In actual production, the performance of the paper directly affects the quality of the printed products, so the paper used for printing should be selected according to the quality requirements, uses, content and customer requirements of the print before printing. The following describes the performance of several types of paper and its impact on printing for your reference.
1. The thickness of the paper
Different papers have different printing pressures. Before printing, the adjustment of the printing pressure between the rubber cylinder and the impression cylinder is based on the thickness of the paper, by adjusting the center distance of the two cylinders or the thickness of the lining. The thickness of the paper of the same batch of printed matter needs to be uniform. If the thickness is not uniform, the ink transfer rate will be different due to different printing pressures, resulting in changes in color shades. In severe cases, images and text information cannot be completely reproduced due to insufficient printing pressure.
When using thin paper for printing, because of its good ink absorption, there may be a phenomenon of through-printing. In order to avoid the problem of through-printing, when printing thin paper, use thicker ink and as little printing pressure as possible. When using cardboard for printing, due to its small tightness and loose structure, the amount of deformation in the vertical direction of the paper during printing is large. It is necessary to increase the maximum compression of the rubber cylinder and the impression cylinder to obtain sufficient printing pressure.
Second, the direction of the thread
The fiber arrangement of the paper is directional. According to the arrangement of paper fibers, it is divided into "longitudinal thread paper" and "horizontal thread paper".
The dimensional elongation caused by the water absorption of the paper is different in the direction of the vertical thread and in the direction of the thread. The elongation in the direction of the vertical thread is about 3 to 8 times the elongation in the direction of the thread; horizontal thread paper Under the effect of printing pressure, trumpet-shaped elongation is easy to occur, and it is especially easy to cause large overprint errors at the two corners of the trailing edge; the tensile strength of the paper in the direction of the thread is better than that perpendicular to the direction of the thread. In general, when printing with a sheet-fed printing machine, it is better to use paper in the longitudinal direction. Because the long side of the paper is always parallel to the axis of the cylinder of the printing press, when the longitudinal paper is used, the long side elongation is small and the short side elongation is large, which can allow the absolute extension of the long and short sides of the paper. The difference is not too big; on the other hand, the misprinting caused by the change of the paper size on the short side with a large elongation can be remedied by adjusting the liner of the plate cylinder, and the axial overprinting of the cylinder Irreparable.
Longitudinal thread paper and horizontal thread paper cannot be mixed together for printing. Because under the same conditions, the two papers with different fiber directions have different long-side and wide-side elongation. If the paper is deformed during the next color printing and the overprint is not accurate, even if you try to take certain measures to remedy It will not be possible because there is no uniform amount of deformation.
3. Paper surface gloss
Gloss refers to the ability of the surface of the paper to reflect incident light at a certain angle, which indicates the degree of gloss of the paper. Generally speaking, the higher the smoothness of the paper, the higher the gloss. The effect of gloss on printing is shown in the following points.
â‘ The gloss of the printed matter will increase with the increase of the gloss of the paper itself, and the increase of the gloss of the printed matter will make the color bright and full. In actual printing, to print high-gloss prints, generally choose high-gloss paper.
â‘¡ Compared with paper with low gloss, paper with high gloss can obtain higher printing density when printed with the same ink film thickness on the paper surface.
â‘¢ Due to the effects of glare, too high gloss often leads to a decrease in reading quality and makes people feel tired, so publications based on text often use low gloss.
Four, smoothness
Smoothness is a technical indicator for evaluating the degree of unevenness on the surface of paper and cardboard. In general, the smoothness of paper under printing pressure is called printing smoothness, and the smoothness in free state is called apparent smoothness.
The smoothness of the paper has a great influence on the printing, mainly in the influence of the dot reducibility and the printing gloss.
â‘ The effect of smoothness on imprint reduction. The smoothness of printing determines the degree of contact between the surface of the paper and the surface of the blanket inked at the moment of imprinting, and is an important factor that affects whether the ink transfer is comprehensive and the graphics are clear.
â‘¡The influence of paper smoothness on the printing gloss. After the ink is transferred from the blanket to the paper and before it is officially dried, it has a certain fluidity on the paper surface. When the ink and printing conditions are constant, the higher the smoothness of the paper, the better the ink leveling and the stronger the printing gloss. In order to obtain a uniform and complete ink transfer and a reasonable solid density on paper with poor printing smoothness, the general measures to be taken are: increase the printing pressure and increase the amount of ink and the fluidity of the ink. [next]
5. Ink absorption of paper
In the printing process, the paper's absorption of ink is related to the tightness of the paper, the surface properties of the papermaking fibers, the content of fillers and rubber compounds, and is also related to the characteristics of the printing ink, printing method, printing speed, printing pressure and other factors . The ink absorption of the paper affects the immobilization and drying speed of the print, and also affects the quality of the printed product.
When the ink absorption of the paper is too high, the binder in the ink is drawn into the paper too much, the binder in the ink layer is reduced accordingly, the pigment particles in the ink float on the paper surface, and the ink layer is dry After lack of gloss. When printing on paper with too little ink absorption, the drying speed of the ink will be slow, and the phenomenon of smudging on the back side is easy to occur. For the paper with too little ink absorption capacity, the thickness of the ink layer should be controlled. Powder and other measures to prevent back rubbing.
6. Water content of paper
The ratio of the mass of water contained in the paper to the mass of the paper is called the moisture content of the paper. Paper moisture content has an important impact on the printing process and product quality, too high or too little moisture content will cause many printing failures.
â‘ If the moisture content of the paper is too high, the surface strength and tensile strength of the paper will be significantly reduced, which will aggravate the roughening and elongation during the printing process; it will reduce the elasticity and stiffness of the paper.
â‘¡ If the moisture content of the paper is too small, the paper will become brittle, hard, and easily damaged, and the insulation of the paper will be improved and static electricity will be easily generated. When printing, double sheets and blank sheets are easy to appear.
â‘¢The change of paper size caused by the change of paper moisture content is the main reason for the inaccurate fault of offset printing. In addition, "lotus edge", "tight edge", and "curl" caused by uneven changes in the moisture content of the paper stack can cause wrinkles in the printed matter, inaccurate overprinting, and difficulty in feeding and receiving paper. Therefore, in normal production, it is necessary to strictly control the changes in the paper's moisture content, including the changes in the paper's water content caused by the surrounding environment of the paper.
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