1. Printing represents the process of copying many sheets from a printing plate on a printing press.
The printing industry is the largest concentrated consumer of paper products.
In order to provide paper products suitable for printing, manufacturers must understand and pay attention to the requirements of printers on paper. The two most important factors must be valued-"runability" and "printability".
1. Runability
The operation of the paper web on the printing press can be considered as a combination of all properties. Poor operation will cause paper breaks, parking, or reduce the speed of the printing press, which will have a greater impact on the economic benefits of printing. The paper's running performance is shown as strength, uniformity, no paper diseases and good mechanical conditions of the paper roll or paper.
Poor runnability appears as a paper break or loose tension on the printing press, causing wrinkling and inaccurate overprinting. For newsprint, when the water content is high, its operability will be improved.
2. Printability
Printability is also called printing quality, but it is the effect of paper on the exact reproduction of the print. Paper properties: whiteness, ink absorption, moisture opacity, color, gloss, smoothness, air permeability and sizing are very important for printing. However, different printing methods require different paper. In general, coated paper is better than uncoated paper because of its smoothness, whiteness, gloss and other physical properties, so it will show a good appearance after printing.
2. There are four main methods of commercial printing, which are classified as follows:
1. Letterpress (Letterpress) transfers on a raised surface, also known as Relief (Relief). A large number of letterpress printing uses rotary presses, including the printing of newspapers, magazines and advertising flyers. This type of printing is on paper The smoothness, ink retention and strength of the paper have high requirements, and no paper breaks, wrinkles or other problems that cause the printer to stop are allowed. At the same time, the paper is not allowed to have lint, powder, blisters, and cannot be There are soluble substances and friction substances.
2, Lithograph (Lithograph) is printed on a flat surface, also known as planographic method (Planographic). Most lithography methods transfer printing images from a printing plate to a rubber cylinder, and then transfer them to the substrate, so this printing method can also be called offset, commonly known as offset printing. Because the ink used in lithography is quite sticky, when the paper is peeled off from the blanket, there is a great pulling force on the surface of the paper to produce a peeling force (Pickingforce), so the paper used for this method of printing must be clean and free of paper wool And the surface strength is stronger. At the same time, this printing method is easy to damage the coating layer of the coated paper. The smoothness of the paper surface is not high, and the rough paper can also print very good results.
3. Gravure transfers ink to the substrate with a concave surface, also known as Intaglio. The notable feature of Gravureplate is that the ink is directly added to the substrate by the concave part of the printing plate, the surface of the printing plate is flat, and only the graphic part is concave. This printing method requires a relatively high degree of paper smoothness. This type of printing is commonly used in cigarette pack printing.
4. Screen printing (Screenprinting) This printing method is mainly used in the production of artworks, and the amount of paper used is insignificant, so it will not be discussed here.
3. Printability of paper
In order to obtain prints with excellent print quality, paper and ink must be suitable materials. That is to say, the paper must be easy to print. In addition to the bright colors and strong coloring power, the ink must also have good and suitable permeability, volatility and conjunctiva capacity. In addition, the rubber rollers, blankets, plates used in printing, the conditions of the printing workshop and the operation of the printing process must also be adapted to the needs of printing excellent works.
The printing suitability of paper means that the paper can adapt to the requirements of ink, printing plate and printing conditions, to ensure the smooth progress of the printing operation and to obtain the necessary conditions for excellent prints. The printability of paper is not a fixed property, but a performance that is compatible with different printing conditions and different print characteristics. (Due to space reasons, a brief introduction to several important paper properties):
1. Brightness: In order to attract the reader's attention to the strong printing contrast, high-brightness paper can be used, while low-whiteness paper is more suitable for reading books that are not easy to cause eyestrain. In Europe and the United States, books printed on low-brightness paper are very popular, and children's books in China are also moving closer to this aspect.
2. Color Colour: There are also different requirements for the color of paper for different purposes. For industrial printing, except for special purposes, the general use is white paper.
3. Gloss: The change of gloss will affect the color uniformity of color printing and the post-print gloss of printed products. Uneven gloss of the paper will cause uneven gloss of the printed matter, resulting in poor reflection uniformity of the visual effect of the image. If the gloss is too low, the gloss of the printed matter will be low, the color reproduction will be poor, and there will be no gloss. The gloss is too high, which affects the penetration of the ink, resulting in different drying speeds, and there will be backprinting problems (and backside rubbing).
4. Opacity: Thin-weight paper weighing less than 105 grams has higher requirements for opacity. If the opacity is too low, it will cause the printed image to pass through easily. When printing on both sides, the front and back images and text will interfere with each other and affect the visual effect. Especially when printing in color, it will cause color shift and make color matching difficult.
5. Ink absorbability: Ink absorbability directly affects the quality of printed products, and is one of the important physical properties of paper. For the same manufacturer, the absorption of each batch of paper products must be uniform, and the paper products of the same brand must also be consistent. Too little ink absorption will lead to problems such as enlarged printing dots, prone to printing blooms, dirty back side and long drying time; too high ink absorption will cause through-printing, ink powdering, color change and deep image, clear Problems such as poor degrees.
6. Surface strength: The higher the surface strength, the better. It is very important for offset printing. It includes IGT table strength and wax pull strength. If the surface strength is too low, the paint or fiber on the surface of the paper is pulled up during printing, which will cause spots and pits, foreign objects on the blanket surface, and foreign objects in the ink fountain. In severe cases, it will cause plate wear. Due to the low surface strength, the waste products have similarities with the waste dust paper, cut paper dust, and dust. The important basis for the difference in appearance is that the former can find the original point on the printed product, and the original point Concave, while the latter does not.
7. Moisture moisture: Moisture content affects the change in size and curling deformation of paper. High moisture content will also increase the runnability of uncoated paper. If the moisture content of coated paper is too high, blistering is likely to occur on commercial rotary machines. At the same time, the moisture content of the paper should match the humidity of the printing environment, otherwise the curling problem of single copper, the ruffled edge and tight edge of double copper double adhesive paper will occur, and the ruffled edge and tight edge will cause overprinting. And printing wrinkles. Excessive paper moisture will cause problems such as paper feeding difficulties, printing wrinkles, backprint, curling, long drying time, and reduced stiffness during printing; too low moisture will cause curling, paper feeding difficulties, printing wrinkles, static electricity, paper strength Reduce and easy to expand and other issues.
8. Smoothness / roughness: Smoothness and roughness are both measures of the microscopic flatness of the paper surface, but the focus is different from the detection method. In gravure printing, the smoothness of the paper is relatively high. If the smoothness is not enough, some dots cannot be transferred to the surface of the paper, resulting in dew point.
9. Bubbling temperature in oil immersion: This property is very important for commercial rotary coated paper. If the bubbling temperature of oil immersion does not meet certain requirements, it cannot be used for rotary printing. Otherwise, blistering phenomenon is likely to occur during printing, resulting in the scrapping of a large number of printed materials.
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