China's newsprint is divided into four grades of A, B, C, and D according to quality standards. Among them, A and B grades are suitable for high-speed rotary offset presses, while color newspaper printing is generally carried out on high-speed rotary presses.
Ink absorption
1. The influence of the absorbency of newsprint on the printing quality of color newspapers
Color newspapers are generally printed on high-speed rotary offset presses. Due to the fast printing speed, newsprint is required to have strong ink absorption, that is, the ink dries faster on newsprint.
There is no coating layer on the surface of newsprint, which is relatively loose and has strong ink absorption. Ink drying is mainly accomplished by the penetration of the solvent. After the ink is transferred to the paper surface, the solvent in it quickly penetrates into the paper.
Make the ink fast dry on the paper surface, but if the newsprint is too loose, the ink absorption is too strong and the viscosity of the ink used is low, the printing pressure is large, the ink will penetrate the back of the paper, the phenomenon of through printing occurs and affects the ink layer. The thickness of the conjunctiva prevents the imprint from exhibiting the desired color saturation on the paper, which affects the quality of the printed matter.
2. Appropriately reduce ink viscosity and improve fluidity
In high-speed operation, this drying method based on penetration requires that the viscosity of the ink must be appropriately reduced and the fluidity must be improved. The viscosity of the ink is too large or too small during printing, which is unfavorable for printing. Excessive viscosity will cause hair loss and powder loss, and it is also easy to cause uneven ink transfer. higher. Unlike adhesion, as the printing speed increases, the viscosity of the ink during printing increases.
3. The dots increase the printing quality of shadow posters
From the main characteristics of newsprint, compared with general coated paper, newsprint has poor image clarity, small color reproduction range, low color purity, bright tone dots are easy to lose, dark tone dots are easy to merge, and tone level The range is small, and it is difficult to print evenly on the flat screen. The reason is that the sizing degree of newsprint is too low (generally not sizing). The ink fluidity is too large and the printing pressure is too large.
Under the effect of printing pressure, the ink will diffuse to different degrees while penetrating the newsprint, making the dot increase rate higher than that of coated paper, which often causes the dark tone dots to be graded, or even paste, to reduce the dark tone effect of the image. , Shorten the tone range and cause the image to be blurry. You can reduce the midtones during image processing, leaving room for the increase of the dots. At the same time, the dark tone level is brightened to compensate for the dark tone level of the image. Sharpening can also improve the clarity of the image.
tensile strength
When printing, the paper emperor must drag the paper roll to rotate, and the high tensile strength can ensure that the paper tape is not broken. The tensile strength of newsprint depends mainly on the binding force between the fibers and the strength of the fiber itself, so the pulp used in papermaking Variety and good beating are the key to achieving higher tensile strength. Of course, adding appropriate reinforcing agents is also beneficial to increase the tensile strength of newsprint.
One of the items that have the greatest impact on print quality among all the printability of newsprint is the paper break rate (the number of times that 100 roll paper breaks during the printing process), which is because of the number of shutdowns every time the paper breaks Minutes sometimes damage the printing plate and blanket, making it impossible to print without adjusting the plate. If the printing speed is reduced to prevent paper breaks, it is necessary to reduce the efficiency of printing operations. Therefore, whether it is a paper break or a reduction in the printing speed to prevent paper breaks, the printing efficiency will be reduced. In order not to cause paper breaks, it is necessary to increase the length of the web break (approximately 3000m according to the standard).
Smoothness
Smoothness is an index to evaluate the degree of unevenness of the surface of the paper, indicating that the surface of the paper is smooth. The degree of smoothness, for newsprint, the smoothness determines the closeness of the contact between the paper and the printing plate and the quality of the color newspaper printing.
1. The smoothness of newsprint affects the clarity and color saturation of color newspaper printing
The higher the smoothness of newsprint, the better the fine reproduction of the dots. If the smoothness is too low, the fine dots will not be printed on the low recesses of the paper surface, making it difficult to print a large area of ​​flat screen uniformly, small dots are easy to lose, and the clarity of the handwriting or screen of the printed matter is not good. Only thicker network cables can be used in the plate making of the picture, which not only reduces the clarity of the picture but also makes it difficult to show delicate layer changes.
The smoothness of newsprint is high, and the oil absorption is low. The color of the printed matter is bright and vivid. The smoothness requirements of newsprint are generally within 90s. However, if the smoothness is too high, and there is not enough ink supply during printing, the transfer rate and transfer amount of the ink are relatively small, resulting in a reduction in the image replication effect and the saturation of the color of the imprint group, and the tone gradation of the color picture color Not clear. As for the reproducibility of the image on the ground, the handwriting and images tend to be "false". If the amount of ink supply and printing pressure are increased, through-printing is likely to occur.
2. The smoothness of the broken paper is poor on both sides
A concept closely related to smoothness is the difference between the two sides of the smoothness, which is an indicator of the difference in the smoothness of the front and back sides of the paper, expressed as a percentage.
Smoothness difference between two sides = (front smoothness-reverse smoothness) / frontal smoothness × 100%
Generally, the performance of the front and back sides of printed products are somewhat different. Since newspapers are printed on both sides, the performance difference between the front and back sides must be limited. Usually, the difference between the two sides of the smoothness should not exceed 5%. Smoothness is an important indicator of the suitability of newsprint printing, requiring newsprint to have higher smoothness and smaller smoothness.
Opacity
Because newspapers are printed on both sides, paper does not allow through printing, so newsprint should have a high opacity.
1. The relationship between opacity and paper
For white paper, opacity indicates the paper's resistance to light transmission. It is related to the paper's stability and the degree of bleaching. The larger the amount, the greater the opacity. For newsprint, as the basis weight decreases, the opacity also drops sharply. Therefore, for newsprint with lower basis weight, special attention should be paid to the problem of opacity. The higher the whiteness of the same thickness and tightness, the lower the opacity. Newsprint paper with more wood pulp and darker color has a higher opacity than white letterpress paper of the same amount, because paper with high whiteness is less light-absorbing than paper with low whiteness. Strong.
2. Printing opacity
Printing opacity is also very important for color newspaper printing. It depends not only on the opacity of newsprint but also on the degree of ink penetration into the paper. That is to say, in addition to the opacity of the original paper, the more easily the ink penetrates the newsprint, the worse the opacity of the printing. As a result, the printing on one side has a greater influence on the printing on the other side. Therefore, when printing newsprint, not only the paper needs to have good opacity, but also good printing opacity.
BaiDu
Whiteness is the value that indicates the optical performance of the paper. According to the national standard (GB1910-89), the whiteness of newsprint is 45% to 52%. In fact, users like to take it higher, and some hope to reach more than 70%. Color, whiteness of about 45% is also acceptable, which is related to readers' reading habits.
Newsprint not only has low whiteness, but the paper base has different degrees of grayscale and color cast. Compared with printing on art paper with the same ink, the color gamut and vividness are reduced compared to printing on coated paper. Affected by the reduction in contrast, the tone range and color reproduction range are also shortened, and the gradation effect is reduced. The contrast, highlights, and sub-highlights of the image can be enlarged during image tone processing, and the whiteness of newsprint can be fully utilized to express the brightness of the highlights of the picture. For colors that are difficult to faithfully reproduce and have high saturation, it is necessary to make full use of the purity of the primary color ink to show the best color saturation on newsprint.
Hygroscopic deformation
Newsprint absorbs a large amount of water. The water absorption and expansion of paper during printing is the main factor affecting the accuracy of overprinting of color newspapers. How to control and eliminate the impact of paper absorption and expansion on the accuracy of printing during printing, especially in the case of unsatisfactory equipment and raw materials Newspapers, this issue is particularly important.
1. Hygroscopic deformation of newsprint
Newsprint fiber is thicker, less dense, and has a greater water absorption and elongation than coated paper and offset paper. Color newspaper printing generally uses offset printing technology. During the printing process, there must be paper expansion and deformation due to water absorption and drying. Big impact.
(1) The lateral water absorption and expansion of newsprint has the most obvious effect on the accuracy of overprinting. In the printing process of newsprint, the longitudinal expansion of the thread is relatively small, about 1 / 3-1 / 2 of the horizontal direction, which is generally eliminated by adjusting the registration adjustment roller between the units, which has little effect on the accuracy of the overprint, and the horizontal expansion and contraction changes Without certain elimination measures, overprint error can reach more than 0.3mm, which greatly affects the printing quality of color newspapers.
(2) The influence of different absorption structures on the accuracy of overprinting is also very different. The printing process of newspapers is to print four colors on paper in sequence. Satellite equipment is very beneficial to color newspaper printing, while combined equipment has a significant impact on accuracy due to the long paper path.
(3) The conveying method of paper directly affects the positioning and anti-deformation of paper in the printing process. It also plays an important role in the accuracy of printing overprint. Newsprint is generally thin paper of about 50g / mZ. The paper is loose and easily deformed by the influence of environmental humidity. When printing in a rotary press, the paper is controlled by tension during transportation, and the accuracy of web newsprint overprint is low.
2. Prevention and remedy of moisture absorption deformation failure of newsprint
By improving the process \ materials \ equipment, you can effectively improve the printing quality of color newspapers.
(1) Control the water-ink balance in the printing process. The amount of water in the first unit during printing is the main factor that determines the effect of paper absorption and expansion on the accuracy of overprinting. The amount of dampening water should be controlled.
(2) The newsprint used for printing the color newsletter should not be too dry, otherwise it will not only break the paper easily during printing, but also have a large water absorption and a high expansion and contraction rate.
(3) Do not store the environment in compliance with the specifications and the storage time should not be too long.
(4) Paper performance is a very important factor. To produce high-quality color papers, use good quality newsprint as much as possible.
(5) The best process structure of the color newspaper printing machine is the tower, because the paper path between the four-color printing of the paper is shorter, which is conducive to improving the accuracy of overprinting.
(6) When ordering equipment, it should be noted that whether it is a double-frame machine or a single-frame machine, a fan-shaped expansion roller must be added to improve the control of paper suction and expansion.
In actual production, according to the performance conditions of the newsprint selected, different printing techniques should be adopted to simultaneously effectively select inks, printing plates, fountain solution, blankets, etc. and continuously improve the operation level of the printing workers to print high-quality Color newspapers to improve the printing quality of color newspapers.
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