I. Problems faced by color newspaper printing in process technology
At present, most of the national color newspapers are printed using newsprint. Compared with the general color printing industry using coated paper, the development time is short, and the process technology is not yet mature. It is mainly reflected in the following aspects:
1. Lack of mature process technology theory and technical standards
In the newspaper printing industry, monochrome printing has been used for decades, and there is a lack of systematic theoretical research results and instructive technical standards in the adaptive research of multicolor printing of newsprint using web offset presses. The production process is difficult to achieve standardization and quantitative control, and it is difficult for the quality of each process to be specifically judged. Many technical parameters are difficult to obtain through experience accumulation in the production process. There must be professional research institutions and personnel to complete the necessary testing methods.
2. Did not form a process management specification adapted to newspaper production characteristics
The production process of color newspapers is very different from the production process of general color prints. Newspapers are highly time-sensitive products. The process of printing newspapers must be subject to the overall requirements of newspaper publishing, which enables the production process management of color newspapers. It is difficult to apply the ready-made pattern of general color prints. In this regard, it is necessary to establish a process management specification that suits the characteristics of newspaper production in order to achieve effective control over the timeliness and quality of its production. At present, this kind of standardized management is still being explored.
3. Lack of professional and technical personnel
The printing presses with the capability of printing color newspapers are mostly equipped with advanced multi-color web offset presses and prepress equipment. The mechatronics and high automation of the equipment put forward higher requirements for the equipment management and operation team. However, after the newspapers have made high investment in equipment, it is difficult to make timely supplements to the corresponding professionals. On the one hand, the existing old workforce needs to adapt and learn color printing knowledge and improve the level of operation technology; on the Other hand, most of the printing professional colleges in China do not mostly target talents in newspaper printing; some printing houses print from single-machines. Skilled technicians use technical backbones to operate multicolor rotary machines. They also need a process of learning and adaptation. Therefore, whether it is experienced old workers or graduates trained by professional colleges, they are faced with the problem of learning and adaptation in the process technology of color newspaper printing. The cultivation of talent lags behind the transformation of equipment. It is a common feature of the printing houses of newspapers. After the colorization of newspaper pages has been achieved, a lot of work needs to be done to stabilize and improve the quality of color newspapers.
4. The market has continuously increased the requirements for printing quality of color newspapers
Although the color newspaper is just in the development stage in China, the market has already used a higher standard to demand it. The printing quality of Caibao is not only one of the important channels for advertising agencies to choose to publish information, but also reflects the strength of each newspaper company's contracting of foreign businesses. This has enabled Caipao Printing to be integrated into fierce market competition.
Judging from the overprinting precision standard of four-color printing of rotary offset printing press, the National Newspaper Association's overprint accuracy for color newspaper printing is set to be less than 0.1mm-0.15mm, which has reached the overprinting precision requirement for the printing of single-platen paper; Many people donâ€™t understand that press and coated paper, rotary presses and single-sheet printing have great differences in process technology and printing effects, and even use the effect of coated paper proofing to measure the print quality of color newspapers, resulting in a fierce market. Competition and user demand for products have brought tremendous pressure on the newspaper printing companies. In the face of pressure, printing companies need to carefully study the stability and improvement of printing quality of color newspapers. Based on the specificity of their process conditions, they will explore technical means for effective control of printing quality.
$Page break $
Second, the effect of printing conditions on the print quality
In the case of the widespread use of newsprint in domestic color newspaper printing, the printing of color newspapers should be based on the premise of addressing the adaptability of high-speed web presses for newsprint printing. It should be understood from the aspects of printing equipment, process management, operation technology, and auxiliary media. Grasp the range of features that are compatible with each other and seek control methods that achieve the best printing results.
1. Paper properties
Newsprint is the main substrate for color newspaper printing. Grasping the characteristics of newsprint plays a leading role in solving adaptation problems in various production processes. The impact of paper performance on print quality is mainly reflected in the following indicators.
The whiteness of paper is a numerical value indicating the optical performance of paper, and is a major indicator for color printing. In general, the whiteness value of coated paper is around 85-100, while the whiteness value of newsprint paper is only 50-60. The whiteness of the paper is high, the contrast between the ink and the paper is high, and the purity of the color is also high. The whiteness of newsprint is not only much lower than other coated papers, and the paper base has different degrees of grayscale and color shift. The same ink is printed on newsprint and compared with printing on coated paper, its color gamut range and vividness are reduced, and the contrast range is reduced and the range of color reduction is also shortened, and the gradation effect is necessarily reduced.
2 surface smoothness
Paper's smoothness is a value that reflects the smoothness of the paper surface. The degree of smoothness of the paper is different, and the degree of contact between the ink and the paper in printing is different. In general, coated paper has a smoothness value of 500-700, or even higher, while newsprint is only about 30-60. The surface of the newsprint is rough, and the ink is imprinted on a rough surface to form fine gap white spots, thereby reducing the density of the ink and reducing the vividness of the colored ink. Moreover, the paper is easy to fall off, so that a large area of â€‹â€‹flat screen is difficult to print evenly, and small outlets are easily lost. In this regard, only thicker screens can be used in photo-printing. This not only reduces the sharpness of the picture, but also makes it difficult to show delicate levels.
3 ink drying method
The performance of the paper determines how the ink dries on the paper. Coated paper has a coating layer on the surface. The drying of the ink is mainly conjunctiva. The oxidized polymer layer formed on the paper not only maintains good saturation, but also has good gloss and good color reproduction. Newsprint surface coating layer, the ink is dried by the amount of penetration into the pores between the paper fibers to complete. In the high-speed operation, this infiltration-based ink method requires the ink to properly reduce the viscosity and improve the fluidity. Under the pressure of the printing cylinder, the ink penetrates into the paper and at the same time forms different degrees of diffusion. The enlargement rate is higher than that of coated paper, which tends to cause the images to darken and merge dots and even paste, thereby lowering the level of image darkness and shortening the range of adjustment.
According to the main characteristics of newsprint used in the printing of newspapers, there is poor image clarity compared with the general coated paper in the printing effect, the color reduction range is small, the color purity is low, the bright spot is easily lost, and the dark spot is easily accessible. At the same time, the range of tone levels is small, and flat screens are difficult to print evenly.
2. Printing machine features
In lithographic offset printing, there are mainly two types of sheet-fed printing methods and web-fed printing methods. Different types of printing methods control print quality. Mainly reflected in the following aspects.
1 paper transport method
The paper transport method directly affects the positioning and deformation resistance of the paper in the printing process, and plays an important role in the printing overlay accuracy. General coated paper is tightly packed. Paper-based thick, not easily deformed, the use of sheet-fed printing methods rely on the transport of paper feeding teeth, the paper in the conveying process has an accurate positioning, version of the roller can do multi-position fine-tuning, therefore, overprinting can achieve higher accuracy. Newsprint paper is generally a thin paper with a weight of less than 50g/m2. Paper is loose, and it is easily deformed by the influence of humidity in the environment. Printing is performed using a web printer. The paper is controlled by the tension during the conveyance. The paper is easily deformed in the pulling process, and the quality is relatively high. The poor web core is not round, and the paper roll easily jumps up and down during feeding, which makes it difficult to overprint. The rotary drum version cannot generally be fine-tuned in many directions. Therefore, the overprint accuracy of web newsprint printing paper is much lower than that of single-sheet printing. Although more advanced web machines are equipped with an automatic registration system, the overprint accuracy of these web presses cannot be overestimated. In addition, the delivery of the rotary press is controlled by the pressure roller. When the printing reaches a certain amount, the pressure roller will be stained with ink, causing the press of the plate to be dirty. The utilization rate of the Chinese sewn layout in domestic newspapers is high, and the pressing of the middle seam position is difficult to avoid, which affects the cleanliness of the layout.
2 printing speed
The speed of printing determines the length of contact time between the ink and the paper and plays an important role in the inking effect. For newsprint printing, which is mainly based on osmotic drying, the faster the printing speed is, the less ink penetrates the paper. As the printing speed increases, the density of ink on the paper tends to decrease. In order to meet the requirements of high-speed printing, it is necessary to improve the permeation performance of the ink. Usually, a high-speed printing ink with small viscosity, small wire drawing, and strong fluidity is selected. However, although the ink permeability is improved, the density of the ink and the saturation of the color are reduced at the same time. Although more modern high-speed printing presses adopt advanced technologies in their ink supply and water supply methods, in terms of actual printing results, the color printing effect of mainframes in similar printing presses is lower than that of minicomputers, and the effect of high-speed machines Below the low speed machine.
3 ink transfer performance
The print quality is also affected by the even transfer of ink on the press. A sheet-fed press has at least four ink fountain rollers and several ink distributor rollers for each color group, whereas a rotary printer has only two ink distributor rollers for each color group. Under normal circumstances, the more the tandem roller, the more uniform the ink in the transfer process, the more uniform the ink transferred to the plate. The stringer's stringing at high speeds is difficult, making the printing uniformity of the large-area color patch and the flat mesh much lower than that of the single-sheet printing machine.
Judging from the characteristics of rotary printing machines used in color newspaper printing, there are certain gaps in the overlay accuracy, ink uniformity, color reduction effect, and layout cleanliness compared with single-sheet machines.
The printing media and printing methods of Caipao determined that it was different from the general color printing. To stabilize and improve the printing quality of color newspapers, it was necessary to specifically discuss the best means to solve and adapt to this particularity. From the above analysis, it can be concluded that to stabilize and improve the printing quality of color newspapers, the main problems to be solved in the process technology should be to improve the clarity of the image, the range of tone levels, the color saturation, and the evenness of the ink.
$Page break $
Third, the main production process of the technological adaptation
1. Picture separation
The key to color printing is prepress, and the key to prepress is color separation. In order to improve the printing quality of color newspapers, within the scope of the printing factory, it is necessary to first effectively control the important steps of color separation of the pictures so that they can adapt to the characteristics of newsprint printing as much as possible.
Classification of 1 manuscript
The manuscript is the basis for color separation and the basis for print reduction. The quality of the original document directly affects the quality of platemaking and printing. Due to the limitation of process conditions, the printing of color newspapers is less effective than general color printing. In order to improve the printing effect of the newspapers and make up for this gap, it should have been more rigorous in selecting manuscripts with high definition, rich gradation, and vivid colors. However, it is still difficult to do this in practical work. First of all, the timeliness of the newspapers has determined that the choice between the content and the quality of the images is difficult. Most of them take the prominent content first. In the actual work, they even encounter clippings with poor results. This gives the picture The color separation brings great difficulty. Secondly, the manuscripts of Cai Pao have come from a variety of sources and are of various types. In addition to reflections and transmissions,
SSS Nonwoven Machine,Automatic PP Spunbond Nonwoven,Automatic Nonwoven Making Machine,Nonwoven Fabric Making Machine
ZHEJIANG YINFEN GROUP , http://www.yingfengmachinery.com