Recognize the printability of newsprint

Newsprint technical indicators

Newsprint is characterized by its soft and flexible paper, good ink-absorbing properties, and ensuring that the ink is fixed on the paper faster; it has a certain degree of mechanical strength and good opacity.

Newsprint technical indicators can refer to GB1910-89 standard, should comply with the provisions of Table 1. The newsprint has a basis weight of 49 to 52 g/m2, which is divided into web and sheet paper. Web width 1575mm, 1562mm, 787mm, 781mm four kinds of specifications, diameter 800 ~ 900mm; sheet size of 787mm × 1092mm, 781mm × 1092mm.

In the selection of newsprint, several aspects such as variety, specification and quality should be included. The choice must be based on the characteristics of the printed material to be printed and the type of printing machine used.

Printability of newsprint

The printability of newsprint includes workability and quality adaptability. Operational suitability mainly affects the printing efficiency, and quality suitability mainly affects the printing effect, but the two cannot be completely separated. The following discusses several major factors and solutions that significantly affect printability.

1. The ink-absorbing effect of newsprint affects the major factors that affect the printability and the solution to the problem.

Newsprint absorbs ink better so that it can adapt to the requirements of high-speed web offset presses. Paper properties determine how the ink dries on the paper. There is no paint layer on the surface of newsprint, which is relatively loose and has strong ink absorption. The drying of ink is mainly accomplished through the penetration of solvents. When the ink is transferred to the paper surface, the solvent therein quickly penetrates into the paper, so that the ink is fast-fixed and fast on the paper surface, which is beneficial to the improvement of the printing speed. However, if the newsprint is too loose and the ink absorption is too strong, when the ink viscosity is low and the printing pressure is large, the ink will penetrate the back of the paper, resulting in the phenomenon of overprinting and affecting the thickness of the ink layer, making the print Can not present the color saturation on paper, affect the quality of printed matter.

In addition, compared with general coated paper, the newsprint used for color newspaper printing is very easy to print due to the fact that the degree of sizing is too low (generally not sizing), together with the high fluidity of the ink and excessive printing pressure. There are disadvantages such as poor image clarity, low color purity, easy-to-lose brightly-tuned network points, easy-to-neutral dark-line network points, a small range of tone levels, and difficulty in even printing on flat screens.

2. The tensile strength of newsprint affects printing efficiency

Tensile strength is an important indicator for newsprint printing. Because the paper sheet is dragged during printing, the high tensile strength ensures that the web is not pulled off. The tensile strength of newsprint mainly depends on the binding force between the fibers and the strength of the fibers themselves, so the type of papermaking pulp and good beating is the key to achieving higher tensile strength. Of course, the addition of suitable reinforcing agents is also beneficial for improving the tensile strength of newsprint.

One of the items that has the greatest impact on paper quality from print quality is the break rate (the number of web breaks that occur during the printing process for 100 webs). Because the printing job is stopped for a few minutes each time the web breaks, the printing plate and the blanket are sometimes damaged, so that the unprinted version cannot be printed. If the printing speed is reduced to prevent the paper breakage, it is necessary to reduce the efficiency of the printing job. Therefore, regardless of the occurrence of paper break or to prevent the paper break to reduce the printing speed, the printing efficiency is reduced.

As the requirements for high-speed rotary presses are constantly increasing, it is necessary to properly increase the length of the webs (in accordance with the standard of 3,000 meters) in order to prevent web breaks.

3. The smoothness of newsprint affects the tone level of print

Smoothness is an index to evaluate the degree of unevenness on the surface of a paper. It is used to indicate the degree of smoothness of the paper surface.
Smoothness determines the closeness and perfection of the contact between the paper and the printing plate, so it has a very close relationship with the quality of the printed matter.
(1) The smoothness of newsprint affects the clarity of prints

The smoother the newsprint is, the finer the reproduction of the dot is. If the smoothness is not high, some of the fine dots will fall on the lower surface of the paper and cannot be printed on. This will make it difficult to print a large area of ​​the flat screen evenly, and the small dots will be easily lost. The handwriting of the printed matter or the clarity of the print Not good. In this regard, thicker screens can only be used when making plates, which not only reduces the sharpness of printed images, but also makes it difficult to exhibit delicate layer changes.

(2) The smoothness of newsprint affects the color saturation of prints

The smoothness of the paper is high, the oil absorption is low, and the color of the printed matter is bright and vivid, which is related to the transfer rate of the ink. Although newsprint requires a high degree of smoothness, it is usually within 100 seconds (a smoothness value of about 30 to 60 seconds). If there is not enough ink supply, the ink transfer rate and transfer amount are relatively small, resulting in an image copy effect. And the saturation of the blotting ink color drops, and the color tone level of the color screen image is not clear. As a result, the poor reproducibility of the image in the field often results in the handwriting and the image being “faint”. If the ink supply amount and the printing pressure are increased, the print-through phenomenon occurs easily.

(3) The smoothness of newsprint is two-sided

A concept closely related to smoothness is the smoothness two-sidedness, which is an index representing the difference between the smoothness of the front and back sides of a paper, expressed as a percentage.

Smoothness two-sided difference = (front smoothness - negative smoothness) / frontal smoothness × 100%

The performance of the general print front and back is often different, but it must be limited, usually not more than 30%.

4. The opacity of newsprint affects the quality of print

Since newspapers are double-sided prints, no paper is allowed to print through, requiring newsprint to have high opacity.

(1) Opacity of white paper

The opacity of white paper indicates the paper's resistance to light transmission. It is related to the paper's quantification and degree of bleaching. Quantitative increase has a significant effect on increasing paper opacity. For newsprint, as the quantitative decrease, the opacity also drops sharply, so especially for newsprint with lower quantification, we must pay special attention to the issue of opacity. Papers with the same thickness and tightness generally have higher whiteness and lower opacity. The dark and dark newsprint paper containing more lignin has an opacity significantly higher than that of a letter paper with a higher whiteness. This is because the paper with high whiteness is not as absorptive as the paper with lightness. Strong absorption.

(2) Opacity of printing

In the actual use of newsprint, the important issue is also the opacity of printing. The opacity of printing depends not only on the opacity of the white paper but also on how far the ink penetrates into the sheet. That is, in addition to the opacity of the original paper, the printing opacity of the paper that the ink penetrates more easily is worse, and the result is that the printing on one side affects the printing on the other side. Therefore, newsprint must not only have good white opacity, but also have good opacity.

5. The whiteness of newsprint affects the color reproduction of prints

Papers with the same thickness and tightness generally have higher whiteness and lower opacity. The dark and dark newsprint paper containing more lignin has an opacity significantly higher than that of a letter paper with a higher whiteness. This is because the paper with high whiteness is not as absorptive as the paper with lightness. Strong absorption.

(2) Opacity of printing

In the actual use of newsprint, the important issue is also the opacity of printing. The opacity of printing depends not only on the opacity of the white paper but also on how far the ink penetrates into the sheet. That is, in addition to the opacity of the original paper, the printing opacity of the paper that the ink penetrates more easily is worse, and the result is that the printing on one side affects the printing on the other side. Therefore, newsprint must not only have good white opacity, but also have good opacity.

5. The whiteness of newsprint affects the color reproduction of prints

The whiteness of paper is a value that represents the optical properties of paper, and is an important indicator for color printing. According to the national standard (GB1910-89), the whiteness of newsprint is 50% to 52%. In fact, users like to take a little higher, and some hope to reach 70% or more. If newsprint is printed in color, about 45% of whiteness is also possible, which is related to readers' reading habits.

Newsprint is not only low in whiteness, but also has a different degree of grayscale and color cast. With the same ink printing, the newsprint has a smaller range of color gamut than the coated paper, and is affected by the decrease in contrast. The tone range and color reduction range are also reduced, and the gradation effect is necessarily reduced.

6. Moisture deformation of newsprint affects the overprint accuracy of printed products

In recent years, China's color newspaper printing has developed rapidly, some newspapers have all achieved colorization, and the printing quality of color newspapers has also greatly improved. Due to the performance characteristics of newsprint, the amount of water absorption is large, and the water absorption and expansion of the paper in the printing process is a major factor that affects the accuracy of overprinting. How to control and eliminate the influence of paper suction and expansion during printing on overlay accuracy, especially when printing equipment and raw materials are not so good, this issue is particularly important.

Newsprint fiber is relatively coarse, has a low density, has a small amount of surface sizing, and has a greater degree of water absorption than coated paper and offset paper. The printing of color newspapers generally uses the offset printing process. In the printing process, the stretching phenomenon caused by water absorption and drying will inevitably exist, which will have a greater impact on the overlay accuracy.

(1) The horizontal water absorption and expansion of newsprint has the most obvious effect on the overlay accuracy, and the impact is most direct. In the printing process of newsprint, the longitudinal stretch of the silk fabric is small, about 1/2 to 1/3 of the horizontal, and is generally eliminated by adjusting the registration rollers between the units, which has little effect on the overlay accuracy. The horizontal direction of the telescopic change has a great influence on the overprinting accuracy, and the overprinting error can reach 0.3 mm or more without taking certain elimination measures, which greatly affects the printing quality of the color newspaper.

(2) The equipment structure is different, and the effect of water absorption and expansion on the overlay accuracy is also very different. The printing process of the newspaper is to print four colors of paper in sequence, and the satellite unit is very advantageous for the printing of the color newspapers. The group printing machine has a very obvious overprint error due to the long paper path.

(3) 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 overlay accuracy of the printing. Newsprint is generally a thin paper of about 50 g/m2. The paper is loose and susceptible to deformation due to the influence of humidity in the environment. When using rotary printing presses, the poor control of the tension in the transport of the paper, the non-circular web core and other factors, can easily cause the overlay accuracy of the newsprint to be much lower than the overlay accuracy of the sheet-fed printing press.

For several of the above issues, through the improvement of processes, materials, and equipment, etc., the overprint accuracy of color newspapers can be effectively improved, thereby ensuring the printing quality of color newspapers. Specifically there are the following aspects.

(1) Control the ink and water balance during the printing process. The amount of water in the first color group during printing is the main factor that determines the effect of the paper's water absorption and expansion on overprint accuracy. In particular, when using newsprint with poor quality, the amount of fountain solution must be controlled.
(2) The new printing press