Discussion on the printability of paper in the mos

2022-08-24
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Preliminary study on the printability of paper in color newspaper printing

at this stage, color newspaper is mainly printed by newspaper rotary offset press. The printability of printing materials such as paper, ink, printing plate, blanket, and fountain solution affects the printing quality of color newspaper. Paper and newspaper rotary offset printing ink are the main materials of color newspaper printing brush. Mastering the printability of paper plays an important role in analyzing the suitability of other printing materials and improving the printing quality of color newspaper

first, the whiteness of paper

the whiteness of paper is an indicator of the optical performance of paper, which is a major indicator for color newspaper printing; Generally, the whiteness value of coated paper is about 85-100, while the whiteness of paper is only 50-60. If the whiteness value of paper is high, the contrast between ink and paper is high and the color is bright; Paper contains more mechanical wood paddles, whiteness is not only much lower than coated paper, but also with varying degrees of bottom ash and color deviation. Compared with printing on coated paper, printing with the same ink on paper reduces the gamut range, contrast, brightness, tone range and hierarchical effect

second, paper smoothness

the smoothness of paper is a performance index that reflects the flatness and smoothness of paper surface. The degree of contact between ink and paper in printing varies with the smoothness of paper. Generally speaking, coated paper such as coated paper has high smoothness and low smoothness

1. The smoothness of the paper affects the clarity and bright color of the pictures and texts of color newspaper printing.

the surface of the paper is rough, and small dots will not be printed if they fall on the low recesses of the paper. Small dots are easy to be lost, and large areas of flat printing are difficult to be printed evenly. Ink imprinting on the rough surface will form small gap white dots, thus reducing the density of the ink, reducing the brilliance of the color ink from the current situation, and it is easy to "de powder and hair". In this regard, only thick lines, generally 100lpi-120 LPI, can be used before color newspaper printing, which not only reduces the clarity of the picture, but also makes it difficult to show delicate hierarchical changes

the smoothness of paper is slightly higher, the reduction of points is fine, and the target color of lubricant is bright and bright; However, it should not be too high, otherwise the oil absorption is low, the ink transfer rate is small, the image reproduction effect is reduced, the color level of the picture is not bright, and the images in the field are often "false"

2. Smoothness difference on both sides

smoothness difference on both sides is an indicator of the smoothness difference between the front and back sides of the paper. Since newspapers are two-sided printed matter, the performance difference between the front and back sides must be limited. Generally, the smoothness difference on both sides must not exceed 5%. Smoothness is an important index of paper printability, which requires paper to have high smoothness and small smoothness difference on both sides

III. ink absorption of paper

at present, color newspapers are generally printed on high-speed rotary offset press, which is a sheet fed offset press with high printing speed. 3 Select 3-5 times of the gear to be calibrated (1000KN or 2000kN); There is no coating layer on the surface of the paper, and the surface is not sized or slightly sized, with loose texture and strong ink absorption; Color newspaper printing uses "cold solid" offset rotary ink. After the ink is transferred to the paper, the solvent in the ink quickly penetrates into the paper and dries. The drying process completes the graphic imprinting through free infiltration and pressure infiltration, mainly permeable drying. Therefore, the paper is required to have strong ink absorption to make the ink dry faster on the paper

1. If the paper is too loose and 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, which will affect the conjunctival thickness of the ink layer and make the image and text imprint not bright; It even leads to too much connecting material in the ink penetrating into the paper, and the smoothness of the graphic ink layer becomes poor, affecting the gloss; What is more serious is that the pigment particles lose the protection of the connecting material and appear the phenomenon of "pulverization", which affects the printing quality of the color newspaper

2. If the paper is tight and the ink absorption is too weak, and then when the ink viscosity is high and the printing pressure is small, the connecting material in the ink cannot quickly penetrate into the paper, so that the ink layer of color newspaper graphic imprinting cannot be dried in time, resulting in the phenomenon of "rubbing dirty"; Especially in the high-speed printing of four-color newspaper, the first color ink layer is not dried in time, and the ink viscosity is still maintained at a low value, which may make the subsequent color ink overprint unable, and even appear the phenomenon of "mixed color", affecting the printing quality of color newspaper

therefore, in high-speed newspaper rotary offset printing, the ink absorption of paper is better, the ink viscosity is lower, and its fluidity is improved. In color newspaper printing, the viscosity of the ink is too large or too small, which will affect the quality of color newspaper printing. Too large viscosity will cause de dusting, hair loss, and also easy to cause uneven ink transmission; If the viscosity is too small, it is easy to cause ink emulsification, weak imprinting, untimely drying, rubbing dirt, etc

IV. water absorption of paper

the water absorption and expansion of paper in the printing process is the main factor affecting the overprint accuracy of color newspapers. How to control and eliminate the influence of water absorption and expansion of paper in the printing process on the overprint accuracy is particularly important, especially in newspapers with unsatisfactory equipment and raw material conditions

1. The moisture absorption deformation of paper

the paper fiber is coarse, the texture is loose, and the degree of water absorption elongation is larger than that of coated paper and offset paper; The color newspaper printing generally adopts the water offset printing process, and there must be paper expansion and deformation due to water absorption in the printing process, which has a great impact on the overprint accuracy

① the horizontal water absorption and expansion of paper have the most obvious influence on the overprint accuracy

in color newspaper printing, the thread direction of paper is consistent with the moving direction of paper tape. The longitudinal expansion and contraction of the strand is small, about 1//2 in the transverse direction. Generally, it can be eliminated by adjusting the registration adjustment roll between the units, which has little impact on the overprint accuracy; The horizontal expansion and contraction change has a large overprint error (up to 0.3mm) without taking certain elimination measures, which greatly affects the printing quality of color newspapers

② the influence of water absorption and expansion on overprint accuracy is also different with different combination forms of printing units

the printing process of color newspaper is to print four-color ink on the paper in turn. Due to the long development of the paper path over the years, the paper absorbs water and expands and deforms greatly, which has a significant impact on the overprint accuracy; Because the paper path of the tower newspaper rotary offset press is short, the water absorption and expansion deformation of the paper is small, which has little impact on the overprint accuracy

2. Countermeasures for paper moisture absorption deformation fault

① 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 influence of paper water absorption and expansion on overprint accuracy. Therefore, the amount of fountain solution in the first color group should be well controlled, and the minimum amount of "water" should be used on the premise of ensuring the quality of color newspaper printing

② the web should not be too dry, otherwise the water absorption is also large, the expansion rate is high, and the water absorption expansion has a great impact on the overprint accuracy; Moreover, the paper becomes brittle and easy to break when printing; Too dry paper is also easy to carry static electricity in the printing process, which affects the folding, receiving and stacking work

③ it is best to use a color tower newspaper rotary offset press, because the paper path between four-color printing is shorter, and the water absorption and expansion deformation of the paper have less impact on the overprint accuracy, which is conducive to improving the overprint accuracy. At present, many local newspaper printing houses are still using the unit type newspaper rotary offset press. The paper path between the units is long and there is no fan-shaped expansion device. When printing color newspapers, the water absorption and expansion of the paper has a significant impact on the registration accuracy. In this case, according to the size of the paper stretch in printing, the influence of the water absorption and expansion of the paper on the overprint accuracy can be eliminated by changing the film position when making up

v. tensile strength of paper

when printing, the paper tape of the paper should drag the paper roll to rotate. A higher tensile strength can ensure that the paper tape will not be broken. The tensile strength of the paper mainly depends on the binding force between the fibers and the strength of the fibers themselves. Therefore, the variety of pulp for papermaking and good beating are the key to obtain a higher tensile strength. Of course, adding a suitable reinforcing agent is also beneficial to improving the tensile strength of the paper. Among all the items of paper printing suitability, one of the items that have the greatest impact on the printing quality is the paper breaking rate (100 Web pages break 1-3 times in the printing process), which is due to the downtime of several minutes for each paper breaking, and sometimes damage to the printing plate and blanket, resulting in the inability to print without adjusting the version. If you reduce the printing speed to prevent paper breaking, it is bound to reduce the efficiency of the printing operation. Therefore, whether it is paper breaking or reducing the printing speed to prevent paper breaking, it will reduce the printing efficiency. In order not to break the paper, it is necessary to appropriately increase the breaking length of the web (about 3000m according to the standard)

in addition, if the quality of the paper roll itself is unqualified or stored improperly, the center of gravity of the paper roll will shift, resulting in eccentricity, and it is also easy to break the paper. It is generally believed that if the eccentricity of the paper roll exceeds 5mm, it is unqualified, and eccentricity is easy to cause fracture. Under normal conditions, the tension and braking force are balanced; When using eccentric paper roll, the paper tape is vibrated and the tension is unstable due to the change of paper roll radius; In each short period of time, the tension of the paper tape changes in the event of startup, operation, joint, braking, etc

VI. paper opacity

color newspapers are printed on both sides, and through printing is not allowed, so it is required that the paper should have a high opacity

1. Paper opacity

opacity refers to the resistance of paper to light transmission. It is related to factors such as the quantitative and bleaching degree of paper. Large quantitative opacity is larger. For paper, with the reduction of the quantity, the opacity also decreases. Therefore, for paper with low quantity (the current paper is about 48g/m2), we should pay particular attention to the opacity. Paper with the same thickness and tightness generally has higher whiteness and lower opacity. The opacity of paper with more wood pulp and dark color is higher than that of letterpress paper with the same amount of white, so paper requires higher opacity

2. Printing opacity

printing opacity is very important for color newspaper printing. It depends not only on the opacity of the paper, but also on the degree of ink penetration into the paper. In other words, in addition to the opacity of the original paper, the more easily the ink is soaked in the paper, the worse the printing opacity is, resulting in a greater impact on the printing of one side on the other side. Therefore, paper printing requires not only good opacity, but also good printing opacity. (end)

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