A preliminary study on the suitability of paper printing in color printing
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At this stage, the color report is mainly printed by newspaper rotary offset printing machine. The printing suitability of printing materials such as paper, ink, printing plate, blanket and dampening solution affects the printing quality of the color newspaper. Newsprint and newspaper offset printing ink are color. Reporting the main materials of printing, mastering the printability of newsprint plays an important role in analyzing the suitability of other printed materials and improving the quality of color printing.
First, the whiteness of newsprint
The whiteness of paper is an indicator of the optical performance of paper. It is a major indicator for color printing. Generally, the whiteness of coated paper is around 85-100, while the whiteness of newsprint is only 50-60. The whiteness 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 pulp, and the whiteness is not only far lower than the coated paper, but also has different levels of bottom ash and color shift. The same ink is printed on newsprint compared to printing on coated paper, the color gamut range, contrast is reduced, the vividness is reduced, the tonal range is shortened, and the layering effect is reduced.
Second, the news paper smoothness
The smoothness of the paper is a performance indicator that reflects the smoothness and smoothness of the paper surface. The smoothness of the paper is different, and the ink is in close contact with the paper in printing. In general, coated paper such as coated paper has a higher smoothness and newsprint has a lower smoothness.
1. The smoothness of newsprint affects the clarity and color of the printed text of the color newspaper.
The surface of the newsprint is rough, the small dots fall on the low concave of the paper, it will not be printed, the small dots are easy to be lost, the large flat mesh is difficult to print evenly, and the ink is printed on the rough surface to form fine gap white spots. Thereby, the density of the ink is reduced, the vividness of the color ink is lowered, and it is easy to "powder and lint". In this regard, only the thicker network cable can be used when the color newspaper is pre-pressed, generally 100Lpi-120 Lpi, which not only reduces the clarity of the picture, but also makes it difficult to show delicate level changes.
The smoothness of newsprint is slightly higher, the reduction of dots is fine, and the color of the color is bright and bright; but it can't be too high, otherwise the oil absorption is low, the transfer rate of ink is small, the copying effect of the image is reduced, and the color tone of the picture is not It is clear that the images in the field are often “defective”.
2. Smoothness two sides
Smoothness Two-sidedness is an indicator of the difference in smoothness between the front and back of the paper. Since the newspaper is a two-sided print, the difference in performance between the front and the back must be limited. Generally, the smoothness must not exceed 5%. Smoothness is an important indicator of the suitability of newsprint printing, requiring newsprint to have a high degree of smoothness and a small smoothness.
Third, the ink absorption of newsprint
At present, the color newspaper is generally printed on a high-speed rotary offset printing machine, and the printing speed is fast, which is 3-5 times that of a sheet-fed offset printing machine; the surface of the newsprint has no coating layer, the surface is not glued or micro-sizing, the texture is loose, and the texture is strong. Ink-receiving; color printing uses “cold-solid” offset printing rotary ink. After the ink is transferred to the paper surface, the solvent in the ink of the reel quickly penetrates into the newsprint and is fixed and dried. The drying process is common through free penetration and pressure penetration. The graphic imprint is completed with osmotic drying. Therefore, newsprint is required to have a relatively strong ink absorption, so that the ink is dried faster on newsprint.
1. If the newsprint is too loose and the ink absorption is too strong, and then the ink viscosity is low and the printing pressure is large, the ink will penetrate into the back of the paper, and the offset phenomenon will occur, which will affect the thickness of the conjunct film of the ink layer, so that the graphic imprint does not Bright; even lead to excessive penetration of the binder in the ink into the newsprint, the leveling of the ink layer of the image is deteriorated and affecting the gloss; more serious is that the pigment particles lose the protection of the binder and appear "pulverized" phenomenon. Affect the printing quality of the color newspaper.
2. If the newsprint is tight in texture and the ink absorption is too weak, and then the ink viscosity is high and the printing pressure is small, the connecting material in the ink cannot penetrate into the news paper quickly, so that the ink layer of the color image can not be dried in time. Leading to the phenomenon of “dirty”; especially in the high-speed printing of four-color newspaper wheel, the first color ink layer is not dried in time, and the ink viscosity is maintained at a low value, which may cause the subsequent color ink to be overprinted, or even “mixed colors” "Phenomenon, affecting the printing quality of the color newspaper.
Therefore, in high-speed newspaper web offset printing, newsprint has better ink absorption, lower ink viscosity and increased fluidity. If the viscosity of the ink in the color printing is too large or too small, it will affect the printing quality of the color report. If the viscosity is too large, it will cause powder removal and hair loss, and it will also cause uneven ink transfer. If the viscosity is too small, the ink will be emulsified and the printing will be weak. Drying is not timely, dirty and so on.
Fourth, the water absorption of newsprint
Newsprint has a large amount of water absorption. The water absorption and contraction of paper during printing is the main factor affecting the accuracy of color printing. How to control and eliminate the influence of paper water absorption and shrinkage on the printing accuracy during printing, especially in the condition of equipment and raw materials. The newspaper, this issue is particularly important.
1. Hygroscopic deformation of newsprint
The news paper fiber is coarser and looser in texture, and the water absorption elongation is larger than that of coated paper and offset paper. The color printing is generally used in the water offset printing process. In the printing process, the paper must be stretched and deformed due to water absorption, and the overprinting precision is Larger impact.
1.1 The lateral water absorption and contraction of newsprint has the most obvious influence on the accuracy of overprinting.
In the color printing, the direction of the silk of the newsprint is consistent with the direction of movement of the tape. The longitudinal expansion and contraction of the wire is small, about 1/3-1/2 of the transverse direction. Generally, it can be eliminated by adjusting the register adjustment roller between the units, and has little effect on the overprinting accuracy; the lateral expansion and contraction changes are not taken. In the case of elimination measures, the overprinting error is large (can reach 0.3mm or more), which greatly affects the printing quality of the color newspaper.
1.2 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 the four-color ink on the paper in turn. The unit-type newspaper rotary offset printing machine has a long paper path, and the news paper absorbs water and expands and deforms greatly, which has obvious influence on the overprinting precision. The tower newspaper rotary offset printing machine has a short paper path. The newsprint has less water-splitting deformation and less influence on the overprinting accuracy.
2. Newsprint moisture absorption deformation countermeasures
2.1 Control the amount of dampening solution in the printing process.
In the color printing, the “water” quantity of the first color group is the main factor that determines the influence of the water absorption and contraction of the newsprint on the accuracy of the overprinting. Therefore, it is necessary to control the amount of the first color group dampening solution well, and to ensure the quality of the color printing. Use the minimum amount of water.
2.2 reel newsprint should not be too dry, otherwise the water absorption is also large, the expansion ratio is high, the water absorption and expansion have a great influence on the overprinting accuracy; and the newsprint becomes brittle, and the paper is easy to break when printing; the too dry newsprint is also easy in the printing process. With static electricity, affecting the report, receiving, and collecting work.
2.3 It is best to use the tower newspaper rotary offset printing machine because the paper path between the four-color printing of the paper is short, and the water-jet stretching deformation of the news paper has little influence on the overprinting precision, which is beneficial to improve the overprinting precision. At present, there are still many newspaper newspaper printing houses in the city newspapers still using the unit-type newspaper rotary offset printing machine. The paper path between the units is long and there is no fan-shaped expansion device. When printing the color report, the water absorption and contraction of the paper has a significant influence on the registration accuracy. In this case, according to the amount of expansion and contraction of the paper in the printing, the influence of the water absorption and contraction of the paper on the overprinting accuracy can be eliminated by changing the position of the film during the imposition.
Fifth, the tensile strength of newsprint
When printing, the paper strip of newsprint should be dragged to rotate the paper roll, and the higher tensile strength can ensure that the paper tape is not broken. The tensile strength of the newsprint mainly depends on the bonding force between the fibers and the strength of the fiber itself, so the papermaking The use of 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 projects that have the greatest impact on print quality in all items of newsprint printability is the paper break rate (100 rolls of paper breaks 1-3 times during the printing process), which is due to each paper break. A few minutes of downtime can sometimes damage the plates and blankets, making it impossible to print without the adjustment. If you reduce the printing speed to prevent paper breaks, it is necessary to reduce the efficiency of printing operations. Therefore, whether it is broken paper or to prevent the paper break and reduce the printing speed, the printing efficiency will be reduced. In order not to break the paper, it is necessary to appropriately increase the crack length of the web (about 3000 m according to the standard).
In addition, the quality of the news paper roll itself is unqualified or improperly stored, the center of gravity of the paper roll is offset, eccentricity is generated, and paper breaks easily. It is generally believed that if the eccentricity of the paper roll exceeds 5 mm, it is unacceptable, and the eccentricity is liable to cause breakage. Under normal conditions, tension and braking force are balanced; when using eccentric paper rolls, the paper tape is oscillated due to changes in the radius of the paper roll, and the tension is unstable; in each short period of time, the machine is turned on, running, joints, brakes, etc., paper The belt tension changes.
Sixth, Newsprint opacity
The color report is a two-sided print, which is not allowed to print through, so the newsprint should be required to have a high opacity.
1. Newsprint opacity
Opacity indicates the resistance of paper to light transmission. It is related to factors such as the amount of paper and the degree of bleaching. The quantitative opacity is large. For newsprint, as the quantitative decrease, the opacity also decreases. Therefore, for the newsprint with lower basis weight (current paper is about 48g/m2), pay special attention to the problem of opacity, the same whiteness of paper of the same thickness and tightness. The higher, the lower the opacity. Newsprint paper with more wood pulp and darker color has higher opacity than the same white paper, so newsprint requires higher opacity.
2. Printing opacity
Print opacity is important for color printing, depending not only on the opacity of the newsprint, but also on the extent to which the ink penetrates into the paper. That is to say, in addition to the opacity of the original paper, the more easily the ink is soaked, the worse the printing opacity, and as a result, the printing on one side has a greater influence on the printing on the other side. Therefore, when printing newsprint, it is required not only to have good opacity of the paper, but also to have good printing opacity.