On the quality control of color reproduction in th

2022-07-24
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Color reproduction quality control in the printing process (Part 1)

1. Color printing quality evaluation

1. there are five key factors for the quality of printing images

① image clarity. That is, whether the change of image edge density is consistent with human visual sensitivity mainly depends on the dot enlargement in plate printing and printing

② image hierarchy. That is, the visually distinguishable density level in the image mainly depends on the contrast expansion and reduction between prepress and printing

③ image color. That is, the reproduction accuracy and neutral gray balance of various colors of an image mainly depend on the color separation mechanism, the quality of photosensitive materials, paper, ink, etc

④ uneven printing. For the phenomenon of uneven picture caused by ink bars, ink spots, uneven ink film and paper failure in the printing process of the image, it is measured by densimeter

⑤ printing repetition rate. In order to ensure the stability of printing quality, the printing quality is required to reach a high repetition rate. In practice, the average quality value is obtained through automatic control technology and expressed by statistical method

2. printing can reflect product quality by measuring nano materials brush quality evaluation method

① the main method of traditional printing quality evaluation printing image quality evaluation method, no matter what method is adopted, will eventually come down to three aspects: image clarity, level and color. The evaluation methods can be divided into subjective evaluation and objective evaluation

subjective evaluation of printing image is a method to evaluate image quality based on experience. It combines aesthetic factors, technical factors and consistency factors to evaluate the image reproduction process and quality, but it can not accurately put forward the process control indicators and process deficiencies, and has poor repeatability and large errors. In recent decades, with the continuous improvement and stability of printing equipment, materials and processes, and the application of detection instrument theory and methods, the method of objectively evaluating printing image quality with data based on experimental research has been widely used. The main evaluation parameters are as follows:

· resolution: it refers to the ability of image reproduction to reproduce small details of the original

· sharpness: it is the speed at which the edge density of printed image details changes, and it is the comprehensive reflection of image details comparison and visual and psychological effects

· K value: also known as relative contrast or printing contrast. K value can not only control the field density, but also control the expansion of hrb:20 ~ 100. It is a better quality control method

· color deviation, gray scale and color efficiency can better evaluate color reproducibility, but they can't do anything about level and definition

· multi index weighted statistical analysis, which is a method to calculate the quality evaluation score according to the set formula according to the measurement data and weight of the printed image

② method of evaluating printing image quality with point characteristics

no matter what method is used for modern printing image, the continuous tone image of the original must be transformed into a halftone point image, that is, the image density must be changed through the change of point size or number of points in the unit area, so as to faithfully reproduce the original. All kinds of image quality factors (such as contrast, sharpness, color, level, uniformity, gloss) depend on a common element - point and the optimization of point reproduction. Point reproducibility refers to the effect of point change increment on the reproduction quality during the point formation and transfer from the original to the printed matter. The most important quality change factor in quality control is point enlargement, and quantitative description of point enlargement is the key, that is, describing point increment data by establishing a mathematical model of point and describing point transfer. The point mathematical model can not only accurately describe the characteristics of the point shape and determine the point area that cannot be measured by the densimeter, but also provide a theoretical basis for finding the best point shape, accurately calculating the point increment and making various precision measurement and control strips

II. Methods to control the color quality of printing images

1. preparation before printing

① inspection of machine equipment

· inspection of paper feeding and receiving system

inspection of paper feeding and receiving system of printing machine is usually checked by listening, looking and trying. "Listening" means that the operator judges whether the paper feeding is smooth and the paper receiving is neat through his own hearing; "Look" means that the operator checks the paper feeding and receiving of the system through his own vision; "Test" is to use waste chips for test run to further check the operation of the paper feeding and receiving systems. "Test" is the key here. Practice has proved that in order to ensure the normal operation of the paper feeder in the printing process, we must pay attention to: a good air pump to ensure the required positive pressure (blowing) and negative pressure (suction). This sentence has become a resonance in the industry. There is no leakage in the air path and pipeline, and the air valve has the correct cooperation with various mechanical parts. Do not change the components after adjustment

· inspection of printing pressure

printing presses transfer ink and fountain solution by pressure. The main pressures are: the pressure between the ink rollers of the inking system and the pressure of the ink rollers on the printing plate; The pressure between the water rollers of the water conveyance system and the water rollers on the printing plate; Pressure between plate cylinder, rubber cylinder and impression cylinder. Only under the correct action of these pressures, can the ink balance, the ink transfer and transfer evenly, and realize the color restoration. The pressure between the three rollers is the key to ensure the printing quality. In practice, we should adhere to the "three leveling" operation method. The so-called "three leveling" means that the blanket should be leveled, the roller axis should be leveled, and the ink roller and water roller should be leveled

② inspection of printing plate

before the printing plate is put into operation, a comprehensive inspection shall be carried out. The inspection of printing plate shall closely focus on the correctness of the contents of the printing plate, the image quality and the overall quality of the printing plate. The inspection is divided into the following four aspects:

. Inspection of appearance, specifications and dimensions

inspection of printing plate appearance. Usually, align the printing plate with the light, gently touch the printing plate with your hand, and check whether the printing plate is scratched, scratched and uneven. If foreign matters are found, clean it. If irregularities can be corrected, correct it. Otherwise, replace the printing plate in time

inspection of specifications and dimensions. The specifications and dimensions shall be checked against the construction sheet. In particular, pay attention to the following requirements: a. the specification and size of the text content and the location of the text on the printing plate, such as whether the central axis is centered. In addition, pay more attention to the fact that the picture and text of a certain color may only be in a local position of the printing plate. B. Necessary processing positions for printing production, such as undercut, shall be reserved on the printing plate. The bite size of printing machines of various specifications is different. The operators of plate printing and printing machines must be familiar with the bite size of the printing machine. The machine personnel shall check whether the bite size on both sides of the printing plate is the same according to the bite size of the machine, and see whether the picture and text are centered

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