The impact of newsprint performance on the quality of color printing
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China's newsprint is divided into four grades A, B, C, and D according to quality standards. Among them, grades A and B are applicable to high-speed rotary offset presses, while lottery printing is generally performed on high-speed rotary machines.
Ink absorption
1. The influence of ink absorption of newsprint on the quality of color newspaper printing
The color report is generally printed on a high-speed web offset press. Because of its fast printing speed, newsprint is required to have a strong ink absorption, that is, the ink dries quickly on the newsprint.
There is no paint layer on the surface of the news paper, which is relatively loose and has strong ink absorption. The drying of the ink is mainly accomplished by the osmosis of the solvent, and after the ink is transferred to the paper surface, the solvent therein rapidly penetrates into the paper.
The ink is quickly fixed and dried on the paper surface, but if the newsprint is excessively loose, the ink absorption is too strong, the ink used is low in viscosity, the printing pressure is large, and the ink penetrates into the back of the paper, which causes offset and affects the ink layer. The thickness of the conjunctiva makes it impossible for the imprint to exhibit the desired color saturation on the paper surface, which affects the quality of the printed matter.
2. Properly reduce ink viscosity and improve fluidity
In high-speed operation, this infiltration-based drying method requires proper reduction of ink viscosity and fluidity. When the viscosity of the ink is too large or too small, it is unfavorable for printing. If the viscosity is too large, it will cause hair loss and powder loss. It is also easy to cause uneven ink transfer. The viscosity is too small, which may cause ink emulsification, imprinting is not strong, and printing gloss is not good. higher. Unlike the adhesiveness, as the printing speed is increased, the viscosity of the ink at the time of printing is increased.
3. The dot gains the quality of the print
From the main characteristics of newsprint, compared with general coated paper, the newsprint image has poor definition, small color reproduction range, low color purity, easy to lose bright dot, and dark tone. The range is small, and the flat net is difficult to print evenly. The reason is that the sizing degree of the news paper is too low (generally no glue), the ink fluidity is too large, and the printing pressure is too large.
Under the action of printing pressure, the ink will penetrate into the news paper at the same time, and will form different degrees of diffusion, so that the dot gain rate is higher than that of the coated paper, which often causes the dark-adjusted dot to be leveled, even paste, and reduce the effect of image darkness. Shorten the range of the tone, resulting in blurred image, which can reduce the midtones during image processing, leaving room for the dot gain, and brightening the dark tone layer to compensate for the dark level of the image, while at the same time Sharpening can also improve the sharpness of the image.
Tensile strength
When printing, the paper Emperor should drag the paper roll to rotate, and the high 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 pulp for papermaking Variety and good beating are 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 the newsprint printability projects is the paper break rate (the number of times a web roll breaks during the printing process), which is due to the number of downtimes per paper break. Minutes, sometimes damage to the plates and blankets, so that the prints are not printed. If you reduce the printing speed to prevent paper breaks, it is necessary to reduce the efficiency of printing operations. Therefore, whether it is a paper break or to prevent the paper break from lowering the printing speed, the printing efficiency is lowered, and 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).
Smoothness
Smoothness is an indicator of the degree of unevenness on the surface of a paper, indicating that the surface of the paper is flat. The degree of smoothness, the smoothness of newsprint determines the closeness of paper contact with the printing plate and the quality of the color printing.
1. The smoothness of newsprint affects the clarity and color saturation of color printing
The higher the smoothness of the newsprint, the better the fine reduction of the dots. If the smoothness is too low, the small dots will not be printed on the lower concave surface of the paper, so that the large-area flat net is difficult to print evenly, the small dots are easily lost, and the writing of the printed matter or the sharpness of the picture is not good. Only thicker network cables can be used in image plate making, which not only reduces the sharpness of the image but also makes it difficult to show delicate level changes.
The smoothness of newsprint is high, the oil absorption is low, the color of the printed matter is bright and bright, and the requirement for smoothness of newsprint is generally less than 90s. However, if the smoothness is too high and there is not enough ink supply during printing, the transfer rate and the transfer amount of the ink are relatively small, resulting in the image copying effect and the saturation of the imprinted group color, and the hue level of the color picture color is Not clear. As for the image reductiveness of the field part, the handwriting and the image are often "faint". If the ink supply amount and printing pressure are increased, the offset phenomenon is likely to occur.
2. The smoothness of the broken net paper is two-sided
One concept closely related to smoothness is the smoothness two-sidedness, which is an indicator of the difference in smoothness between the front and back of the paper, expressed as a percentage.
Smoothness two-sided difference = (front smoothness - reverse smoothness) / front smoothness × 100%
The performance of the front and back of the general print is somewhat different. Since the newspaper is a two-sided print, the performance difference between the front and the back must be limited. Generally, the smoothness of the two sides must not exceed 5%. Smoothness is an important indicator of the suitability of newsprint printing, requiring newsprint to have high smoothness and small smoothness.
Opacity
Since the newspaper is a two-sided print, the paper is not allowed to be printed, so the newsprint should be required to have a high opacity.
1. Opacity and paper relationship
For white paper, opacity indicates the resistance of the paper to light transmission. It is related to the setting of the paper and the degree of bleaching. The greater the quantitative, the greater the opacity. For newsprint, the opacity also drops sharply with the decrease in the quantitative amount. Therefore, especially for the less quantitative newsprint, the problem of opacity should be paid attention to. For the same thickness and tightness, the higher the whiteness, the lower the opacity. For newsprint papers containing more wood pulp and darker colors, the opacity is significantly higher than that of the same white paper, because the paper with high whiteness absorbs light less than the paper with low whiteness. Strong.
2. Printing opacity
Print opacity is also important for color prints, 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.
BaiDu
Whiteness is the value indicating the optical performance of paper according to the national standard (GB1910-89). The whiteness of newsprint is 45% to 52%. In fact, users like to take higher, and some hope to reach more than 70%. Color, about 45% of whiteness is also possible, which is related to readers' reading habits.
Newsprint not only has low whiteness, but also has different levels of grayscale and color shift. The same ink printed on newsprint has a lower color gamut and less vividness than printing on coated paper. Due to the contrast reduction, the range of tone and color reproduction is also shortened, and the layer effect is reduced. The contrast, high light, and sub-high light of the image can be widened during the image tone processing, and the whiteness of the news paper can be fully utilized to express the brightness of the highlight of the image. For colors that are difficult to faithfully reproduce and have high saturation, the purity of the primary ink should be fully utilized to show the best color saturation on newsprint.
Hygroscopic deformation
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 expansion 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 thicker, the density is small, the water absorption elongation is larger than that of coated paper, and the offset paper is generally used. The printing process generally adopts the offset printing process. In the printing process, the paper has the expansion and contraction due to water absorption and drying. Great impact.
(1) The horizontal water absorption and contraction of newsprint has the most obvious influence on the accuracy of overprinting. In the process of printing newsprint, the longitudinal expansion and contraction of the silk thread is small, about 1/3-1/2 of the transverse direction. Generally, it is eliminated by adjusting the registration adjustment roller between the units, which has little effect on the overprinting precision, and the lateral expansion and contraction changes. The overprinting error can reach 0.3mm or more without taking certain elimination measures, which greatly affects the printing quality of the color newspaper.
(2) Different equipment structure The influence of water absorption and expansion on the accuracy of overprinting is also very different. The printing process of newspapers is to print four colors of paper in sequence. Satellite equipment is very beneficial for color printing, and the combined equipment has obvious influence on precision due to the long paper path.
(3) The paper conveying mode directly affects the positioning and deformation resistance of the paper in the printing process, and plays an important role in the printing overprinting precision. Newsprints are generally thin papers of about 50g/mZ. The paper is loose and easily deformed by the influence of environmental humidity. When the rotary printing machine is used, the paper is controlled by tension during transportation, and the overprinting accuracy of the web newsprint is low.
2. Prevention and remediation of newsprint hygroscopic deformation failure
By improving the process, materials, and equipment, the printing quality of the color newspaper can be effectively improved.
(1) Control the ink and water balance during the printing process. The amount of water in the first unit at the time of printing is the main factor that determines the influence of the amount of water absorption and contraction on the accuracy of overprinting.
(2) The newsprint used for printing color newspapers should not be too dry, otherwise it will not only be easy to break the paper, but also have a large amount of water absorption and a high expansion ratio.
(3) Do not store the environment in accordance with the requirements of the specification and the storage time should not be too long.
(4) Paper performance is a very important factor. To produce high-quality color reports, try to use good quality newsprint.
(5) The best process structure of the color printing press is the tower type, because the paper path between the four-color printing of the paper is short, which is beneficial to improve the overprinting precision.
(6) When ordering equipment, be aware that whether it is a double-frame machine or a single-frame machine, a fan-shaped expansion roller must be added to improve the control of water absorption and contraction of the paper.
In actual production, according to the performance conditions of the selected newsprint, different printing processes should be adopted to effectively select inks, plates, fountain solutions, blankets, etc., and continuously improve the operation level of the printing workers, and print high quality products. Color newspapers to improve the print quality of the color newspaper.