1 printing control strip measurement and control principle and control parameters
Based on the theory of platemaking and print quality control, the quality of the printed image is affected by a number of process parameters. These parameters are often not independent variables but interact with each other. Maintaining the print color throughout the printing process is primarily dependent on three parameters: ink layer thickness, dot coverage, and trapping rate. Each of these three parameters has its own physical quantity that can be measured objectively. Figure 1 shows the correspondence between the three quality parameters and related physical quantities, that is, the basic principle of quality control for printing images.
The printing control bar is based on the basic principle of quality control, designing corresponding control blocks for these parameters, and printing and outputting together with the images during actual production. The key to the control strip's ability to control printing, proofing and printing quality is that it can express the value of changes in the transmission process and correctly reflect the delivery status of the outlets. The control bars of different manufacturers will be different, but in general they all control the quality of the entire print by controlling the following parameters.
1. 1 Field density
Solid density is the basis for print quality control. It not only directly reflects the degree of color reduction but also indirectly controls the thickness of the various ink layers. Because when the ink layer thickness gradually increases, the density also rises. When the thickness of the ink layer reaches a certain limit, the color begins to saturate, and the density can no longer rise. Therefore, controlling the solid density of each color ink is controlling the thickness of each color ink layer. Different printing methods have different requirements for field density. Solid density values ​​are not universal data and are generally affected by differences in paper, ink, printing sequence, and densitometer calibration, as well as operational errors. In order to effectively control the reproduction of tone values ​​in copying, the solid density of each color is controlled. At the same time, the relative contrast can also be calculated using solid patches for each printing primaries. Because of the importance of field density in print quality control, any print control strip contains a single color solid control block of various colors and is usually arranged in the first place.
1. 2 printing relative contrast
The relative contrast K can be expressed by the formula: K = (DV - D0.75 (0.8))/DV. Where DV is the solid density value of the color ink; D0. 75 (0.8) is the density value of the dot block with a dot area coverage of 75% or 80%. China uses a dot density of 75%, and Europe often uses 80. % dot density to calculate. It can be seen from the formula that when the K value is large, it means that the density difference between D0.75 (0.8) and DV is larger, that is, 75% to 80% of the dot area of ​​the dot area is expanded less, the sample step is pulled open, the level is clear, and the color is Saturation is high; K value is small, indicating that 75% to 80% of the dot area of ​​the dot area is expanded more, the density of the field is small, the sample level adjustment is not open, the level is fuzzy, and the color saturation is low. The correct relative contrast is an important indicator to measure whether the density of the foot print is clear or whether the level of the dark tone is clear. It is of great significance to improve the quality of proofs and prints.
Any print control strip also contains a monochrome 75% or 80% dot-block control segment, which is usually immediately after the density block for ease of calculation. In production control, you need to use a density meter or a colorimeter to measure the density of the corresponding color patch first and then measure the dot mass of 75% or 80%. The relative contrast can be obtained by formula calculation.
1. 3 dot gain value
The dot enlargement value refers to the difference between the actual area coverage of the web site of the substrate and the actual area coverage of the web site corresponding to the plate. For stressful printing, dot gain is unavoidable. As mentioned above, when the relative contrast exceeds the range, it is precisely to indicate that the dot area coverage (75% or 80%) has been expanded or The problem of shrinking, so that in the production process, it is easy to have a process failure of the stencil or stencil.
The control of dot gain is very important for the control of print quality. In the lithographic offset printing process, dot gain has a greater effect on copy hue change than any other variable. Each type of printing control strip has elaborately designed dot gain control sections, and is often the core part of the entire control strip. Generally, the reproduction effect of thin dots and fine dots is used to reflect the dot gaining situation. Generally, only magnifying glass is used in production. Visual inspection can determine the dot gain value. Figure 2 is a Bruner fine control section.
Figure 3 is the GATF star, which consists of 36 black and white equal wedge lines centered on a very small white point. By visually measuring the changes of the white point and the wedge line in the center of the star, it is possible to determine the conditions of the dot enlargement, dot distortion, and ghosting in the printing process. The same applies to the resolution block in the UGRA/FOGRA Postscript control bar.
1. 4 Overprinting rate
Overprint rate is the ability to indicate that the ink is attached to another ink. The purpose of controlling the trapping rate is to avoid the disadvantages of too low a trap rate when the wet or wet stacking is dry, so as to meet the requirements for tone reduction. The higher the value of the trap rate, the better the effect of overprinting.
The trap rate T is a value between 0% and 100%. When T = 100%, it shows that the superimposition rate of the post-printing color is excellent, just as it is printed directly on the substrate. If T = 0, there is an extreme "wet" or "dry" process problem that must be avoided and resolved. Different overprinting methods generally have different overprinting rates. A single-color offset printing press is a wet-press dry-overprinting. Generally, the overprinting rate is relatively high. The four-color printing is wet and wet printing, and the overprinting rate is relatively low.
The level of trapping rate has become a test item for judging the quality of printing, and the grade of printing product quality can be divided according to the size of the overprinting rate. The print control bar is generally designed to control the overprint rate by designing square two-color overprint solid blocks.
1.5 neutral gray balance
Printed neutral gray is an important indicator to monitor whether the dot gain value of each color is balanced. Accurate reproduction of neutral gray is very important for printing. The three-color overprint color block refers to a neutral gray color block generated by halftone overprinting of the yellow, magenta, and cyan three primary colors, used to print black with the same color value, and visually can quickly estimate the gray balance. The degree of change. Since the dot enlargement, ghosting, or overprinting will cause changes in the three kinds of printing chromatic colors, this change can be manifested by neutral grey and reddish, bluish, and yellowish grays.
The printed control strips are usually designed with gray tone of midtones (C50%, M39%, Y39%) and 3-color overprinted solid blocks (C100%, M100%, Y100%), which are recommended by the national printing standards. The gray balance data (C50%, M39%, Y39%), the value of the midtones mainly controls the intermediate gray balance. Due to differences in paper, ink, and printing conditions used, the gray balance data varies. The main changes were M (39% to 44%) and Y (39% to 44%).
2 Traditional Printing Control Strips
2. 1 Traditional Printing Control Strip
Traditional print control strips are now commonly used in the form of film control strips. Brunel's print control strips are used most often.
The traditional printing control strip is designed for the traditional printing process. In the color separation and filming stage, the control strip film is affixed to the film and printed together. Therefore, the print control strip not only records the quality information of the print, but also reflects the quality of the print.
2. 2 Disadvantages of traditional printing control strips Traditional printing control strips have the following disadvantages in practical use:
(1) In traditional printing, the control strip appears in the form of a film. The control strip of this physical carrier is not easy to keep and is easily lost. For a long time, scratches appear on the surface of the film, affecting the accuracy of information transmission;
(2) Although the price of traditional printing control strips is not high (generally between 3,000 and 4,000 yuan), in fact, some printers use copied after-printing alternatives in order to save costs, because the traditional form of printing control strips in the form of film At the same time, strict optical inspections have been carried out and the accuracy requirements are very high, so even a copy of the control bar has lost control of the print quality.
(3) Traditional printing control strips need to use manual labor. The technical worker first wipes the control strip with a cotton cloth and then sticks it on the film, and pays attention to the distance and position of the control strip and the tightness of the paste.
3 Digital Control Strips and Application Status
3. 1 digital printing control strip
With the development of electronic and computer technologies, CTP and digital printing have become the development trend of the printing industry. Because the CTP technology eliminates the process of color separation, the traditional print control strips cannot play a role in controlling quality. Any industrial production needs to be carried out in accordance with the stipulated quality standards. The quality standards for digital printing products can be implemented with reference to the standards formulated by traditional printing processes. However, there is no way for digital printing light to have product quality inspection standards, and specific means for product quality inspection and control are also required. This requires the design of digital control strips that can both control the printing process and control the quality of platemaking.
The digital print control strip is a print control strip that exists in the form of an electronic file and can usually be obtained by two methods of design. One is drawn using Coreldraw graphics software or written in Postscript language, while the latter has higher precision and more. advantage.
Different from the traditional printing control strips, digital printing control strips are added to electronic manuscripts and then printed out with traditional printing or direct platemaking, or even direct printing. From this it can be easily seen that the digital printing control strip has advantages over the traditional printing control strip:
(1) The digital control strip eliminates the insufficiency of the film carrier and ensures that the control strip information is not distorted permanently, so that the quality of the printed matter can be controlled more accurately and stably. At the same time, digital printing control strips in electronic form are convenient for transmission and copying, and are more suitable for use by printing companies.
(2) It can be seen from its use method that the scope of application is wider and more convenient and flexible. The digital control strip can be used not only for traditional printing but also for digital printing. In traditional printing, the customer puts the control strips together with the jobs to be printed on the computer, outputs the film, and then performs the traditional printing and printing. If it is more simple to use in digital printing, it is only necessary to make the control bar and the work piece imposition and directly change the digital information into a print. The digital control strip can be arranged or printed together with the job to be printed, so it can control the printing process more accurately than the traditional control bar. At the same time, the digital control bar can output any resolution. It is especially suitable for the detection and control of direct platemaking and printing quality. It can control the consistency of the color of the printed matter, stabilize the quality of the printed matter, and bring higher social benefits and economy to the printing enterprises. benefit.
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