A Study on Printing Applicability of Printing Paper in Color Newspaper Printing

At this stage, color newspapers are mainly printed by newspaper offset presses. The printability of printing materials such as paper, ink, printing plates, blankets, and fountain solutions affects the printing quality of color newspapers. Newsprint and rewind inks are color Reporting the main materials for printing, mastering the printability of newsprint plays an important role in analyzing the suitability of other printing materials and improving the printing quality of color newspapers.

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 newspaper printing. In general, the whiteness value of coated paper is about 85-100, and 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 lower than the coated paper, but also has different degrees of bottom ash and color shift. The same ink is printed on newsprint and printed on coated paper. The range of color gamut and contrast are reduced, the vividness is reduced, the range of gradation is shortened, and the gradation effect is reduced.

Second, newsprint smoothness

The smoothness of the paper is a measure of the smoothness and smoothness of the surface of the paper. The degree of smoothness of the paper is different, and the degree of contact between the ink and the paper in printing is different. In general, coated paper such as coated paper has higher smoothness and lower newsprint smoothness.

1. The smoothness of newsprint affects the clarity and vividness of printed newspapers and graphic arts

The surface of newsprint is rough. Small dots will fall on the lower surface of the paper and will not be printed. Small dots are easy to lose. Large areas of flat mesh are difficult to print evenly, and ink is imprinted on a rough surface to form fine gap white dots. Therefore, the density of the ink is reduced, the vividness of the color ink is reduced, and it is easy to “bleach and lose hair”. In this regard, the use of thicker screens in pre-press screenings is generally 100 Lpi-120 Lpi, which not only reduces the clarity of the picture, but also makes it difficult to show delicate levels.

The smoothness of newsprint is slightly higher, the reproduction of dots is fine, and the color newspapers are bright and bright; however, they cannot be too high. Otherwise, the oil absorption is low, the transfer rate of the ink is small, the copying effect of the image is reduced, and the color tone of the screen is not adjusted. Obviously, the image of the field part is often "embossed."

2. Smoothness difference between two sides

Smoothness Two-sidedness is an indicator of the difference between the smoothness of the front and back sides of paper. Since newspapers are printed on both sides, the performance difference between the front and back must be limited. Usually, the difference between the two sides of smoothness must not exceed 5%. Smoothness is an important indicator of the printability of newsprint, and it requires newsprint to have a high degree of smoothness and a small degree of smoothness on both sides.

Third, newsprint ink absorption

At present, color newspapers are generally printed on high-speed web offset presses. The printing speed is 3 to 5 times faster than that of sheet-fed offset presses. There is no paint layer on the surface of newsprint, and the surface is not sized or micro-sized, and the texture is loose and has strong Ink absorption; Color newspaper printing uses “cold-solid” offset printing transfer ink. After the ink is transferred to the paper surface, the ink in the wheel ink quickly penetrates into the newsprint paper and is fixed and dry. The drying process is common through free penetration and pressure infiltration. Photographic imprinting is completed with permeability drying. Therefore, newsprint paper is required to have relatively strong ink absorption so that the ink can be dried faster on newsprint.

1. If the newsprint is too loose and the ink absorption is too strong, and then the ink is low in viscosity and the printing pressure is high, the ink will permeate the back of the paper, and the phenomenon of overprinting will occur, affecting the thickness of the conjunctiva of the ink layer, so that the image is not printed. Brightness; even lead to excessive penetration of the ink in the newsprint into the newsprint, the graphic level of the ink layer deteriorated and affect the gloss; more serious is the pigment particles lose the protection of the connection material and the phenomenon of "powdering", Affect the printing quality of color newspapers.

2. If the newsprint is dense and the ink absorption is too weak, then the ink viscosity is high and the printing pressure is low. The ink in the ink cannot penetrate into the newsprint quickly, so that the color printing graphic imprinting ink layer cannot be dried in time. Causes "dirty" phenomenon; especially in the high-speed printing of four-color reels, the first color ink layer is not dried in time, and the ink viscosity is still maintained at a relatively low value, which may cause subsequent color inks not to be overprinted, or even appear to be "color mixing". "The phenomenon affects the print quality of Caibao.

Therefore, in high-speed newspaper web offset printing, newsprint paper has better ink absorption, lower ink viscosity and improved fluidity. Too much or too little ink viscosity in the printing of newspapers will affect the printing quality of the newspapers. Excessive viscosity will cause de-powdering and lint-free, and it will also cause uneven ink transfer. When the viscosity is too small, ink emulsification and imprinting will be caused. Drying is not timely, dirty damage and so on.

Fourth, newsprint water absorption

Newsprint paper has a large amount of water absorption, and the water absorption and shrinkage of paper in the printing process are the main factors affecting the accuracy of color newspaper overlays. How to control and eliminate the impact of paper absorption and expansion during printing on the accuracy of overprinting, especially in the case of unsatisfactory conditions of equipment and raw materials The newspaper, this issue is particularly important.

1. Hygroscopic deformation of newsprint

Newsprint fibers are thicker and looser in texture, and the water absorption elongation is greater than that of coated paper and offset paper. However, there is a water-based offset printing process in color newspaper printing. In the printing process, there is bound to be stretching and deformation of paper due to water absorption. Accuracy exists in overprinting. Big impact.

1 The horizontal water absorption and expansion of newsprint has the most obvious influence on the overlay accuracy.

In color newspaper printing, the direction of the newsprint paper is consistent with the direction of the tape movement. The longitudinal stretching of the silk thread is relatively small, about 1/3-1/2 of the transverse direction, which can be eliminated by adjusting the registration rollers between the units. The influence on the overlay accuracy is not significant; the horizontal stretching change is not taken in a certain degree. In the case of elimination measures, overprint errors are large (up to 0.3 mm or more), which greatly affects the print quality of the color newspaper.

2 The combination of printing units is different, and water absorption and expansion have different effects on overprint accuracy.

The printing process of the color newspaper is to print four colors of ink in order on the paper. The type of newspaper offset web offset press has a long paper path. The newsprint paper absorbs water and expands greatly, which has a significant influence on the accuracy of overprinting; the tower type newspaper web offset press has a shorter paper path. The newsprint paper absorbs less stretching deformation, which has less effect on overprint accuracy.

2. Measures against the failure of newsprint moisture deformation

1 Control the amount of fountain solution in the printing process.

In color newspaper printing, the “water” quantity of the first color group is the main factor that determines the impact of newsprint water absorption and expansion on overprinting accuracy. Therefore, it is necessary to control the amount of fountain solution of the first color group well, under the premise of ensuring the printing quality of color newspapers. Use the minimum amount of water.

2Reel newsprint should not be too dry, otherwise the water absorption is larger, the expansion and contraction rate is higher, and the water absorption and expansion have a great influence on the overlay accuracy; and the newsprint becomes brittle, and the paper breaks easily during printing; too dry newsprint is also easy during the printing process. With static electricity, affect the depreciation, closing, accumulation work.

3 The best color is to use a tower type newspaper offset press, because the paper path between the four-color printing of the paper is shorter, and the water-sinking deformation of the newsprint has little influence on the overlay accuracy, which is beneficial to improving the overlay accuracy. At present, there are many local newspaper printing houses still using a rotary newspaper press with a unit type newspaper. The paper path between the units is long and there is no fan-shaped expansion device. The water absorption and expansion of the paper in the printing color report has a significant impact on the accuracy of registration. In this case, the influence of the paper suction and expansion on the overprint accuracy can be eliminated by changing the position of the film during the imposition according to the amount of stretching of the paper in the printing.

Fifth, the tensile strength of newsprint

When the newsprint paper tape is dragged to rotate, the high tensile strength ensures that the tape is not broken. The tensile strength of the newsprint mainly depends on the binding force between the fibers and the strength of the fiber itself. Pulp varieties and good beating are the key to achieving higher tensile strength. Of course, adding suitable reinforcing agents can also be beneficial to improving the tensile strength of newsprint. One of the items that have the greatest impact on print quality in all newsprint printability projects is the break rate (100 web breaks occur 1-3 times during the printing process). This is because once every break A few minutes of downtime can sometimes damage the plates and blankets, rendering them unprintable. If the printing speed is reduced to prevent paper breakage, it is necessary to reduce the efficiency of the printing job. Therefore, whether it is a paper break or to prevent the paper break to reduce the printing speed, it will reduce the printing efficiency. In order to prevent paper breakage, it is necessary to properly increase the breaking length of the web (about 3000 m in terms of standard).

In addition, the quality of the newsprint paper itself is not qualified or stored improperly, the center of gravity of the paper roll shifts, resulting in eccentricity, and it is also easy to break the paper. It is generally believed that when the eccentricity of the paper roll exceeds 5 mm, it is unqualified, and the eccentricity is liable to cause breakage. Under normal circumstances, tension and braking force balance; with eccentric paper roll, due to changes in the radius of the paper roll, the tape is subject to oscillation, tension is not stable; in each short period of time boot, operation, joints, brakes and other events, paper The belt tension changes.

Newsprint Opacity

The newspapers are printed on both sides and are not allowed to pass through. Therefore, newsprint should have high opacity.

1. Newsprint opacity

The opacity indicates the paper's resistance to light transmission. It is related to the paper's quantification and bleaching degree, etc. The quantitative and large opacity are relatively large. For newsprint, as the quantitative decrease, the opacity also decreases, so for newsprint with low quantification (about 48g/m2 for current paper), in particular, attention should be paid to the problem of opacity. The same thickness and tightness of the paper is generally whiteness. The higher, the lower the opacity. Newsprint papers with more wood pulp and darker colors have higher opacity than letter papers with higher quantification. Therefore, newsprint requires higher opacity.

2 printing opacity

Printing opacity is 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. In other words, in addition to the opacity of the original paper, the newsprint which is more easily saturated with ink has a poorer printing opacity, and as a result, the printing on one side has a greater influence on the printing on the other side. Therefore, newsprint printing requires not only good opacity, but also good opacity.