from the original to control the influence of the flat concave conversion. (3) Use better clarity as much as possible. (4) Pre-compensation of the conversion relationship between flat and concave using the function of the electronic color separation machine. In a word, gravure plate making should pay attention to its mutual connection and influence in the electronic color separation, and reasonably use various functions of the electronic color separation machine to compensate and correct the influence of various factors in the color image reproduction to ensure the excellent image reproduction quality.
(to be continued)
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