The color printed image is printed by the cyan, magenta, and yellow primary inks, and various colors are displayed, so that the color original image is reproduced. In the reproduction, due to the influence of various processes and production factors, the color of the printed image cannot restore the color of the original well. In order to obtain satisfactory print image quality, the color of the image must be detected in print production, and the color reproduction quality can be controlled by adjusting certain printing variables.
1. The main factors affecting the quality of color reproduction
When the picture and text are copied on the printing machine, there are many factors that affect the color of the copied image, such as: dot expansion, printing color sequence and overprinting, ink hue and solid density, ink temperature and viscosity, ink supply (offset), paper properties, printing plate Layout depth, printing pressure, etc.
1. Expansion of outlets
In printing, dot expansion always occurs, but when the expansion exceeds a certain range, many quality problems will occur. This kind of dot expansion will reduce the contrast of the image, and make the entire image darker. The dark tone dots will be dead, and the hue of the copy will change drastically. When the dots of the image of each color in the printing are expanded at the same time, the entire image will become darker. When only one of the color dots is expanded, the copied image will produce a color cast. For example: the dot coverage of the magenta version will be expanded in the mid-tone. When the 50% dot becomes 55%, the printed image will become reddish and the flesh color will become partial. Red, neutral colors become light red, and green becomes dirty. Both printing pressure and solid ink density affect dot expansion. A small change in printing pressure will cause a significant change in the entire printed image; when the solid ink density increases, the dot expansion will increase, which will have a great impact on the midtone and dark tone colors of the image.
2. Printing color sequence and overprint
In color image printing, the ink is overprinted one by one, and poor overprinting will cause color deviation, color mixing and turbulence. The printing color sequence arrangement has a great influence on the overprint color effect. For multi-color printing presses, the printing time of each color ink is short, and the post-printing ink is overprinted on the surface of the first-printing ink, which belongs to the "wet stack wet" printing state. In the overprint color, 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 order of the two colors is reversed, the hue, lightness and saturation of the overprinted colors may be different. For example, cyan and magenta inks are overprinted, cyan is printed first and then magenta is printed, and the overprinted color is cyan; when magenta is printed first and then cyan, the overprinted color is reddish. In order to obtain a good overprinting effect, it is necessary to arrange the viscosity of each color ink reasonably after the color sequence is scheduled.
3. Ink hue and solid density
The inks used in printing production all have different degrees of color shift, so that the printed image has a color shift. As far as possible, ink with less color shift should be used for color printing. The solid ink density on the surface of the printed image determines the tone and tone reproduction range of the printed image. The higher the solid density, the wider the tone and tone reproduction range. The low solid density reduces the image color saturation and weakens the overprint color.
4. Ink temperature and viscosity
In the reproduction of printed images, ink viscosity is a very important parameter. Generally, liquid ink is used for gravure printing. There is no ink leveling and channeling mechanism in the ink delivery device. The viscosity of the ink is controlled by adding an appropriate amount of solvent to the ink tank. Offset and relief printing generally use viscous ink, which has a high viscosity. In order to transfer the ink from the printing plate to the surface of the paper evenly, both offset and relief printing machines have ink leveling and channeling devices. The ink is squeezed, sheared and separated when it is transferred and evened between the rollers of these devices. The roller works to overcome the internal friction of the ink, and the surface temperature of the ink roller is raised. During the process of being transferred and evened, it is squeezed, sheared and separated. In order to overcome the internal frictional force of the ink, the roller makes the surface temperature of the ink roller increase, and the viscosity of the transferred ink decreases accordingly. After the ink becomes thinner, the amount of ink carried on the surface of the roller is reduced, and the amount of ink transferred to the surface of the paper is reduced, which changes the tone and tone of the printed image and destroys the consistency of the printed image. Studies have shown that the reason why the color deviation of the printed image is 60% after the printing press is turned on is caused by the temperature change of the ink roller.
5. Offset printing ink balance
The water-ink balance of offset production directly affects the quality of image reproduction. A small amount of water will make the printing plate dirty and paste; the water emissary will emulsify the ink and reduce the color saturation of the printed image.
Second, the detection of printed image color
The color of the color printed image is formed by the superposition of three primary inks of yellow, magenta and cyan in different proportions. Generally, when measuring the color of a printed image, the color on the screen is not measured, but the quality control bar printed simultaneously with the printed image. The control strip is generally placed at the trailing edge of the printed sheet. Testing the corresponding color blocks of the control strip with a measuring instrument can obtain print quality information, such as the solid density, overprint rate, dot expansion, dot density, neutral gray reduction, contrast and other parameters of each primary color ink, to judge the image tone and tone reproduction.
There are three measurement methods when measuring the color of printed images, namely density meter measurement, colorimeter measurement and spectrophotometer measurement.
1. Densitometer measurement
Density meter 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 the printing industry. The density meter is cheap and widely used. When the density meter measures the color surface, it can only obtain the relative amount of a certain primary color ink in printing. It cannot indicate the hue of the color being measured. The measured value of the densitometer is not related to various color-measuring systems, so it is impossible to describe the measured color in color language. In color measurement and evaluation, the density meter has certain limitations, it is not a standard color measuring instrument.
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