Eco Friendly Paper Bags Printing image color control and detection method (1)

Eco Friendly Paper Bags Printing image color control and detection method (1)

 

In print reproduction, the number of color printed images has a considerable proportion. Color printing image is through the blue, magenta, yellow three primary color ink superposed printing to present a variety of colors, 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 can not be well restored to the color of the original manuscript. In order to obtain satisfactory printing image quality, the image color must be detected in the printing production, and the color reproduction quality must be controlled by adjusting some printing variables.

 

First, the main factors affecting the quality of color reproduction

When copying on the printing press, there are many factors that affect the color of the copied image, such as: dot expansion, printing color sequence and overprinting, ink hue and field density, ink temperature and viscosity, water supply (offset printing), paper properties, plate layout depth, printing pressure and so on.

 

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 is short, and the post-printing color ink is overprinted on the surface of the first printing wet ink, which belongs to the printing state of "wet overlapping 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

The inks used in printing production have different degrees of color bias, so that the printed image has color bias, and the ink with less color should be used as far as possible for color printing. The field ink density of the printed image surface determines the reproduction range of the order and tone of the printed image. The higher the field density, the wider the range of order and tone reproduction. The small field density reduces the color saturation of the image and weakens the overprint color.

 

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

The balance of ink in offset printing directly affects the quality of image reproduction. Small amount of water will make the printing plate dirty, paste plate; The amount of water emulsifies the ink and reduces the color saturation of the printed image.

 

Second, color detection of printed images

The color of the color printing image is formed by the superposition of yellow, magenta and blue inks in different proportions. Generally, when measuring the color of the printed image, it is not to measure the color on the screen, but to measure the quality control strip printed at the same time as the printed image. The control strip is generally placed at the trailing tip of the sheet. Using measuring instruments to detect the corresponding color blocks of the control strip can obtain printing quality information, such as the field density of each primary color ink, overprint rate, dot expansion, dot density, neutral gray reduction, contrast and other parameters to judge the order and tone reproduction of the image.

 

There are three ways to measure the color of printed images, namely densitometer 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 been the most commonly used objective evaluation of quality in the printing industry, density meter is cheap, widely used. When the density meter measures the color surface, it can only obtain the relative amount of a certain primary color ink in the printing, and it cannot indicate the hue of the color to be measured. The measured value of the densitometer is not associated with various color systems, so the color language cannot be used to describe the measured color. In color measurement and evaluation, densitometer has certain limitations, it is not a standard color measurement instrument.

 

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