At this stage, color newspapers are mainly printed on newspaper offset presses. The printability of printing materials such as paper, ink, printing plates, blankets, fountain solution and other printing materials affects the printing quality of color newspapers. Newsprint and newspaper offset printing ink The main materials of newspaper printing, mastering the printing suitability of newsprint plays an important role in analyzing the suitability of other printing materials and improving the printing quality of color newspapers.
1. The whiteness of newsprint
The whiteness of the paper is an indicator of the optical performance of the paper, which is a major indicator for color newspaper printing; the whiteness value of general coated paper is about 85-100, while the whiteness of newsprint is only 50-60. The whiteness value of the paper is high, the contrast between the ink and the paper is high, and the color is bright; the newsprint contains more mechanical wood paddles, the whiteness is not only much lower than the coated paper, but also has different degrees of bottom gray and color deviation. 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 surface. Small dots are easy to lose. Large flat screens are difficult to print evenly. Ink printing on rough surfaces will form small gaps. , Which reduces the density of the ink and reduces the vividness of the color ink, and it is easy to "powder and fluff". In this regard, only thicker network cables can be used for screen printing before color newspaper printing, generally 100Lpi-120Lpi, which not only reduces the clarity of the picture, but also makes it difficult to show delicate gradation changes.
The smoothness of newsprint is slightly higher, the reproduction of the dots is 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 picture is not Vivid, images on the ground are often "false".
2. Smoothness difference on both sides
The smoothness difference between the two sides is an indicator of the difference between the smoothness of the front and back sides of the paper. Since the newspaper is a two-sided print, the performance difference between the front and back sides must be limited. Generally, 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, with a fast printing speed, which is 3-5 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 Ink absorption; color newspaper printing uses "cold-set" offset rotary ink. 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 combined with free penetration and pressure penetration. The completion of the graphic imprint is mainly permeable drying. Therefore, newsprint is required to have relatively 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 Bright; even cause the binder in the ink to penetrate too much into the newsprint, 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 "pulverization" phenomenon, Affect the printing quality of color newspapers.
2. If the newsprint has a tight texture and the ink absorption is too weak, if the ink viscosity is high and the printing pressure is low, the connecting material in the ink cannot quickly penetrate into the newsprint, so that the ink layer of the color graphic printing can not be dried in time. Causes "smudge" phenomenon; especially in the high-speed printing of four-color newspaper wheels, the ink layer of the first color is not dried in time, and the viscosity of the ink is still maintained at a low value, which may cause the subsequent color ink to be overprinted, and even "mixing" "Phenomenon, affecting the printing quality of color newspapers.
Therefore, in high-speed newspaper rotary offset printing, newsprint has better ink absorption, lower ink viscosity and improves its fluidity. The viscosity of ink in color newspaper printing is too large or too small will affect the quality of color newspaper printing. Excessive viscosity will cause powdering and hair loss, and it is also easy to cause uneven ink transfer; too low viscosity will easily cause ink emulsification and unbearable printing. Insufficient drying, dirty 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 fibers are thicker, looser in texture, and have a greater degree of water absorption than coated paper and offset paper; while color newsprint printing generally uses water-based offset printing technology. During the printing process, there must be expansion and contraction of the paper due to water absorption. 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 / 3-1 / 2 of the horizontal direction. Generally, it can be eliminated by adjusting the registration adjustment roller 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 effect 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 ink on 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 short paper path The newsprint has less deformation due to water absorption and expansion, and 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 color newspaper printing, the amount of "water" in the first color group is the main factor that determines the impact of newsprint water absorption and expansion on the accuracy of overprinting. Therefore, the amount of fountain solution in the first color group should be well controlled to ensure the quality of the color newspaper printing Use the minimum "water volume".
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 have a greater impact on the accuracy of overprinting; and the newsprint becomes brittle, and it is easy to break during printing; too dry newsprint is also easy to print With static electricity, it affects the work of folding, receiving and stacking.
â‘¢It is best to use a tower-type 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 less effect on the accuracy of overprinting, which is conducive to improving the accuracy of overprinting. At present, many printing presses of city newspapers still use unit-type newspaper rotary offset presses. 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 have 1-3 paper breaks during the printing process), which is because each paper break A few minutes of downtime can sometimes damage the printing plate and blanket, making printing impossible without adjusting the plate. If the printing speed is reduced to prevent paper breaks, it is necessary to reduce the efficiency of printing operations. So whether it is a paper break or to reduce the printing speed to prevent paper breaks, it will reduce the printing efficiency. In order not to break the paper, it is very necessary to increase the length of the web break (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 of more than 5 mm are considered unqualified, and eccentricity is likely to cause breakage. Under normal circumstances, the tension and the braking force are balanced; 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; in each short period of time, such as starting, running, joints, braking, etc. The belt tension changes.
6. Newspaper opacity
The color newspaper is a two-sided printed matter, which does not allow through-printing, so newsprint should have a high opacity.
1. Newspaper opacity
Opacity indicates the paper's resistance to light transmission. It is related to the basis weight of the paper and the degree of bleaching. The greater the basis weight, the greater the opacity. For newsprint, as the basis weight decreases, the opacity also decreases, so for newsprint with a lower basis weight (the current paper is about 48g / m2), special attention should be paid to the problem of opacity. Papers with the same thickness and tightness are generally white. The higher, the lower the opacity. Newsprint paper with more wood pulp and darker color has higher opacity than white letterpress paper of the same amount, so newsprint requires higher opacity.
2. Printing 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 newsprint, not only the paper needs to have good opacity, but also good printing opacity.
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