Second, paper

Paper is one of the important things that prepress workers need to pay attention to. It determines the difference in the status of printed products.

1, the composition of the paper

Paper is a kind of material material that is processed and synthesized from plant fibers by adding fillers, rubber compounds, pigments, and the like. The raw materials that make up paper are straw, bamboo, cotton, and available waste. Depending on the raw materials, the nature of the formed paper will also be different.

The "filler" is a material used to increase the flexibility of the paper, reduce the transparency and stretchability of the paper, and make the surface of the paper even and even, such as talc for general printing paper, kaolin for high-grade paper and barium sulfate. The use of filler should be appropriate, usually about 20% of the paper, too much will reduce the resistance and flexibility of the thread, and will hinder the absorption of ink, resulting in the powder off when printing.

The “sizing” is used to fill the small holes in the paper to improve the water resistance of the paper, as well as to improve the gloss and strength of the paper and prevent the paper from fluffing. Commonly used rubber compounds are rosin, alum and starch.

"Color" is used to enhance the color purity of paper. Most of them use inorganic pigments or organic dyes.

2, the specifications of the paper

Paper according to the different uses of the printing can be divided into flat paper and web, flat paper is suitable for general printing machine, web is generally used for high-speed rotary press.

The size of the paper is generally produced according to the standards set by the country. The base paper sizes for printing, writing and drawing papers are:

Web width is divided into four types: 1575mm, 1092mm, 880mm and 787mm;

The base paper size of the flat sheet paper is divided into six sizes according to size: 880 mm×1230 mm, 850 mm×1168 mm, 880 mm× 1092 mm, 787 mm× 1092 mm, 787 mm×960 mm, and 690 mm×960 mm.

The standard for books and magazines and paper size is specified by the state of 880mm × 1230mm, 900mm × 1280mm, and 1000mm × 1400mm uncut paper sizes.

Due to equipment, production, supply, and other reasons, the original 787mm × 1092mm, 850mm × 1168mm size paper, can still continue to use, but it should be noted that this size is to be phased out of the old standard format, after the paper size will Transition to new national standards.

In the coastal areas, because of the large number of foreign-funded printing companies, many printing organizations still widely use some old versions of paper. Their specifications and sizes after cutting are shown in the figure. (missing picture)

The weight of the paper is expressed in terms of weight and weight. It is generally expressed in terms of quantification, which is what we commonly call "weight."

Quantitative refers to the mass relationship of the unit area of ​​paper, expressed in g/m2. For example, 150g of paper means that the weight per sheet of this type of paper is 150g.

Paper with a weight of 200 g/m2 or less (including 200 g/m2) is called "paper", and a paper with a weight exceeding 200 g/m2 is called "cardboard."

The order weight refers to the total mass of paper per order (1 sheet for 500 sheets) and the unit is calculated in kg (kg). According to the paper's basis weight and the size of the web, the weight can be calculated by using the formula of weight (kg) = paper size (m2) × 500 × g/m2.

3, printing on the commonly used paper

Paper can be classified into industrial paper, packaging paper, household paper, and cultural paper depending on the application.

Among them, cultural paper includes writing paper, art and drawing paper, and printing paper. In the printing paper, it is divided into newsprint, letterpress printing paper, offset printing coating paper, dictionary paper, map and sea drawing, gravure printing paper, painting newspaper, weekly newspaper, whiteboard paper, writing paper, etc. according to the performance and characteristics of the paper. . In addition, some high-grade prints are also widely used in art-drawing papers.

Newsprint: Newsprint is mainly used for the printing of newspapers and some letterpress publications. It is soft and elastic, has strong ink-absorption capacity, and has certain mechanical strength. It can be adapted to various high-speed web presses. This kind of paper uses wood paddles as raw materials for manufacturing, contains more lignin and impurities, and is prone to yellowing and brittle papers, and has poor water resistance, so it is not suitable for long-term preservation.

Letterpress Printing Paper: This is a special paper used for letterpress printing. The properties of paper are similar to that of newsprint paper. Water resistance, color purity, paper surface smoothness are slightly better than newsprint paper, ink absorption is more uniform, but strong ink capacity is better than newsprint paper. To be bad.

Offset printing paper: Offset printing paper is a kind of paper used for offset (flat) printing, and it is divided into single-sided offset paper and offset-type offset paper. Single-sided offset paper is mainly used to print posters, packaging boxes, etc. Double-sided offset paper is mainly used for printing pictures, pictures, etc. Offset paper is compact in texture, less in flexibility, and strong in water resistance, and can effectively prevent the paper from being deformed, misplaced, napped, or removed from powder during multicolor overprinting. Can give the print better color purity.

Offset coated paper: also known as coated paper, is a kind of high-grade paper coated with a layer of inorganic coating on the surface of paper and then super-calendered. The surface of the paper is smooth and flat, and the color purity is high. Delicate and smooth network points can better reproduce the layered feeling of the manuscript, and are widely used in art pictures, albums, and commercial leaflets.

Gravure Printing Paper: Rotogravure printing paper is white and firm, with good smoothness and water resistance. It is mainly used for the printing of banknotes, stamps and other high quality and difficult to imitation prints.

Whiteboard paper: Whiteboard paper is a kind of paper with relatively uniform fibrous structure, surface layer with filler and rubber compound and coated with a layer of paint, and is produced by multi-roller calendering. The paper surface has high purity and relatively uniform color quality. The ink-absorption property has a good folding-resistance. It is mainly used for the packaging of goods, the lining of products, and the picture-charts.

Synthetic paper: Synthetic paper is made of chemical raw materials such as olefins and then added with some additives. It has a soft texture, strong tensile strength, high water resistance, light resistance to heat and cold, and can resist the corrosion of chemical substances without environmental pollution. Breathable, widely used in advanced art, maps, pictures, high-grade books and other printing.

4, paper performance requirements

Whether or not paper can be used for printing with high quality depends on the printability of the paper. The requirements for printing on paper are: the hue of the paper should reach the purity of the same texture as much as possible, the degree of dust of the paper must not exceed the allowable range, have the minimum light transmittance, and ensure the mechanical strength during normal printing, and the thickness of the paper. The tightness, structural characteristics, etc. should be the same in the same print batch. The edge of the paper should be right angle and the slope error cannot exceed ±3mm.

The smoothness of the paper: The smoothness of the paper directly determines the print quality of the print. The paper with high smoothness can make contact with the plate better during embossing, so that the ink layer on the plate can be evenly transferred to the surface of the paper. The paper with low smoothness will be uneven on the surface during printing. The uneven contact between the surface of the plate and the paper during imprinting causes uneven transfer of the ink layer to the surface of the paper. If the paper touches a paper with low smoothness, the pressure during printing may be appropriately increased during printing. Can properly make up for the phenomenon of imprints appearing on the surface because of its uneven surface.

Ink absorption of paper: The absorption of ink by paper depends mainly on the tightness of the fibers of the paper (the size of the gap). When the gap between the paper fibers is small, due to too little deuterated fibers, the effect of the fiber capillary is affected, resulting in the paper's ink absorption; if the gap between the paper fibers is too large, it will be caused by excessive absorption of the connecting material. Ink pigments are also absorbed, making prints appear through the phenomenon.

Paper's elasticity and plasticity: Paper can undergo various changes due to the surrounding environment during various processes such as printing and printing. As the external force, will instantly change their shape and size, and when the external force stops, the paper will return to the original shape and size, this deformation process is called sensitive elastic deformation; when the paper is subjected to external force , change its shape and size within a certain period of time, when the external force ceases to function, it slowly returns to the original shape and size, this process of deformation is called the elastic deformation; when the external force is cancelled, the paper remains The shape and size of a deformed state caused by an external force are called plastic deformation. Sensitive elastic deformation, elastic deformation is a reversible deformation, and plastic deformation is an irreversible deformation.

Paper surface strength: The surface strength of the paper determines the decisive influence of the surface on the surface of the paper during printing, such as wear resistance, resistance to powder fall, and fluff resistance. When printing, in order to get a clearer dot, some inks with higher viscosity will be used. If the surface strength of the paper is not enough, it will be easy to appear powder off, hair loss, and adhere to the surface of the plate; if you use a lower viscosity Ink, in lithographic printing, is emulsified by the ink and fountain solution, causing dirt in the blank area of ​​the printing plate.

The moisture content of the paper: How much water does the paper directly affect the quality of the printed matter. Excessive water content will reduce the strength of the paper. Under the action of the external force, the paper fibers will be pulled out and the plastic deformation will be enhanced. The drying speed is affected; if the water content is too low, the paper will be brittle and easily damaged, resulting in static electricity. Since the moisture content of the paper has a great correlation with the surrounding environment, the humidity and temperature of the printing room should be properly arranged to maintain the balance of water content of the paper.