In printing reproduction, the number of color printing images has a considerable proportion. The color printing image is produced by superimposing the three primary colors of cyan, magenta and yellow, so that the color original image can be reproduced. In the process of reproduction, due to the influence of various processes and production factors, the color of the printed image cannot restore the color of the original. In order to obtain satisfactory printing image quality, the image color must be detected in printing production, and the color reproduction quality can be controlled by adjusting some printing variables. 1、 The main factors that affect the quality of color reproduction are: dot enlargement, color sequence and overprint, ink hue and field density, ink temperature and viscosity, water supply (offset), paper properties, plate depth, printing pressure, etc. 1. Dot enlargement in printing, there will always be dot enlargement, but there will be a lot of quality problems when the expansion exceeds a certain range. This kind of dot enlargement will reduce the contrast of the image, darken the whole image, paste the dark dot, and make the copy hue change rapidly. When only one of the dots is enlarged, the reproduction image will produce partial color. For example, the red dot coverage will expand in the middle tone, and 50% of the dots will turn 55%. The color of the printed image is reddish, the flesh color becomes reddish, the neutral color turns pale red, and the green becomes dirty. Printing pressure and ink density affect dot gain. Slight changes in printing pressure will make the entire printing image change significantly. When dot ink density increases, the dot gain will increase, which will have a great influence on the tone color of the image. 2. In printing color sequence and overprint color image printing, the ink is one color one color overprint. Poor overprint will produce color deviation, color mixing and level disorder. The color sequence arrangement has a great influence on the color effect of overlay printing. For multi-color printing machine, the printing interval of each color ink is short, and the post printing ink is overprinted on the surface of the first printing wet ink, which belongs to the printing state of "wet lamination". In overlay printing, the ink printed on the paper surface has an advantage over the ink printed on the wet ink layer. Two colors of ink as long as reverse the color order, superimposed color hue, lightness and saturation may be different. For example, cyan and magenta ink overprint, first printing cyan and then magenta, stack printing color is cyan; first printing magenta and then cyan, overlay printing color is red. In order to obtain a good overprint effect, the color sequence should be arranged reasonably after the ink viscosity. 3. Ink hue and ink used in field density printing have different degrees of color deviation, resulting in color deviation of printed image. Ink with less color deviation should be used for color printing as far as possible. The density of the printing ink determines the range of the printing ink tone. The higher the field density, the wider the range of tone reproduction. The field density is small, so that the image color saturation is reduced, and the overprint color becomes weak. Viscosity and printing ink viscosity is a very important parameter. Generally, liquid ink is used in gravure printing. There is no uniform ink and ink channeling mechanism in the ink conveying device. The ink viscosity is controlled by adding appropriate amount of solvent to the ink tank. Offset printing and embossing generally use viscous ink, which has a high viscosity. In order to make the ink evenly transferred from the plate to the paper surface, offset and embossing machines have uniform ink and ink channeling device. The ink is transferred between the rollers of these devices and is even in the process of extrusion, shearing and separation. In order to overcome the internal friction of the ink, the roller works so that the surface temperature of the ink roller rises. In the process of being transferred and being even, it is squeezed and sheared and separated. The roller works in order to overcome the internal friction of the ink, so that the surface temperature of the ink roller increases, and the viscosity of the transferred ink decreases. When the ink becomes thinner, the ink load on the roller surface decreases and the ink transferred to the paper surface decreases, which changes the order and tone of the printing image and destroys the consistency of the printing image. Some studies have shown that 60% of the color deviation of the printing image is caused by the temperature change of the ink roller. 5. Offset ink balance offset production ink balance directly affects the image reproduction quality. A small amount of water will make the printing plate dirty and paste, and the water will emulsify the ink and reduce the color saturation of the printed image.2、 The color of color printing image is composed 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 bar printed simultaneously with the printed image. The control strip is generally placed at the trailing end of the printing sheet. Printing quality information, such as field density, overprint rate, dot enlargement, dot density, neutral gray reduction, contrast and other parameters of each primary color ink, can be obtained by measuring the corresponding color blocks of the control strip, so as to judge the image level and tone reproduction. There are three ways to measure the color of printed image: densitometer, colorimeter and spectrophotometer. 1. Densitometer measurement densitometer is the main instrument in color separation, plate making and printing. This measurement method has always been the most commonly used form of objective quality evaluation in printing industry. Densitometer is cheap and widely used. When measuring the color surface, densitometer can only obtain the relative quantity of a certain primary color ink in printing, it can not indicate the hue of the color to be measured. The measured value of densitometer is not related to various surface color systems, so it can not be described by color language. In color measurement and evaluation, densitometer has some limitations, it is not a standard color measurement instrument. 2. Colorimeter colorimeter is to obtain the visual response which is proportional to the three stimulus values x, y and Z by measuring the color surface directly. After conversion, the X, y, Z values of the tested color can be obtained, and these values can also be converted into color parameters of other uniform color spaces. Colorimeter is a special densitometer with three wide band filters. The absolute accuracy of color measurement value is not good because of the error in the device and principle of the instrument. But because of its low price, it is still a widely used color measuring instrument. 3. Spectrophotometer (spectrophotometer) color measuring spectrophotometer measuring the reflectance of color surface to light of various wavelengths in visible spectrum. The color surface is illuminated by visible light at a certain step distance (5nm, 10nm, 20nm), and then the reflectivity is measured point by point. By tracing the relationship between the reflectance of each wavelength and the wavelength, the spectrophotometer curve of the color surface can be obtained. Each spectrophotometer curve uniquely represents a color. The measured values can also be converted into other surface color system values. Spectrophotometer is a kind of flexible and ideal color measuring instrument. At present, the spectrophotometer is used in some foreign printing machines for color quality inspection.
Dongguan Beite packaging materials Co., Ltd. - located in Dongguan City, the world's manufacturing city, is a subsidiary of Beite packaging (China) Co., Ltd., with a plant area of 6000 square meters. It is a flexible packaging enterprise integrating R & D, production and sales. It has a complete set of domestic advanced production equipment, a scientific and humanized management system and a group of experienced technical personnel For backing, to provide customers with quality products and first-class service.
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