Control and detection of the hottest printing imag

Control and detection of printing image color (Part 2)

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

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 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 (spectrophotometer) color measurement

spectrophotometer measures the reflectivity of the color surface to the light of each wavelength of the visible spectrum. Irradiate the visible spectrum light on the color surface - sub total at a certain step distance (5nm, 10nm, 20nm), and then measure the reflectivity point by point. By tracing the relationship between the reflectivity value of each wavelength and each wavelength, the spectrophotometric curve of the measured color surface can be obtained. Each spectrophotometric curve uniquely represents a color. The measured value can also be converted into other color system values. Spectrophotometer is a flexible and ideal color measuring instrument. At present, some foreign printing machines are equipped with color measuring instruments for printing color quality inspection, which use spectrophotometer

III. methods to control printing color quality

in printing, paper, ink, printing plate and other printing image quality constraints. When these factors have been determined, how to control the color quality of printed images when image reproduction production is carried out on the printing machine

1. adjust the printing pressure according to the production of the upper body

adjust the printing pressure according to the printing image reproduction quality requirements, paper thickness, printing plate and other conditions, so that the image points can be best reproduced

2. Reasonably arrange the color sequence

at present, multi-color printing generally adopts ink, cyan, magenta and yellow sequence or cyan, magenta, yellow and black sequence. This color sequence makes color reproduction easier and more accurate. The ink layer thickness is arranged from small to large. For example, when black, cyan, magenta and yellow sequences are used in offset printing, the ink layer thickness is: black ink is 0.8 μ m. Green ink is 0.9 μ m. Magenta ink is 1.0 μ m. Yellow ink is 1.1 μ m； The viscosity of ink is arranged from large to small. For example, black ink Wanhua chemical (Beijing) Co., Ltd. has developed paper honeycomb glass fiber reinforced (PHC) polyurethane system materials with excellent performance, which are 1188pa, 488pa for green ink, 477pa for magenta ink and 147pa for yellow ink. When the ink viscosity cannot meet the arrangement requirements, the ink viscosity can be adjusted in advance

3. Calibrate the printer

adjust the printer according to the proofing sheet and the established printing conditions to make the printing image effect consistent with the printing sheet

IV. in the printing process of printing machine, give customers more real introduction to control the quality of color reproduction

in the printing process, there are many variables that affect the quality of printing image. In order to make up for the adverse effects caused by these variables, the printing machines produced by more advanced printing machines such as Heidelberg, Roland and and Mitsubishi in Japan mainly adjust three parameters: the temperature of inking device, the amount of ink supply and the amount of water supply

1. Temperature control of inking device

some printing machines are equipped with or can be optionally equipped with temperature control device of inking device. In the process of inking, the ink channeling roller rotates and moves, so its temperature rises the most. During temperature control, the cooling water is usually introduced into the shaft core of the ink channeling roller to reduce the temperature, so that the surface temperature of the ink channeling roller remains stable, so as to ensure a stable ink supply. In addition to the ink channeling roller, cooling water is introduced into the ink bucket roller of some printing machines to cool down

2. ink supply control

the field ink density on the surface of the printed image directly affects the reproduction color quality, and the field ink density is affected by the ink supply (the thickness of the ink layer on the surface of the printed image). As the ink layer thickness increases, the field density also increases. After reaching a certain point, the field density will not increase with the increase of ink layer thickness. The ink supply amount (ink layer thickness) can be adjusted according to the quality information of the printed image surface control strip

at present, the automatic adjustment of ink volume of printing machine mainly includes two parts, namely, the overall ink output of ink bucket roller and the adjustment of ink volume in each local area of ink roller. The overall ink volume adjustment ensures the correct reproduction of the order and tone of the whole picture of the printed image. Due to the different distribution of the printed image on the whole page, the demand for ink volume in each area is also different. Therefore, the ink bucket roller is divided into several small ink areas along the axis direction of the ink roller. The ink volume sent out by different ink areas is different to meet the different demand for ink volume in each local area of the image

3. water supply control

the water volume of offset printing plates must be controlled. The amount of water should vary with the printing speed and ink supply of the printing machine, so as to ensure the balance of water and ink, so that the printed image will not be blurred, the color saturation is good, and the plate will not be pasted. Adjusting the amount of water is achieved by changing the rotation speed of the bucket roll

v. conclusion

in order to meet the quality requirements of color image printing and reproduction, it is necessary to detect and control the printed image. Modern printing machines make the printing image quality meet the requirements by controlling the ink temperature, ink supply and water supply

information source: packaging and printing

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