Color image printing color reproduction quality control and evaluation

1. Color print image quality evaluation

Quality evaluation is divided into subjective evaluation, objective evaluation and comprehensive evaluation.

The subjective evaluation is based on the technical indicators stipulated in the “Standards” by the evaluator, and focuses on the factors related to quality. The evaluation conclusions are obtained through comprehensive and systematic evaluation and comparison of the quality of printed proofs or printed matter. The subjective evaluation is subject to the educators’ own knowledge. The impact of technical literacy, aesthetic perspectives, hobbies, and other aspects of psychological impressions often varies from person to person and it is unlikely that a unified conclusion can be drawn. For example, depending on the location and the surrounding environment, in particular, the visual difference caused when the illumination conditions of the copy (compared to the original) is observed. The subjective evaluation cannot fully reflect the quality characteristics of the printed matter, but it is the final arbiter of the quality of the printed matter. At the present stage, the method of appraising the printing quality is mostly based on subjective evaluation. However, subjective evaluation factors should be objectively interpreted to make it scientific and consistent with objective evaluation.

The objective evaluation is based on the determination of the physical properties of the printed matter. The physical properties of the printed matter are quantitatively inspected through the use of instruments or tools combined with print quality standards, and then the conclusions are drawn through mathematical statistics and necessary comprehensive analysis. The objective evaluation excludes human interference and makes the evaluation result more scientific and consistent. However, the objective evaluation requires a higher implementation condition. The objective evaluation method is fully used to evaluate the quality of printed matter. There are still some specific problems and difficulties. Some printing companies have now adopted this evaluation method throughout the process design and production process, and randomly and automatically control the printing quality. Such as electronic pre-proofing in the process of plate making, plate detection device during the printing process, water supply in the printing process, remote control device for ink supply.

The comprehensive evaluation is based on the means of objective evaluation plus subjective evaluation and verification of various factors. It is an effective method for evaluating the quality of printed matter. According to the provisions in the “Standards”, it is possible to evaluate prints more objectively based on the use of some test supplies (such as signal strips, test strips, control strips, and ladders) combined with human color psychology, physiology, and other factors. Quality.

There is no unified comprehensive evaluation standard. People are constantly exploring and developing. Therefore, in the current work of evaluating the quality of prints, it should be based on the technical indicators specified in the "Standards" and pay attention to the mutual connection between them. In the evaluation, we should focus on the overall evaluation of the quality of the printed matter from technical indicators such as tone value, color, dot, and relative contrast.

2. Method of controlling the color quality of printed images

In the production of printing, paper, ink, and printing plates are the limiting factors for the quality of printed images. However, when these factors have been determined, how to control the color quality of the printed image when the image is reproduced on a printing press?

1) Adjust printing pressure according to specific production conditions

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 black, 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 1.0 μm, yellow The ink is 1.1 μm; the ink viscosity is arranged from large to small, such as black ink 1188Pa, green ink 488Pa, magenta ink 477Pa, and yellow ink 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

Adjust the printing press according to the printing proofs and the given printing conditions so that the printed images are as consistent as the proofs.

4) Inking device temperature control

Some printers may be equipped with or may 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 ink fountain roller with cooling water.

5) 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.

6) 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.

Source: China Print Chemicals Network