Common quality control strips in printing (1)
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When printing, it is often possible to see some small colored rectangular blocks on the printed edge (outside of the cutting line). Except for the most common ink block or color mark indicating the color of the printing plate, the other does not know what to do. So, what do these seemingly useless things that are ultimately cut off are used?
The objective criteria for assessing the quality of color image prints are mainly the tone reproduction and color reproduction of printed matter. Since the images are different, it is very difficult to directly detect a certain quality item on the printed sheet and control the printing quality. Therefore, selecting several major quality assessment indicators, designing some standard images, and combining the various standard images in different ways constitutes the print quality control bar. The print quality control bar mentioned here is what we mentioned as "useless".
The standard image constituting the print quality control strip may be an original image pixel of the image, such as a solid block or a network block; or a standard pixel that is not printed, such as a star or a line block that checks the deformation of the dot. The quality control strip is generally divided into two types: a signal strip and a test strip. The signal strip is used for visual inspection to provide qualitative information. The test strip can measure density data using a densitometer. At present, most of the printing quality control strips are composed of signal strips and test strips, which can detect the deformation of the dot, and cooperate with the density meter to check the solid density, dot gain, relative contrast, and ink overprint rate of each color printing ink. And other major quality indicators.
As shown in the figure, the GATF star is composed of 36 black wedge lines arranged in a circle with equal spacing distances. Due to the geometrical nature of this arrangement, it can be more obvious for various imprint defects such as "ghosting" and "dot increase" on the printed sheet. When “ghosting” or “dot increase” occurs, the wedge line will behave as shown below, producing an equal expansion width. Just like the expansion points of different sizes, the wedge tip is the smallest dot, and the wedge tail is the largest dot. The expansion or reduction of the same amount is clearly displayed at the center of the circle where the wedge tips are relatively concentrated.
In the printing process, if the ink supply amount is too large, the printing package lining is not suitable or the speed difference between the rollers, "ghosting", "paste plate", "mesh point deformation", etc., when the dot diameter increases by 0.025 mm In the middle of the circle, the diameter is reduced by 0.575 mm, which is about 23 times (a magnification of 23 times). This amplification of the star can help the print operator find the problem in time. There are several types of faults in the following picture, and the stars have obvious changes.
Usually, the dots on the printed sheet are not deformed or ghosted. When the ink supply amount of the printing plate is appropriate, the center of the star is whitish. If the ink supply is too large, the center of the star is a large black circle, and the black circle is larger, indicating that the larger the ink supply, the more serious the dot gain. If the ink supply of the printing plate is small, the center will have a relatively large white circle. If the dot is longitudinally deformed, the lateral deformation of the star, and vice versa, the longitudinal deformation of the star, indicating the lateral deformation of the dot. If the dot appears ghost, the middle part of the star disappears, and the remaining outline is a GATF star. Although it is a high-precision star mark, after repeated copying, the accuracy of the original star is difficult to guarantee, but The effect is not ideal when using stars, and is often used with GATF digital bars.
GATF digital signal strip. The GATF digital signal strip is shown in the figure. It has a number from 0 to 9 on the left side, and each number is a step. The number of dots in each of 1 to 7 digits increases from 3% to 5%, and the number of dots in 7 to 9 digits increases to 5% or more per step. From 0 to 9, the digital color gradually fades. The dot on the number is equivalent to 80 lines/cm, and the substrate portion is equivalent to 25 lines/cm. The dot gain value is judged by the fact that the fine dot is increased more sensitive than the coarse dot.
When printing, adjust the printing conditions and control the printing quality by observing the depth of the color display of the digital numbers around the standard digital. For example, a factory stipulates that “3” is a standard number according to the printing process conditions. In the actual printing process, when printing is normal, as shown in the figure, “3” indicates that it is not clear. The figure shows that the dot is increasing, and the number 4 is deeper and clear. Figure C shows that the dot is reduced, and the number 2 or even 1 is shallow. According to the digital deepening or shallowing steps, the dot gain value can also be roughly estimated. When the dot on the printed matter is deformed, the density of the vertical line on the signal bar will be different. "SLUR" is darker than the ground color, indicating that the dot is longitudinally deformed. As shown in the figure, the lighter than the ground color indicates the lateral deformation of the dot. .
Test strip.
GATF stars and GATF digital signal strips can only provide people with qualitative quality information, but can not provide people with quantitative data. In order to assess the amount of dot enlargement or reduction, relative contrast, and the like. Densitometers are needed to accurately measure the density of the field and the density of the dots. Therefore, there are test strips consisting of field test blocks and dot test blocks to control the print quality.
Field test blocks, generally use circular dots. There are 20%, 25%, 40%, 75%, 80% and so on. The solid block and the selected dot step block are connected together, and the solid density and dot density are measured by a reflection densitometer to calculate the dot gain and relative contrast in printing.
In addition to this, there are multicolor overprint solid blocks for measuring ink overprinting ratios, gray balanced color patches, and the like.
Let's introduce the GATF color signal strip. The GATF color signal strip is 0.25 inches wide and 20 inches long. Each 5 inches is a unit. Each unit consists of 25 1/4 inch x 3/16 inch signals. Block composition. Each signal and test block includes the following contents:
Blocks 1-7: solid blocks of monochromatic or two-color overprinting;
Block 8: Three-color overprinted solid block;
Section 9: Yellow and magenta overprint blocks of 40% network cable;
Block 10: Gray balance patches of 40%, 30% magenta and yellow;
Block 11: Black block of 40% network cable;
Block 12: Gray blocks of 40% cyan, 30% yellow, and 20% magenta;
Section 13: Black solid block;
Blocks 14-19: Fields of each color of cyan, yellow, and magenta and 40% dot color patches;
Blocks 20 to 22: Overprinted color blocks of two color 40% dots;
Blocks 23-26: GATF star.