On the control and detection of printing image col

2022-10-15
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On the control and detection of printing image color

in printing reproduction, the number of color printing images has a considerable proportion. The color printing image presents various colors through the superposition printing of cyan, magenta and yellow primary inks, so that the color original image can be reproduced. In reproduction, due to the influence of various processes and production factors, the color of the printed image cannot be well restored to the color of the original. In order to obtain satisfactory printing image quality, we must detect the image color in printing production, and control the color reproduction quality by adjusting some printing variables

first, the main factors affecting the quality of color reproduction

when copying on the printing machine in text and text, there are many factors affecting the color of the copied image, such as dot enlargement, printing color sequence and overprint, ink hue and field density, ink temperature and viscosity, water supply (offset printing), paper properties, printing plate layout depth, printing pressure, etc

1. dot expansion

in printing, dot expansion will always occur, but when the expansion exceeds a certain range, many quality problems will occur. This kind of dot enlargement will reduce the contrast of the image, and make the whole image darker, and the dark spots will be blurred, so that the duplicate hue will change sharply. When the dots of various colors of the image are enlarged at the same time in printing, the whole image will become darker. When only one of the color dots is expanded, the copied image will be biased. For example, the magenta version dot coverage will be expanded in the middle note. When 50% of the dots become 55%, the printed image color will be biased red, the flesh color will become biased red, the neutral color will become light red, and the green will become dirty. Printing pressure and field ink density both affect point expansion. Small changes in printing pressure will make the whole printing image change significantly; When the ink density on the spot increases, the dot enlargement increases, which has a great impact on the harmonic and dark colors in the image

2. printing color sequence and overprint

in color image printing, the ink is one color one color overprint, and poor overprint will produce color deviation, color mixing and hierarchy disorder. The arrangement of printing color sequence has a great influence on the effect of overprint. For multi-color printing machines, the printing interval of each color ink is short, and the post printing ink is overprinted on the surface of the first wet ink, which belongs to the "wet overprint" printing state. In overlay printing, the ink printed on the paper has an advantage over the ink printed on the surface of the wet ink layer. As long as the color sequence of two colors of ink is reversed, the hue, lightness and saturation of overprint color may be different. For example, cyan and magenta inks overprint, first cyan and then magenta, and the overprint color is cyan; Printing magenta first and then India. The development and Reform Commission, the Ministry of finance, and the Ministry of industry and information technology jointly issued the implementation plan for the upgrading of key materials, which is blue, and the overprint color is red

in order to obtain a good overprint effect until they break, the viscosity of each color ink should be reasonably arranged after the color sequence is arranged

3. ink hue and field density

the inks used in printing production have different degrees of color deviation, which makes the printed image color biased. The inks with less color deviation should be used for color printing as far as possible. The ink density on the surface of the printed image determines the level and tone reproduction range of the printed image. The higher the field density is, the wider the range of tone reproduction is. The field density is small, which reduces the color saturation of the image and weakens the overprint color

4. Temperature and viscosity of ink

together with the two departments, the notice on launching the first insurance compensation work of new materials in 2017 (ljxz [2017] No. 188) was issued. In the reproduction of printed images, ink viscosity is a very important parameter. Generally, gravure printing adopts liquid ink. There is no ink homogenizing and ink channeling mechanism in the ink conveying device. The ink viscosity is controlled by adding an appropriate amount of solvent to the ink tank. Offset printing and embossing generally use viscous ink, which is very viscous. In order to transfer the ink evenly from the printing plate to the paper surface, offset printing and embossing machines have ink leveling and ink channeling devices. In the process of transferring and homogenizing the ink between the rollers of these devices, it is squeezed, sheared and separated. The roller works to overcome the internal friction of the ink, and the surface temperature of the ink roller rises. In the process of being transferred and being beaten, it is squeezed and sheared to separate. The roller works to overcome the internal friction of the ink, which increases the surface temperature of the ink roller and reduces the viscosity of the transferred ink. After the ink becomes thinner, the ink load on the roller surface decreases, and the amount of ink transferred to the paper surface decreases, which changes the order and tone of the printed image and destroys the consistency of the printed image. Studies have shown that 60% of the color deviation of the printed image after the printing machine starts printing is caused by the temperature change of the ink roller

5. Offset ink balance

offset production ink balance directly affects the quality of image reproduction. Small amount of water will make the printing plate dirty and paste; The amount of water will emulsify the ink and reduce the color saturation of the printed image

II. Detection of printing image color

the color of color printing image is formed by the superposition of yellow, magenta and cyan inks in different proportions. Generally, when measuring the color of a printed image, it is not the color on the screen, but the quality control strip printed at the same time as the printed image. The control strip is usually placed at the trailing tip of the printing sheet. Using measuring instruments to detect the corresponding color blocks of the control strip can obtain printing quality information, such as the field density, overprint rate, dot expansion, dot density, neutral gray reduction, contrast and other parameters of each primary color ink, so as to judge the image gradation and tone reproduction

there are three measurement methods when measuring the color of printed images, namely densitometer measurement, colorimeter measurement and spectrophotometer measurement

1. densitometer measurement

densitometer is the main instrument for color separation and manufacturing, which has led many PLA material manufacturers to rush into this industry. This measurement method has always been the most commonly used form of objective evaluation of quality in the printing industry. Densitometer is cheap and widely used. When the densimeter measures the color surface, it can only obtain the relative amount of a primary color ink in printing, and it cannot indicate the hue of the measured color. The measured value of densimeter is not related to various surface color systems, so the measured color cannot be described in color language. In color measurement and evaluation, densitometer has certain limitations. It is not a standard color measurement instrument

2. colorimeter color measurement

colorimeter directly measures the surface of the measured color to obtain the visual response proportional to the three stimulus values x, y and Z of the color. After conversion, the X, y and Z values of the measured color can be obtained, and these values can also be converted into the color parameters of other uniform color spaces. Colorimeter is a special densitometer with three broadband color filters. The absolute accuracy of the color measurement value is not good because the device and principle of the instrument are thick in a certain error. However, because of its low price, it is still a widely used color measuring instrument

3. spectrophotometer

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