1. Dot expansion

In printing, there will always be network expansion, but the expansion of more than a certain range will produce a lot of quality problems. This kind of dot expansion will reduce the contrast of the image, and make the entire image darker, dark dot paste dead, so that the copy hue changes sharply, when the printing color image dot expand at the same time, the image deepens as a whole. When only one of the color dot expansion, the copied image will produce color bias, for example: magenta plate dot coverage in the medium tone occurs expansion, 50% of the dot becomes 55%, the printed image color becomes red, the flesh color becomes red, the neutral color becomes light red, and the green becomes dirty. Printing pressure and field ink density both affect dot expansion. A small change in printing pressure will make the entire printed image significantly change; When the density of the field ink increases, the dot enlarges and increases, which has a great influence on the color of the image.

 

2. Printing color sequence and overprint

In color image printing, the ink is overprinted one color after another, and the poor overprinting will produce color deviation, color mixing and layer disorder. The printing color sequence has a great influence on the overprint color effect. For multi-color printing presses, the printing interval of various colors of inks is short, and the post-printing ink is overprinted on the surface of the previous printing ink, which belongs to the printing state of "wet overlap wet". Inks printed on paper in superimposed colors have an advantage over inks printed on the surface of wet ink layers. Two colors of ink as long as the color order is reversed, the color of the overprint color, brightness and saturation may be different. Such as cyan and magenta ink overprint, first print green and then print magenta, overprint color partial cyan; First print magenta, then print green, overprint color red. In order to obtain a good overprint effect, the viscosity of various colors of ink should be reasonably arranged after the color sequence is arranged.

 

3. Ink hue and field density

 

4. Temperature and viscosity of the ink

In printing image reproduction, ink viscosity is a very important parameter. Generally, liquid ink is used in gravure printing, and 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 printing and relief printing generally use viscous ink, this ink viscosity is very large. In order to make the ink evenly transferred from the plate to the surface of the paper, offset printing and letterpress printing machines have ink leveling and ink channeling devices. The inks are squeezed and cut apart as they are passed and beaten between the rollers of these devices. The roller does work to overcome the internal friction of the ink, and the surface temperature of the ink roller rises, and is squeezed and cut apart during the process of being transferred and beaten. In order to overcome the internal friction of the ink, the roller does work, and the surface temperature of the ink roller increases, and the viscosity of the ink transferred decreases. After the ink becomes thinner, the ink load 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 order and tone of the printed image and destroys the consistency of the printed image. Studies have shown that after the printing press is turned on, 60% of the color deviation of the printed image is caused by the temperature change of the ink roller.

 

5. Offset ink balance

 

 

 

1. Densitometer measurement

 

2. Colorimeter measures color

Colorimeter is to directly measure the color surface to obtain the visual response proportional to the color stimulus value X, Y, Z, after conversion to get the measured color X, Y, Z values, these values can also be converted into other uniform color space color parameters. A colorimeter is a special density meter with three wide-band color filters. The absolute accuracy of the color measurement value is not good because of the error of the device itself and its principle. However, because of its cheap price, it is still a widely used color measuring instrument.

 

3. Spectrophotometer (spectral spectrophotometer) measures color

A spectrophotometer measures the reflectance of a color surface to light at various wavelengths of the visible spectrum. The visible spectrum of light is irradiated at a certain step (5nm, 10nm, 20nm) on the color surface, and then the reflectivity is measured point by point. The relationship between the reflectance value of each wavelength and each wavelength can be traced to obtain the spectrophotometric curve of the measured color surface. Each spectrophotometric curve uniquely represents one color. The measured values can also be converted to other color system values. The spectrophotometer is a flexible and ideal instrument for color measurement. At present, some foreign printing machines equipped with printing color quality inspection of the color measuring instrument is the use of spectrophotometer.

 

 

1. Adjust the printing pressure according to the production condition of the upper body

 

2. Arrange the color order reasonably

At present, multi-color printing generally uses ink, cyan, magenta and yellow sequence or cyan, magenta, yellow and black sequence. This color sequence makes color reproduction easier to achieve precisely. The thickness of the ink layer is arranged from small to large, such as when the black, blue, magenta and yellow sequence is used in offset printing, the ink layer thickness is: black ink is 0.8μm, green ink is 0.9μm, magenta ink is 1.0μm, yellow ink is 1.1μm; Ink viscosity is arranged from large to small, such as black ink 1188Pa, green ink 488Pa, magenta ink 477Pa, yellow ink 147Pa. When the ink viscosity can not meet the arrangement requirements, the ink viscosity can be adjusted in advance.

 

3. Calibrating press

 

 

1. Temperature control of ink delivery device

 

2. Ink supply control

 

 

3. Water supply control