Quality control scheme of CTP printing plate
The CTP system is part of the digital workflow, so digital control methods are essential for quality assurance. The digital control plate can control the image quality of the CTP printing plate reasonably and effectively.
(1) Standard digital plate-making measurement and control strip
The digital measurement and control strips used for CTP plate control mainly include GATF digital plate control strips, Ugra / Fogra digital plate control strips, Kodak digital plate control strips, and Heidelberg digital plate control strips. Among them, the most widely used and the most important are Ugra / Fogra digital plate control strips and GATF digital plate control strips.
â‘ Ugra / Fogra digital plate control bar
The control bar contains six function blocks and control areas:
Information area: including output device name, PS language version, screen line number, dot shape, etc.
Resolution block: contains two semicircular areas. The lines are emitted from one point and are arranged in the form of rays. The density of the rays is consistent with the theoretical resolution of the output device. Form a more or less, open or closed quarter circle in the center of the line. The smaller and rounder the two quarter circles, the better the quality of focusing and imaging. The left side is the Yang line, and the right side is the Yin line.
Linear block: It is composed of horizontal and vertical micro lines, which are used to control the resolution of the printing plate.
Checkerboard area: Checkerboard grid cells composed of 1 × 1, 2 × 2, 3 × 3 and 4 × 4 (pixels × pixels). Control the resolution of the printing plate and show the differences in exposure and development techniques.
Visual Reference Ladder (VRS): Control the image transfer of the printing plate.
Mesh Scale: Mainly used to determine the tone transfer characteristics of the printing plate through measurement. At the same time, the 1%, 2%, 3% and 97%, 98%, 99% color blocks provided can also be used to visually judge the tones that can be copied in the high-tone and dark-tone areas.
Among them, the visual reference ladder (VRS) is a special feature of the Ugra / Fogra digital plate control bar. It is the basic element for image transfer control, controlling the stability of the printing plate, and standardizing the production process of digital printing plates. The VRS contains a pair of thick wire reference blocks, and around it is a fine screening area. There are a total of 11 VRS in the control bar, and it increases in 5% increments in the network area from 35% to 85%. In the ideal state and linear replication, the two regions in VRS4 should visually have the same tone value. But in reality, the VRS of the two regions with the same tone is higher or lower than that of VRS4, depending on the type of printing plate and the selected calibration conditions. VRS is a very ideal process control block. It can visually indicate the difference from the selected conditions for visual inspection without using measurement.
â‘¡ GATF digital plate-making control strip
Information area: including output device name, PS language version, screen line number, dot shape, etc.
Horizontal and vertical thin lines of positive and negative images: test the resolution of the system and control the exposure intensity.
Checkerboard area: Checkerboard grid cells composed of 1 × 1, 2 × 2, 3 × 3 and 4 × 4 (pixels × pixels).
Micron arc area: positive and negative micron arcs. Using the smallest set size to detect the system with arc segments, the micron arc pattern is the most severe challenge to the system. If a system maintains good details on the arcs of the positive and negative images at the same time, it indicates that the system has good exposure conditions.
Star object: test system's exposure intensity, resolution and tone transfer characteristics.
The remaining part is two sets of matching step ladders. The difference between the two tone scales is that the upper one bypasses the RIP compensation procedure applied to other files, while the lower one does not bypass the compensation setting. The comparison of the two ladders clearly shows the impact of the compensation procedure. To use the gradation scale, you must first use a magnifying glass to observe the limitation of the highlight and dark tone of the image system, and then use a density meter to measure the gradation scale from 10% to 90%, thereby constructing a dot expansion curve. [next]
(2) Design and manufacture digital control plate
In the actual use process, in addition to the above standard measurement and control strips, users can also design some patterns as measurement and control tools according to their own system characteristics, such as designing commonly used microwires and stars for test exposure and resolution. Users can create microwires and stars in CorelDraw or Illustrator, or edit them in postscript language.
A. Microwire Microwire consists of straight or curved Yin and Yang lines of different thickness. The size of the line is determined by points, from 1 point to 0.01 points, where 1 inch = 72 points = 25.4mm, 1 point ≈ 0.35mm. The line type includes four directions: horizontal, vertical, 45 degrees, -45 degrees, and semicircles. The semicircular lines and horizontal lines meet at the junction of the Yin and Yang diagrams.
Microwires can reflect the performance and imaging of the platesetter in all directions. If the thinner curve can be accurately restored, it means that the exposure imaging system of the platesetter has good performance. In addition, this micro-line can reflect whether the exposure is appropriate. If you can clearly see that the Yin line and the Yang line are well connected on the positive pattern plate, it means that the exposure is appropriate; if the Yin line is obviously thicker than the Yang line, or the part of the Yang line disappears at the joint, it means that the exposure is overexposed and the exposure should be reduced; The female line is obviously thinner than the male line, or some of the female line at the joint disappears, which means that the exposure is insufficient and the exposure should be increased.
B. Star
The star is composed of wedge-shaped strips with a width of 5 degrees and an interval of 5 degrees. A total of 36 pieces are evenly distributed in a circle. The tip of the wedge faces the center of the circle and is the smallest dot; the tail of the wedge faces the round edge and is the largest dot. The wedge is thinnest at the center and eventually disappears to form a blank center. The size of the blank center is determined by the amount of image detail lost during the plate making process. Due to the geometric characteristics of this arrangement, the star can reflect the resolution of the system. The higher the resolution of the system, the clearer the center of the star, but when the system resolution is too high, it will cause the expansion of the dot; on the contrary, the expansion of the center of the star indicates that the system's low resolution or inappropriate media exposure, Cause the loss of outlets.
In addition, the geometric characteristics of the star can also show printing failures such as ghosting or overprinting. The center of the non-circular star indicates the difference in the directionality of the system. If ghosting occurs, the center of the star is an ellipse, and the short axis indicates the direction of ghosting; if overprinting occurs, the center is an "8" shape, and the short axis indicates the direction of double ghosting.
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