At this stage, color newspapers are mainly printed on newspaper offset presses. Since the printability of paper, ink, printing plates, blankets, fountain solution and other printing materials affects the printing quality of color newspapers, newsprint and newspaper offset offset printing inks are the main materials for color newspaper printing. Therefore, master the printing of newsprint Suitability plays an important role in analyzing the suitability of other printed materials and improving the printing quality of color newspapers. This article discusses the adaptability of newsprint printing in the process of color newspaper printing based on the practice of color newspaper printing and the theoretical teaching of newspaper printing technology.
1. The whiteness of newsprint
The whiteness of paper is one of the optical performance indicators of paper. It is a major indicator for color newspaper printing; the whiteness value of general coated paper is about 85 to 100, while the whiteness of newsprint is only 50 to 60. The whiteness value of the paper is high, the contrast between the ink and the paper is high, and the color is bright; newsprint contains more mechanical wood paddles, not only the whiteness is much lower than that of coated paper, but also with varying degrees of background gray and color cast. Compared with printing on art paper with the same ink, the color gamut and contrast are reduced, the vividness is reduced, the tone range is shortened, and the gradation effect is reduced.
Second, the smoothness of newsprint
The smoothness of the paper is a performance index reflecting the smoothness and smoothness of the paper surface. The smoothness of the paper is different, the degree of contact between the ink and the paper during printing is different. Generally speaking, coated paper and other coated paper have higher smoothness, while newsprint has lower smoothness.
1. The smoothness of newsprint affects the clarity and vividness of printed graphics of color newspapers
The surface of newsprint is rough, and the small dots will not be printed on the low depressions of the paper. Small dots are easy to lose. It is difficult to print evenly on a large area of ​​flat screen. Ink imprinted on the rough surface will form small gaps , Which reduces the density of the ink, reduces the vividness of the color ink, and is easy to de-powder and fluff. In this regard, only thicker network cables can be used for screen printing before color newspaper printing, generally 100 to 120 lines, but this not only reduces the clarity of the picture, but also difficult to show delicate gradation changes.
The smoothness of newsprint is slightly higher, which can make the reproduction of the dots fine, the color of the color newsletter is bright and vivid, but it should not be too high, otherwise the oil absorption is low, the transfer rate of the ink is small, the copy effect of the image is reduced, and the color gradation of the screen Unclear, images on the ground tend to be dull.
2. Smoothness difference on both sides
The difference between the two sides of smoothness is an index indicating the difference in smoothness between the front and back sides of the paper. Since newspapers are printed on both sides, the performance difference between the front and back sides must be limited, and 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.
3. The absorbency of newsprint
At present, color newspapers are generally printed on high-speed rotary offset presses, and the printing speed is faster, which is 3 to 5 times that of sheet-fed offset presses; there is no coating layer on the surface of newsprint, no sizing or micro-sizing on the surface, and the texture is loose and has a strong The ink absorption of color printing; cold press offset printing ink is used for color newspaper printing. After the ink is transferred to the paper surface, the solvent in the newspaper ink quickly penetrates into the newsprint and fixes it. The drying process is completed by free penetration and pressure penetration The permeability of the graphic print is dry. Therefore, newsprint is required to have strong ink absorption, so that the ink dries faster on newsprint.
1. If the newsprint is too loose, the ink absorption is too strong, and then the ink viscosity is low, and the printing pressure is large, the ink will penetrate the back of the paper, and the through printing phenomenon occurs, affecting the thickness of the conjunctiva of the ink layer, making the graphic imprint It is bright, and even leads to excessive penetration of the binder in the ink into the newsprint, and the leveling of the graphic ink layer deteriorates and affects the gloss; more seriously, the pigment particles lose the protection of the binder and appear powdered, which affects the color The print quality of the newspaper.
2. If the newsprint has a tight texture and the ink absorption is too weak, and then the ink viscosity is high and the printing pressure is low, the linking material in the ink cannot quickly penetrate into the newsprint, so that the color printing graphic ink layer cannot be dried in time. Dirty smearing phenomenon; especially in the four-color newspaper rotary high-speed printing, the first color ink layer is not dried in time, the ink viscosity is still maintained at a low value, may cause subsequent color ink overprint, or even mixed color phenomenon, Affect the printing quality of color newspapers.
Therefore, in high-speed newspaper web offset printing, newsprint has better ink absorption, lower ink viscosity and improves its fluidity. Too much or too little viscosity of ink in color newspaper printing will affect the printing quality of the newsletter: too much viscosity will cause de-powdering and hair loss, and it is also easy to cause uneven ink transfer; too low viscosity will easily cause ink emulsification and unbearable imprint , Drying is not timely, rubbing faults, etc. [next]
Fourth, the absorbency of newsprint
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, looser in texture, and has a greater degree of water absorption than coated paper and offset paper; water color offset printing is generally used in color newspaper printing. There must be expansion and contraction of the paper due to water absorption during the printing process. Greater impact.
â‘ The lateral water absorption and expansion of newsprint has the most obvious effect on the accuracy of overprinting. In the printing of color newspapers, the thread direction of newsprint is consistent with the direction of movement of the paper tape. The amount of longitudinal expansion and contraction of the parallel strands is relatively small, about 1/3 to 1/2 of the horizontal direction. Generally, it can be eliminated by adjusting the register adjustment rollers between the units, and it has little effect on the accuracy of overprinting. In the case of eliminating measures, the overprint error is large (more than 0.3mm), which greatly affects the printing quality of the color newspaper.
â‘¡The combination of printing units is different, and the degree of influence of water absorption and expansion on the accuracy of overprinting is also different. The printing process of the color newspaper is to print four-color inks on the paper in sequence. The unit-type newspaper rotary offset press has a long paper path, and the newsprint absorbs water and expands and deforms greatly, which has a significant impact on the accuracy of overprinting. The tower newspaper rotary offset press has a paper path The shorter, the smaller the deformation of the newsprint's water absorption and expansion, which has less effect on the accuracy of overprinting.
2. Newspaper moisture absorption deformation failure countermeasures
â‘ Control the amount of fountain solution in the printing process. In the printing of color newspapers, the amount of water in the first color group is the main factor that determines the impact of the absorption and expansion of newsprint on the accuracy of overprinting. Therefore, the amount of fountain solution in the first color group should be well controlled and used under the premise of ensuring the quality of the color newspaper printing. The minimum amount of water.
â‘¡The reel newsprint should not be too dry, otherwise the water absorption will be larger, the expansion rate will be higher, and the water absorption and expansion will also have a greater impact on the accuracy of the overprint; and the newsprint will become brittle and easy to break during printing; too dry newsprint will also be easy during the printing process With static electricity, it affects the work of folding, receiving and stacking.
â‘¢It is better to use the tower newspaper offset press for color printing, because the paper path between the four-color printing of paper is shorter, and the newsprint's water absorption and expansion deformation has little effect on the accuracy of overprinting, which is conducive to improving the accuracy of overprinting. At present, there are still many city newspaper printing plants that still use the unit-type newspaper offset press. The paper path between the units is long, and there is no fan-shaped expansion device. When printing color newspapers, the paper's water absorption and expansion have a significant effect on the registration accuracy. In this case, according to the amount of paper expansion and contraction in printing, the effect of paper absorption and expansion on overprinting accuracy can be eliminated by changing the film position during imposition.
5. Tensile strength of newsprint
When printing, the paper tape of newsprint must be dragged by the paper roll to rotate. The higher tensile strength can ensure that the paper tape is not broken. The tensile strength of newsprint depends mainly on the bonding force between the fibers and the strength of the fiber itself, so papermaking The type of pulp used and good beating are the key to achieving higher tensile strength.
Of course, adding a suitable reinforcing agent is also beneficial to increase the tensile strength of newsprint. One of the items that have the greatest impact on the printing quality of all the printability of newsprint is the paper break rate (100 webs break 1 to 3 times during the printing process), because it is necessary to break every time A few minutes of downtime can sometimes damage the printing plate and blanket, making printing impossible without adjusting the plate. If you reduce the printing speed to prevent paper breaks, it will inevitably reduce the efficiency of the printing job. Therefore, whether it is a paper break or reducing the printing speed to prevent paper breaks will reduce the printing efficiency. In order not to break the paper, it is necessary to increase the length of the web (approximately 3000m) according to the standard.
In addition, the quality of the newsprint roll itself is unqualified or improperly stored, and the center of gravity of the paper roll is shifted, resulting in eccentricity, and the paper is easily broken. It is generally believed that paper rolls with an eccentricity exceeding 5 mm are considered unqualified, and eccentricity is likely to cause breakage. Under normal circumstances, the tension should be balanced with the braking force; when using an eccentric paper roll, due to the change in the radius of the paper roll, the paper belt is oscillated and the tension is unstable; within each short period of time, starting, running, joints, braking, etc. Under, the tension of the tape changes.
6. The opacity of newsprint
The color newspaper is a two-sided printed matter, which does not allow through-printing, so newsprint should have a high opacity.
1. The opacity of newsprint
Opacity indicates the paper's resistance to light transmission. It is related to the basis weight and bleaching degree of the paper. The larger the basis weight, the greater the opacity. For newsprint, as the basis weight decreases, the opacity also decreases. Therefore, for newsprint with a lower basis weight (currently about 48g / m2), special attention should be paid to the opacity. The paper with the same thickness and tightness is generally white The higher the degree, the lower the opacity. Newsprint paper with more wood pulp and darker colors has a significantly higher opacity than white letterpress paper of the same amount. Therefore, newsprint requires higher opacity.
2. Printed opacity
Printing opacity is 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 on newsprint, not only the paper needs to have good opacity, but also good printing opacity.
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