The nature of reel newsprint generally refers to its suitability for printing, including some performance characteristics during transportation and storage. The printability refers to the comprehensive reflection of the physical and chemical properties that should be possessed when copying a certain quality of printed matter under the necessary conditions. The nature of web newsprint must reflect the best printing quality and color effects. During the operation of the roller, the paper can smoothly transfer the ink absorbed on the printing plate to its surface, and obtain a complete, full, clear and rich graphic imprint.

The printability of web newsprint includes: the pore structure and ink absorption of the paper; the smoothness and compressibility of the paper; the surface strength of the paper; the acidity and alkalinity of the paper and the water content and elasticity of the paper.

1. Pore structure and ink absorption of paper

Paper is a network structure made of various plant fibers intertwined with each other. In this network structure, the pores between the fibers are filled with some fillers, glues, and colors to meet the paper itself. Features. Adding filler is to improve the whiteness, smoothness and opacity of the paper; adding glue is to make the paper have a certain water resistance and is not easily wetted by water or other liquids; adding color is to make the paper have a higher whiteness. The pore structure of the paper is the basis of the paper's ability to absorb ink, and it is one of the key factors for the permeation and drying of printed crystals in newspapers and periodicals. Therefore, to understand the ink absorption of paper, we must first understand the pore structure of the paper. Because the pore structure of the paper is closely related to the amount of ink penetration, the penetration speed, the smoothness of printing, the solidification and drying of the ink, and the quality of the printing.

The pore structure of the paper mainly includes the porosity of the paper. The porosity of the paper represents the ratio of the volume of air in the paper to the volume of the paper. The porosity of the paper can be obtained by calculation method and experimental measurement method.

The porosity of the paper = the volume of air in the paper (unit area) / the volume of paper (unit area) x 100%

As can be seen from the above equation, the larger the volume of air in the paper, the higher the porosity of the paper, which also verifies the condition of the paper's pore structure. The fewer the paper fibers, the more interwoven the sparseness, the more the inter-fiber voids The larger, the poorer the pore structure of the paper. On the contrary, the more paper fibers, the denser the interweave, the smaller the gap between the fibers, the better the pore structure of the paper.

The solidification and drying of the ink on the paper surface accelerates with the increase of the porosity, and its ink absorption also increases, but the gloss of the ink on the paper surface decreases with the increase of the porosity.

The paper has a pore structure. The porous structure of the paper determines the paper's breathability, moisture absorption, absorption and compressibility, which is the basis for the paper to absorb water, ink and other substances. In order to ensure better printing quality and printing effect, the printing paper needs to have better ink absorption. The ink absorbency of paper refers to the property that after the paper contacts the ink and transfers the ink to the surface of the paper, the ink penetrates into the paper and is absorbed. The larger the porosity of the paper and the looser the structure, the more the amount of ink absorbed, and the less is the opposite. This is why we often say that the paper has good ink absorption.

2. Smoothness and compressibility of paper

Paper smoothness refers to the degree of uniformity, smoothness and smoothness of the paper surface. The smoothness of the paper depends on the processing method adopted during the processing of the paper, the distribution of fibers in the paper, and the application of fillers.

Paper smoothness includes paper smoothness and printing smoothness. The smoothness of the paper surface refers to the time required for the surface of the paper to pass a certain volume of air under a certain degree of vacuum through the gap between the paper surface and the glass under a certain pressure, expressed in seconds. The smoothness of the paper is generally concentrated between 50 and 100 seconds, and with the increase of the smoothness, the solid density of the printing field increases significantly. Printing smoothness refers to the ratio of the contact between the surface of the paper and the surface of the smooth cylinder under conditions equivalent to printing pressure and printing speed.

The determination of printing smoothness requires special advanced instruments, and the actual operability is not strong. At present, paper smoothness is generally used to indicate paper smoothness.

The reproduction of printed matter is to express its hue and gradation through dots. The carrier of the picture and text is the blanket. The contact between the blanket and the paper directly affects the transfer of the dots of the text. Under the same pressure and blanket conditions, the smoothness of the paper This is the prerequisite for the transfer of graphics and text. The smoother the paper, the more the graphics and text dots are attached to the paper surface. The closer the ink speed of the absorption dots on the paper is, the closer the size of the graphic dots and the ink absorption are. More realistic. On the contrary, the picture and text restoration is even worse.

The effect of paper compressibility on printing is that the paper deforms in thickness during imprinting, and the paper surface is flattened, which facilitates good contact between the paper surface and the blanket surface, and provides better conditions for the transfer of ink . Because newsprint is flexible, it is easily compressed under the action of printing pressure, so the more complete the contact between the paper surface and the blanket surface. If printing on a strong paper surface, due to the thickness of the paper and the uneven surface, when the blanket is in contact with the paper surface, the force on each part of the paper surface is uneven, and the blanket is transferred to the surface of the paper. The text has distortion phenomena such as blurring or deformation and loss of dots.

3. The surface strength of the paper

The surface strength of the paper is a property of the binding force between the fiber, filler and rubber on the surface of the paper. This property has a direct impact on the physical properties of the paper surface. The strength of the fiber pore structure of newsprint is relatively poor, especially some shorter wood paddle fibers and impurities affect the pore structure of the fiber. During the copying, in order to increase the whiteness, opacity, smoothness of the newsprint, and increase the water resistance of the paper , Add more or less a certain amount of fillers and rubber compounds. The amount of filler and glue added directly affects the surface strength of the paper. In addition, the calendering of the calender directly affects the surface strength of the paper. In the actual printing process, the ability of the paper to resist powdering and anti-brushing is collectively referred to as the surface strength of the paper.

At present, the measurement of the surface strength of the paper has been expressed by the pulling speed value. The so-called paper-pulling speed refers to the printing speed. As the printing speed increases, the outward tension of the ink acting on the paper surface is greater than the binding force of the paper, which causes the paper surface to begin to de-powder. It is the speed of paper pulling. From this value of drawing speed, we can understand the correlation of paper surface strength. Under the conditions of constant printing pressure and constant ink, the faster the printing speed, the greater the reverse adhesion of the ink to the paper, and the higher the paper's pulling speed. To increase the strength of the paper surface, in addition to increasing the binding force of the paper surface, reducing the viscosity of the ink is also an effective method.

4. The acidity and alkalinity of paper

Any paper itself has a certain pH. The pH is usually expressed by PH value.

The acidity and alkalinity of the paper itself is due to the different pulping methods in the papermaking process. Some use the acid method for pulping, and some use the alkaline method for pulping. In addition, the ingredients of the coating are added, and finally the paper is finished. It has a certain acidity and alkalinity. The acidity and alkalinity of the paper itself have an effect on the drying speed of the ink during printing, especially the oxidized conjunctive type drying ink, because its drying method is completed by the oxidized form, and the acidity and alkalinity of the paper itself is specified within a certain pH The strength of the acidity and alkalinity of the paper directly affects the dryness of the ink. In addition, the printing ink adheres to the paper surface, and the vividness of the ink color will also be affected by the pH of the paper.

There are certain control requirements for the acidity and alkalinity of paper, as is newsprint. In the printing process of newspapers, the weak acidic wetting powder is generally added to the water to form a wetting liquid, so that the PH value of the wetting liquid is controlled between 5-6, which is convenient for the formation of water-ink balance on the printing plate, and the paper has its acid and alkali The sex is best controlled between weak acids and bases, so as not to damage the acid-base balance on the printing plate.

5. Moisture content and elasticity of paper

The moisture content of the paper is a physical quantity representing the ratio of the weight of the paper reduced to the weight of the base paper when the paper is dried to a constant temperature of 100 ° C to 105 ° C, and is expressed as a percentage.

In fact, the paper has always contained moisture since it was copied, and it is always in a balance. Its moisture content changes with the change of the temperature and humidity of the air in the environment. The paper is placed in the air at a certain temperature and humidity, in its own fibers The water content of the water and the water content in the air have been undergoing dynamic water exchange, always maintaining a relative balance.

The moisture content of the paper is inversely proportional to the temperature, that is, the moisture content of the paper decreases when the temperature increases, otherwise, the moisture content of the paper increases when the temperature decreases. For every change of the ambient temperature of ± 5 ℃, the moisture content of the paper changes by 0.15%, which changes inversely relationship. Therefore, it is better to control the temperature in the printing workshop not more than ± 3 ℃. During the overprinting process, the moisture content of the paper does not change more than ± 0.1%, otherwise it will affect the accuracy of the overprinting.

Relative humidity has an effect on the moisture content of the paper. Due to climate change, the relative humidity in the air is not fixed. The paper must absorb and drain water with the change of humidity, so the paper has different moisture content under different humidity. The relative humidity in the air has a certain relationship with the change of the paper's water content. The paper's water content increases as the air's relative humidity increases, and vice versa, decreases as the relative humidity decreases. When the relative humidity in the air does not exceed 80%, every time the relative humidity changes by 100%, the paper moisture content will have a corresponding change of 1%, but when the relative humidity exceeds 80%, the change in water content will increase. Generally increased to 1.5%-2%.

The elasticity of the paper is directly related to the moisture content of the paper. Due to the uneven moisture content, the paper fibers often expand and shrink, causing the paper size to increase or decrease, resulting in inaccurate overprinting.

Paper stretchability refers to the relative change of paper size when the paper is immersed in water at a certain temperature and air-dried after being immersed in water. The ratio of the increase or decrease of the paper size to the size of the base paper is expressed as a percentage.

The paper fiber expands after absorbing water. The expansion of the fiber diameter is much larger than the longitudinal expansion, usually 2-8 times. The expansion and contraction of the paper causes many quality failures in printing. For example, when the moisture content of both sides of the roll paper is higher than that of the middle, "folds" will be formed on both sides, and "folds" with tight edges and raised edges will be formed along the circumferential direction. Once this paper is printed on the machine, it will produce wrinkles and creases If it is serious, it will affect the normal operation of the paper tape, and it will tear directly. When it encounters the printing of color newspapers, its color overprinting cannot be carried out. Therefore, we must conscientiously grasp the changing law of paper moisture content and strictly control the expansion and contraction of paper. One of the raw materials, the quality of paper storage will directly affect the quality and use of paper. Pay attention to the inspection and acceptance of the roll paper when purchasing and entering the factory for storage and standby, especially the outer packaging should be more strict, and the storage site is very particular. In addition to ventilation, moisture and light, it should also pay attention to fire, electricity and waterproof And other security risks. In the management of newsprint, the following management requirements should be noted:

1. Storage: The stacking of the roll paper should not be too high. If possible, the roll paper can be erected to ensure the roundness of the roll paper, and at the same time prevent the roll paper from wrinkling and the core tube from being deformed.

2. Moisture-proof: The paper has strong hygroscopicity, and is more sensitive to the temperature and humidity of the environment. The paper is easily deformed after absorbing moisture, which brings great trouble to the use. Especially when the packaging paper is removed or the packaging paper is partially broken, etc., we should pay attention to its moisture-proof requirements. The storage location should be ventilated, moisture-proof on the ground, and dry on the walls. It is best to make a professional paper warehouse for long-term mobile storage and use of newsprint. The relative humidity of the storage environment is best controlled at 50-55%.

3. Sunscreen: The roll paper should be stored in a sunscreen to prevent the paper roll from being exposed or exposed to sunlight. Because the paper itself will have a certain amount of moisture, the moisture in the roll paper package is relatively balanced. Upon exposure, the roll paper will heat up and the paper moisture balance will be destroyed. The paper will release water to the air, causing the paper to shrink and deform, and the paper fibers will become brittle , Loses its flexibility and cannot be printed normally.

4. Heat protection: Although the paper is in an environment where the temperature requirements are not as sensitive as humidity, the moisture content changes rapidly, but when the temperature exceeds certain requirements, its mechanical strength will decrease significantly, and the storage temperature is generally controlled at 18 ℃ -25 ℃.

5. Transportation, loading and unloading: Due to the large volume and large weight of the roll paper, the packaging requirements must be paid attention to during the transportation process, and the above precautions must be fulfilled at the same time. Especially in rainy weather, it must be protected from rain and moisture, otherwise, the roll paper surface is directly threatened by quality. When loading and unloading, professional forklifts or trucks are used. The only requirement is not to damage the outer packaging of the roll paper during the loading and unloading process, otherwise it will cause the loss of the roll paper and threaten the storage and use of the roll paper.

6. Use: When using newsprint, we must strictly control the temperature and humidity of the printing workshop to avoid the phenomenon of moisture absorption of the paper. After the paper absorbs moisture, the tensile strength is reduced, and the paper breaks easily during the printing process. In addition, changes in temperature and humidity will cause the paper to expand and contract, affecting the accuracy of printing. Newsprint has strong ink absorption properties, which can quickly absorb and cure ink after printing, and is suitable for high-speed printing. In printing, it is necessary to put forward requirements on the permeability and viscosity of ink to suit the characteristics of newsprint. Newsprint contains a lot of wood After being exposed to light, lignin easily oxidizes and turns yellow and becomes brittle. Therefore, it is not suitable for long-term storage or printing for long-term retention.

As a commonly used printing paper, reel newsprint, as long as we have a good grasp of its characteristics and performance, control its printing suitability, and store and use it correctly, we will be able to maximize its role and increase its printing speed And quality, bring greater economic benefits.

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