Color printing image color control and detection method
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
The color printed image is printed by various colors of cyan, magenta, and yellow primary inks, and the color original image is reproduced. In copying, due to various processes and production factors, the color of the printed image does not restore the color of the original. In order to achieve satisfactory print image quality, image color must be detected in print production and color reproduction quality controlled by adjusting certain print variables.
First, the main factors affecting the quality of color reproduction
When copying on a printing press, there are many factors that affect the color of the copied image, such as: dot enlargement, printing color sequence and overprint, ink hue and solid density, ink temperature and viscosity, water supply (offset), paper properties, plate The depth of the layout, printing pressure, etc.
1. Network expansion
In printing, dot gain will always occur, but when the amount of expansion exceeds a certain range, many quality problems will occur. This dot enlargement will reduce the contrast of the image, and make the whole image darker, dark-adjust the dot, and make the copy hue change abruptly. When the dot of each color image is enlarged at the same time, the image becomes dark as a whole. When only one of the dots is enlarged, the copied image will produce a color cast. For example, if the magenta dot coverage is expanded in the middle, and the 50% dot becomes 55%, the printed image is reddish and the flesh color becomes partial. Red, neutral color turns light red, green becomes dirty. Both printing pressure and solid ink density affect dot gain. A small change in printing pressure will cause a significant change in the entire printed image; when the density of the ink in the field increases, the dot gain will increase, which has a great influence on the color of the image in the image.
2. Printing color sequence and overprint
In color image printing, the ink is overprinted in one color and one color, and the overprinting may cause color deviation, color mixing and level disorder. The printing color sequence arrangement has a great influence on the overprinting effect. For multi-color printing presses, the printing interval 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 printing state of "wet-wet". The ink printed on the paper in the overprinted color has an advantage over the ink printed on the surface of the wet ink layer. As long as the ink of the two colors is reversed, the hue of the overprint, the brightness and saturation may be different. For example, cyan and magenta ink overprints, first printed green and then printed magenta, overprinted color is cyan; first printed magenta and then printed green, overprinted color is reddish. In order to obtain a good overprinting effect, it is necessary to arrange the viscosity of each color ink after the color sequence is scheduled.
3. Ink hue and solid density
The inks used in printing production have different degrees of color shift, which causes the printed image to have a color cast. Color printing should be performed using ink with less color shift as much as possible. The solid ink density of the printed image surface determines the print image tone and tone reproduction range. The higher the solid density, the wider the range of tone and tone reproduction. The density in the field is small, the image color saturation is lowered, and the overprint color is weakened.
4. Ink temperature and viscosity
Ink copy viscosity is a very important parameter in print image reproduction. Generally, gravure printing uses liquid ink, and there is no ink leveling and ink squeezing mechanism in the inking device, and the viscosity of the ink is controlled by adding an appropriate amount of solvent to the ink tank. Offset and embossing generally use viscous inks, which are very viscous. In order to allow the ink to be uniformly transferred from the printing plate to the surface of the paper, the offset printing and embossing machines have a leveling and squeezing device. The ink is separated by being squeezed and sheared during transfer and smoothing between the rolls of these devices. The roller works to overcome the internal friction of the ink, and causes the surface temperature of the ink roller to rise, and is separated by being squeezed and sheared during the process of being transferred and being evenly distributed. In order to overcome the internal friction of the ink, the roller works to increase the surface temperature of the ink roller, and the viscosity of the ink to be delivered decreases. After the ink is thinned, the amount of ink 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 for the 60% color deviation of the printed image after the printing machine is turned on is caused by the temperature change of the ink roller.
5. Offset ink balance
Offset production of ink and wash balance directly affects the quality of image reproduction. The small amount of water will make the plate dirty and paste; the amount of water will make the ink emulsified, which will 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 superimposing yellow, magenta, and cyan primary inks in different proportions. Generally, when measuring the color of a printed image, instead of measuring the color on the screen, a quality control strip printed at the same time as the printed image is measured. The control strip is typically placed at the tip of the sheet. Using the measuring instrument to detect the corresponding color block of the control strip, the printing quality information can be obtained, such as the solid density, the overprinting rate, the dot enlargement, the dot density, the neutral gray reduction, the contrast, and the like of the primary color inks, so as to judge the image tone and the tone reproduction.
There are three ways to measure the color of a printed image, namely density meter measurement, colorimeter measurement, and spectrophotometer measurement.
1. Densitometer measurement
Densitometers are the main instruments in color separation, plate making and printing. This type of measurement has always been the most commonly used form of objective evaluation quality in the printing industry. The density meter is inexpensive and widely used. When a densitometer measures a color surface, only the relative amount of a primary color ink in the print can be obtained, which does not indicate the hue of the color being measured. Densitometer measurements are not associated with various color systems and therefore cannot be described in color language. Densitometers have certain limitations in color measurement and evaluation. They are not standard color measuring instruments.
2. Colorimeter color measurement
The colorimeter is obtained by directly measuring the surface of the measured color to obtain a visual response proportional to the color three-stimulus values X, Y, and Z. After conversion, the X, Y, and Z values of the measured color are obtained, and these values can also be obtained. Convert to the color parameters of other uniform spaces. A colorimeter is a special density meter with three broadband filters. Due to the inherent error of the instrument's own device and principle, the absolute accuracy of the color measurement is not good. However, due to its low price, it is still a widely used color measuring instrument.
3. Spectrophotometer (spectral spectrophotometer) color measurement
The spectrophotometer measures the reflectivity of the color surface to light of each wavelength of the visible spectrum. The light of the visible spectrum is irradiated to the color surface at a certain step (5 nm, 10 nm, 20 nm), and then the reflectance is measured point by point. A spectrophotometric curve of the surface of the measured color can be obtained by plotting the relationship between the reflectance value of each wavelength of light and each wavelength. Each spectrophotometric curve uniquely expresses a color. The measured values can also be converted to other color system values. The spectrophotometer is a flexible, ideal color measurement instrument. At present, some color measuring instruments for color quality inspection of printed products equipped with foreign printing machines use a spectrophotometer.
Third, the method of controlling the quality of printing color
Constraints on the quality of printed images such as paper, ink and plates in printing production. When these factors have been determined, how to control the color quality of printed images when performing image reproduction on a printing press?
1. Adjust printing pressure according to upper body production
The printing pressure is adjusted according to the quality requirements of the printed image reproduction, the thickness of the paper, the printing plate, etc., so that the image dot is optimally reproduced.
2. Reasonable arrangement of color sequence
Currently, multicolor printing generally uses ink, cyan, magenta, and yellow or cyan, magenta, yellow, and black sequences. This color sequence makes color reproduction easier and more accurate. The thickness of the ink layer is arranged from small to large. For example, when the black, cyan, magenta and yellow sequences are used in the offset printing, the thickness of the ink layer is 0.8 μm for black ink, 0.9 μm for cyan ink, and 1. 0 μm, yellow ink is 1.1 μm; ink viscosity is arranged from large to small, such as black ink 1188Pa, blue ink 488Pa, magenta ink is 477Pa, yellow ink is 147Pa. When the viscosity of the ink does not meet the alignment requirements, the viscosity of the ink can be adjusted in advance.
3. Calibration press
The press is adjusted according to the proof sheet and the predetermined printing conditions to make the printed image effect consistent with the printed sheet.
Fourth, control the color reproduction quality during the printing process of the printing press
In the printing process, there are many variables affecting the quality of printed images. In order to compensate for the adverse effects of these variables, the printing presses produced by more advanced printing machines such as Heidelberg, Roland and Mitsubishi of Germany mainly adjust three parameters: the inking unit Temperature, ink supply and water supply.
1. Ink unit temperature control
Some presses are equipped with or with optional inking unit temperature control. In the process of inking, the ink roller rotates and sways, so its temperature rises most. When the temperature is controlled, the cooling core is introduced into the core of the ink roller to lower the temperature, so that the surface temperature of the ink roller remains stable. To ensure a stable supply of ink. In some printing machines, in addition to the ink roller, the ink fountain roller is cooled by cooling water.
2. Ink supply control
The density of the ink on the surface of the printed image directly affects the quality of the reproduced color. The density of the ink in the field is affected by the amount of ink supplied (the thickness of the ink layer on the surface of the printed image). The solid density also increases as the thickness of the ink layer increases. After reaching a certain point, the solid density will not increase as the thickness of the ink layer increases. The amount of ink supply (thickness of the ink layer) can be adjusted in accordance with the quality information of the surface of the printed image control strip.
At present, the automatic adjustment of the ink volume of the printing machine mainly includes two parts, namely, the total ink discharge amount of the ink fountain roller and the adjustment of the ink amount in each partial region of the ink roller. The overall ink amount adjustment ensures the correct reproduction of the tone and tone of the entire image of the printed image. Since the printed image is distributed throughout the layout, the demand for the amount of ink varies from area to region. Therefore, the ink fountain roller is divided into several sections along the axial direction of the ink roller. In the small ink zone, the amount of ink sent by different ink zones is different to meet the different requirements of the ink volume in each partial region of the image.
3. Water supply control
The amount of offset printing plate water must be controlled. The amount of water should vary with the printing speed and ink supply of the printing machine to ensure the balance of ink and water, so that the printed image is not smudged, the color saturation is good, and the paste is not paste. Adjusting the amount of water is achieved by changing the speed of the bucket roller.
V. Conclusion
In order to meet the quality requirements of color image printing, it is necessary to detect and control the printed image. The modern printing press meets the requirements of the printed image quality by controlling the ink temperature, the amount of ink supplied, and the amount of water supplied.