Effect of Paper Performance on Newspaper Printing Quality (II)

Third, the paper moisture absorption and dimensional stability of the print quality

Phototype offset printing has gained popularity in newspaper production. Offset uses the principle of immiscible oil and water, and the principle of water-immiscible printing is used for printing on the PS plate surface. When printing, the PS plate needs to be mixed with water before printing. After ink is applied, transfer is performed. . Water is an indispensable requirement for offset printing. The non-printing area of ​​the positive PS plate absorbs water and ink, and the printing area does not absorb water and ink. During the printing process, the water is transferred to the paper. Paper is mainly composed of natural plant fibers. After absorbing water, it will tend to stretch and cause the paper to stretch. After drying, the paper will shrink due to the shrinkage of the fibers. Due to the tightness of newsprint paper and the high proportion of paper plant fibers, its hygroscopicity is strong and its dimensional stability is lower than other papers. Grasping the hygroscopicity and dimensional stability of paper is the key to improving the quality of overprinting.

There are many factors that cause overprinting errors in the production of color newspapers, such as the error of film repetitive precision, plate-making errors, tension instability, printing pressure problems, improper adjustment of printing press overprinting, overprint of printing presses, and poor dimensional stability of paper sheets. The overprint error caused by various factors is not the same, there are fixed overprinting errors and variable overprinting errors; there are transverse overprinting errors, overprinting errors in the circumferential direction and overprinting errors in the diagonal direction; there are full versions. Overprinting errors, but also partial overprinting errors. In addition to overprint errors caused by equipment reasons, plate-making reasons, photosetting reasons, etc., overprint errors caused by paper causes are not to be overlooked. The overprint errors caused by paper are mainly caused by the deformation of the paper caused by the change of the water content during the operation of the paper ribbons. Among them, it is difficult to control the dimensional stability in the transverse direction, that is, the moisture generated after the paper absorbs moisture. Fan-shaped and fan-shaped shrinkage after drying. Since the performance of various papers in terms of moisture absorption and dimensional stability is not consistent, and the paper used for different printing products may be different, the control of overprinting errors due to the paper properties is also more complicated.

In the production of newspapers, the paper must absorb water after many imprinting processes, and the paper is dried during the printing process between the various color groups, which in turn leads to a certain amount of stretching in the printing process. According to statistics, the horizontal and vertical water-absorbing and retracting amounts of paper fibers are different, and the lateral width is generally about 2 times of the longitudinal dimension. Paper production In order to ensure the stability of the paper tension, the paper must be printed in the direction of the silk, and the offset error in the direction of the silk must be eliminated by the expansion of the fan-shaped expansion roller and the line of imposition. This point is very important for printing machines with long paper path between printing units. The size and volatility of the dimension in the cross-thread direction are large, and the control is difficult. At present, the layout design of color newspapers is becoming more and more complicated, and higher requirements are imposed on the accuracy of overprinting. In high-speed four-color printing up to 30,000 to 5,000 rpm, it is required to be able to achieve an overlay error of 0.1 mm or less, so paper is required in particular. Achieve a higher level of dimensional stability.

The hygroscopicity and dimensional stability of the paper have a certain correlation, the size of the moisture absorption of the paper determines the amount of water absorbed by the paper, and the size of the water absorption is an important factor in determining the amount of paper deformation. The drier the paper is, the smaller the tightness is, and the greater the hygroscopicity, the greater the change in the amount of water absorption and contraction of the paper and the more pronounced the effect on the print register. It is an important part of paper management to ensure that the paper's moisture content meets the requirements of the index, to reduce the paper's storage time, and to protect the outer packaging during storage.

Fourth, the paper absorb ink on the print quality

The ink absorption of paper refers to the absorption capacity of paper ink or the penetration ability of ink to paper. It is not only related to paper but also related to the performance of ink. The following only discusses the effect of paper ink absorption on the quality of printed paper.

Newsprint newspapers are printed with cold-setting inks. During the printing process, the combination of ink and paper is the process by which the binder in the ink penetrates into the paper and the solvent evaporates. The ink absorption ability of paper is also the penetrability and speed of the ink on the paper. It has a great influence on the composition and physical properties of the ink on the paper after drying. In other words, the ink absorption ability of the paper has a great influence on the print quality of the final printed product.

The most fundamental point in improving printing quality is to ensure the accurate reproduction of outlets. The higher the value of outlet expansion, the lower the amount of outlets will be, and the level adjustments reflected by outlets will be affected to varying degrees. The paper's ink absorption performance and to a certain extent determine the printing dot enlargement rate, which determines the number of hanging meshes and the setting of some color separation parameters for prepress production of printed products. In general, the ink absorption performance of paper is good, and the printing dot enlargement rate is small; otherwise, the printing dot enlargement rate is large. Due to the fact that the ink drying in newsprint printing is mainly based on osmotic drying and the printing speed is fast, the ink-absorbing performance of newsprint has a more direct effect on the quality of printed matter. The rate of expansion of general newsprint printing outlets is about 30%, and the expansion rate of newsprint outlets of various brand lot numbers is not the same. The expansion rate of good imported newsprint outlets is below 25%, while the ratio of some poor quality domestic paper outlets will exceed 30%. We have the experience of printing with newsprint paper with ink-absorbing properties. The layout of the text is delicate, the dots are neat, the gradation is rich, the lines are smooth, and the colors are accurate. However, newspapers produced by newsprint with poor ink absorption performance have poor quality at many points. The picture is lacking in level, and the text is also detrimental. For the printing of color newspapers, it is particularly important to improve the quality of multicolored overprinted color pictures to ensure a lower dot gain rate in the printing process.

The ink absorption properties of the paper affect the drying speed of the ink and the brightness of the ink on the paper, thus greatly affecting the quality of the print. If the ink absorption of the paper is too high, too much of the material is absorbed into the paper fibers, the oil content in the ink layer will be greatly reduced, the pigment particles will be suspended on the paper surface, the printed material will lack luster after the conjunctiva is dried, and some newspapers may even When the ink is blotted by hand, the ink has flaking or pulverization; the ink absorption performance of the paper is too low, and the ink may not be fully dried and absorbed after the newspaper is folded, causing problems such as insufficient ink brightness and dirty paper. Since paper production, printing, and folding are completed at one time, paper is required to have better ink absorbing performance in order to prevent sticking during folding and newspaper accumulation.

The effect of the ink absorption performance of the paper on the density of the ink layer on the printed product is also significant. With paper with good ink absorption, the density of the ink on the print can reach a more desirable level, and the ink color is solid. Absorption is too low, will make the ink transfer in the printing process is not sufficient, the thickness of the ink on the surface of the paper is insufficient, the problem of paper off powder and hair removal in the printing process is more prominent; and the thickness of the conjunctiva of the ink-absorbing film, the ink layer can not make the paper reach Some color saturation, and some even appear different degrees of print phenomenon.

Fifth, the impact of paper strength on print quality

The strength of the paper includes tensile strength (breakage), burst resistance, folding resistance, tear strength, abrasion resistance, and the like.

Newsprint Newspaper printing is the continuous printing of tapes. The tape tension is a necessary process condition for ensuring the smooth printing. The tensile strength of the tape is the basic basis for the tension of the tape. The tensile strength of the paper is insufficient, and the tension cannot be given too large when printing. Insufficient printing tension may cause problems such as poor reproduction of printing dots, inaccurate overprinting, and ghosting. In the color printing process, due to the need for multiple imprints, the effect of insufficient printing tension will be more pronounced. With the Y-structured printing press production, due to the presence of secondary imprinting, if the tension is too small, serious ghosting problems will occur.

Process problems involving the surface strength of the paper are problems that often occur in print production, and the effect of paper off and fluffing on the print quality is particularly pronounced. The ability of the paper to resist delamination and tearing perpendicular to the paper surface is called Z-strength. It mainly includes paper's ability to resist powder dropping and resistance to pulling. Because the surface treatment of newsprint does not basically use calendering paint or inorganic fillers, the phenomenon of hair loss is mainly manifested during printing. In the transfer of the ink layer to the paper, it will also have a certain peeling effect on the surface of the paper. Some coating particles and paper fibers will be peeled off on the surface of the paper, and will stick to the blanket and be mixed in the ink so that white spots appear on the print. . The large accumulation of paper powder and paper on the surface of the blanket can seriously affect the full transfer of ink during printing. The high printing speed of newspaper printing equipment, the low surface strength of newsprint paper, and the increase in the viscosity of ink in recent years have made the problem of powder falling out and become a very important issue for improving the printing quality of newspapers.

Color newspaper printing is a multi-color wet-wet overprinting process. Each time the paper strip is imprinted, there will be a shift of moisture to the paper surface, which will further reduce the surface strength of the paper, the so-called wet nap phenomenon. Guaranteeing the printing quality of the Caibao and solving the problems of paper falling and linting, especially the multi-color overprinting process, is a problem that must be paid close attention to in the printing of color newspapers.

Too long paper storage time or too low paper moisture content and other reasons can also result in reduced paper strength. The paper is crunchy, has low bursting resistance and low folding endurance and easily breaks in the folding process. At the same time, long-term paper storage will reduce the tensile strength of the paper and cause it to break easily.

Sixth, strengthen paper management and process cooperation to improve the printing quality of newspapers.

The ability of newspaper printing companies to control the quality of paper products is relatively low. Various products are, in most cases, customer-owned paper. How to improve newspaper printing quality under certain paper conditions is a very important process issue. Doing a good job in paper management, rationally arranging the process flow according to the situation of the paper and rationally adjusting the process parameters are important means for improving the printing quality of newspapers.

First of all, we must choose the appropriate paper according to the product quality requirements and the performance of the paper. To have a good printing product, you must use a good amount of paper, even if the process is adjusted to do better, it can not fundamentally improve the quality of printed products. The quality requirements of the printed matter are high, and the quality of the paper must also be correspondingly improved. The printing color newspaper should be higher than the performance requirements of printing the black and white paper. Printing companies should improve their ability to evaluate and judge the impact of paper quality on product quality, and promptly and accurately indicate to customers the characteristics of paper and analyze the impact on the quality of printed products, and cooperate with customers in the selection and matching of paper.

Second, it is necessary to do a good job of craftwork in accordance with the situation of paper. Under certain paper conditions, improving the printing quality through process adjustment is an important part of the printing process management, and it is also an inevitable requirement for newspaper printing companies to go to market. The cooperation of the printing process mainly includes the following aspects:

1, prepress parameter settings

A significant number of parameters in the prepress system are determined based on the condition of the paper. The number of hanging meshes in prints is mainly determined by the performance of the paper. We often say that coated paper 175 mesh, offset paper 133 mesh, and newsprint 87 mesh are only an approximate range. In production, we must also make appropriate adjustments according to the specific conditions of the paper. In order to obtain a better force film level effect, we should increase the number of nets as much as possible. In previous years, we heard that some newspapers printed a color newspaper with a net of 150 mesh. The number of hanging meshes is determined by the smoothness of the surface of the paper and the ink absorption performance. The number of paper meshes for better papers can be around 100 mesh, while the number of hanging meshes for some poor papers should be around 87 mesh, otherwise Reduce the level of the picture.

In order to obtain a good level of color control, we need to set the dot gain rate, the maximum ink volume, etc. when the picture is separating color from RGB to CMYK. These depend on the ink absorption performance of the paper. Generally, the dot gain rate of newsprint is about 25%-30%. The maximum ink volume is generally about 280-300%. There are also some differences in the values ​​of different papers, in order to obtain better results. Printing effect, the setting of such parameters should also be properly adjusted according to the situation of the paper.

Newsprint printing does not have a relatively good control system for coated paper printing, and the color difference of newsprint is relatively large. In order to obtain a good picture-level effect, we should try to increase the number of items to be screened. We have heard of some newspapers in previous years. Print color newspaper with 150 mesh. The number of hanging meshes is determined by the smoothness of the surface of the paper and the ink absorption performance. The number of paper meshes for better papers can be around 100 mesh, while the number of hanging meshes for some poor papers should be around 87 mesh, otherwise Reduce the level of the picture.