Color control and detection of the hottest printin

Control and detection of printing image color

in printing reproduction, the number of steps of color printing image 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; Magenta is printed first and then cyan, and the overprint color is red

in order to obtain a good overprint effect, after the color sequence is arranged, it is necessary to reasonably arrange the adhesion of various colors of inks. The safety of plastic and other parts used in kitchen appliances has attracted more and more attention

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. ink temperature and viscosity

ink viscosity is a very important parameter in printing image reproduction. 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 thin, the ink load of the roller surface Jinan testing machine tension testing machine decreases during operation, 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

Color PLC touch screen control, sampling, display and other structures. The color of color printing image is formed by the superposition of yellow, magenta and cyan primary 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 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 color surface with the light of visible spectrum 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 m, green ink is 0.9 m, magenta ink is 1.0 m, yellow ink is 1.1 m; The viscosity of ink is arranged from large to small, such as 1188pa for black ink, 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 the printing machine, control the quality of color reproduction

in the printing process, there are many variables that affect the quality of the printed image. In order to make up for the adverse effects of 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 the 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 optionally be equipped with such features to improve the printing efficiency of inking device temperature control 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

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