Color Control and Detection of Printed Images (Part 2)

There are three measurement methods for measuring the color of printed images, namely densitometer measurements, colorimeter measurements, and spectrophotometer measurements.

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 evaluation of quality in the printing industry. Densitometers are inexpensive and widely used. When the densitometer measures the color surface, only the relative amount of a certain color ink in the printing can be obtained. It cannot indicate the hue of the measured color. Densitometer measurements are not associated with a variety of color systems and therefore cannot be described in color language. The density meter has certain limitations in color measurement and evaluation. It is not a standard color measurement instrument.

2. Colorimeter Color Measurement

The colorimeter is to obtain the visual response proportional to the color tristimulus values ​​X, Y and Z by directly measuring the surface of the measured color. After conversion, the X, Y and Z values ​​of the measured color can be obtained, and these values ​​can also be obtained. Convert color parameters into other color spaces. The colorimeter is a special density meter with three broadband filters. Due to the fact that the instrument itself and the principle are thick in certain errors, the absolute accuracy of the color measurement value is not good. However, due to its low price, it is still a widely used color measuring instrument.

3. Spectrophotometer (Spectrophotometer) Color Measurement

Spectrophotometers measure the reflectance of light at each wavelength of the visible spectrum of the color surface. The light of the visible spectrum is irradiated with the color surface at a certain step distance (5 nm, 10 nm, 20 nm), and the reflectance is measured point by point. The spectrophotometric curve of the surface of the measured color can be obtained by plotting the reflectance value of each wavelength of light with each wavelength. Each spectrophotometric curve uniquely expresses a color. The measured value can also be converted to other color system values. Spectrophotometer is a flexible, ideal color measurement instrument. At present, color measuring instruments for color quality inspection of some printing machines equipped abroad are using spectrophotometers.

Third, control printing color quality method

In printing, constraints on the quality of printed images such as paper, ink, and printing plates. When these factors have been fixed, how can the color quality of the printed image be controlled when the image is reproduced on a printing press?

1. According to the upper body production situation, adjust the printing pressure

According to the print image reproduction quality requirements, paper thickness, printing plate and other conditions to adjust the printing pressure, so that the image network points to get the best reproduction.

2. Reasonably arrange the color sequence

At present, multi-color 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 black, cyan, magenta, and yellow are used in offset printing, the thickness of the ink layer is: black ink is 0.8 μm, cyan ink is 0.9 μm, 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 is 477Pa, yellow ink is 147Pa. When the ink viscosity does not meet the alignment requirements, the viscosity of the ink can be adjusted in advance.

3. Calibration printing machine

The printing press is adjusted according to the proof sheet and the predetermined printing conditions so that the printing image effect is consistent with the print sheet.

Fourth, the printing press, control the color reproduction quality

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, currently more advanced printing presses such as Heidelberg, Germany, Roland, and Mitsubishi, Japan produced three printing presses that regulate the intrinsic parameters: Temperature, ink supply, and water supply.

1. Inking device temperature control

Some printers are equipped with or can be equipped with an inking device temperature control device. In the inking process, the ink roll is rotated and oscillated, so that its temperature is increased most. When the temperature is controlled, cooling water is generally introduced into the core of the ink roller to reduce the temperature, and the surface temperature of the ink roller is kept stable. , thereby ensuring a stable supply of ink. Some printing presses, in addition to the ink roller, also cool the water in the ink fountain roller.

2. Ink supply control

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

At present, the automatic adjustment of the ink volume of the printing press mainly includes two parts, that is, the overall ink output volume of the ink fountain roller and the adjustment of the ink amount in each local area of ​​the ink roller. The overall ink volume adjustment ensures the correct reproduction of the whole picture gradation and hue of the printed image. Since the printed image is distributed differently across the entire page, the requirements for the amount of ink in each area are also different. Therefore, the ink fountain roller is divided into several parts along the axis of the ink roller. In the small ink area, the ink volume sent by different ink areas is different to meet the different requirements for the amount of ink in each local area of ​​the image.

3. Water supply control

Offset printing plate water volume must be controlled. The amount of water should be different with the printing press speed, ink supply, etc. to ensure the balance of ink and water, so that the printed image is not blooming, the color saturation is good, and the paste is not pasted. Adjusting the amount of water is achieved by changing the speed of the fountain roller.

V. Conclusion

In order to meet the quality requirements of color image printing and copying, the printed image must be detected and controlled. Modern presses control the quality of printed images by controlling the ink temperature, ink supply, and water supply.

Source: "Packaging and Printing Online"