Collection of paper faults and troubleshooting in

2022-08-13
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Collection of paper faults and troubleshooting in printing technology

first, the expansion and contraction of paper

usually in a printing plant, the thread of paper is determined based on the direction of the paper fiber and the axis of the drum

any printed paper whose fiber arrangement direction is parallel to the axis of the drum is called longitudinal strand paper, while the paper whose fiber arrangement direction is perpendicular to the axis of the drum is called horizontal strand paper

in offset printing, longitudinal thread paper should be used for printing. This is because: from the perspective of paper feeding, the longitudinal strand paper is relatively hard in the coming and going (axial) direction, and it is not easy to tear along the radial direction, which is convenient for blowing and conveying. From the perspective of overprint, after a dry fiber is completely wetted, its diameter may increase by 30%, while its length direction may increase by%. Therefore, the horizontal strand paper is more elongated than the vertical strand paper, which directly affects the overprint accuracy. The longitudinal strand paper has more up and down (radial) elongation, but the overprint can be accurate by adjusting the lining or other methods

[case]

a factory uses a monochrome Offset Press to print a batch of three open four-color moving parts. The paper used is a single sheet of offset paper, and the cutting method is a T-shaped cutting method. When the first color is printed and the second color is overprinted, irregular overprint inaccuracies are found on the left and right sides

[case analysis in addition]:

this phenomenon often occurs. The offset machine operator mistakenly believes that this is the poor quality of the paper. The actual reason is that the T-shaped cutting method is adopted in the previous process, and the horizontal and vertical strands of the paper are mixed, so that the paper expansion and contraction are inconsistent. Therefore, the left and right overprint is inaccurate and irregular

[expert tips]:

all cut sheets of paper must be clamped with warning strips. When printing, separate the vertical and horizontal strands of paper, and do not mix them

paper is a kind of changeable material. The fiber that makes up the paper catches the plastic garbage that accumulates in one dimension with the movement of water. It has a hygroscopic effect, and the air contains a certain amount of moisture. Therefore, as long as the paper is in contact with the air, it will constantly balance with the air humidity. When the air humidity in the surrounding environment is higher than the humidity contained in the paper, the paper will absorb the moisture in the air and expand and stretch. On the contrary, when the air humidity in the surrounding environment is lower than the humidity contained in the paper itself, the paper will release moisture and shrink and shorten, so as to achieve the balance with the environmental humidity. The so-called tight edges and ruffles of the paper are related to the moisture content of the paper. In order to prevent inaccurate image overprint caused by paper expansion, try to use longitudinal strand paper (called horizontal strand paper in paper mills) to control the size of page moisture during printing. If the paper is not good, the overprinting requirements are high, and the monochrome Offset Machine is used for multi-color overprinting, try to press once with water as much as possible

second, the swing angle of the paper

the swing angle refers to the phenomenon that one side of the trailing tip of the printed paper is not nested properly. In recent years, most of the corner throwing failures occur in multicolor offset printing. The main reasons are:

(1) the front gauge is not on a parallel straight line, resulting in less paper when the paper reaches the front gauge

(2) there is a large difference in the thickness of the paper

(3) the pressure of each color group shows an increasing trend

(4) the blanket cylinder lining is uneven

(5) there is a difference in the tightness of the two sides of the teeth of the imprinting cylinder

(6) individual tooth pads on both sides of the teeth of the imprinting cylinder are smooth or too low

(7) due to the large blank area on both sides of the rear of the printing plate, the amount of wetting fluid in this area is large, resulting in the paper adhering to more wetting fluid, causing the extension of the trailing part, Form a swing angle

(8) the different amount of liquid used in the blank areas on both sides of the printing plate will also form a swing angle on one side

similar failures occur in monochrome Offset Presses

[case]

a factory uses a four-color offset press to print a map book, which is printed with 80g/㎡ offset paper. Some red and blue symbols are often found at the trailing tip, especially some special symbols, such as some city symbols are blue circle with red. This phenomenon mostly occurs in the latter half of the printing format

[case analysis]:

the above example is an obvious angle swing phenomenon. The main reason for this phenomenon is that the decrease of printing pressure is not enough, so it is necessary to re measure and adjust the printing pressure. In addition, it is also related to the tightness of the paper and the tightness of the paper teeth here

[case]:

in the process of debugging cp2000-cd102 machine, a factory carried out color printing test after all indicators were tested to be qualified, and added character portraits at 50% flat positions of black, blue, magenta and yellow plates respectively. The test results show that the coated paper and paperboard above 128g/㎡ are normal, while when the thinner 80g/㎡ coated paper is tested, the corner is thrown at the lower right corner of the printed matter, and the angle gauge overprint is inaccurate

[case analysis]:

this kind of angle throwing fault is generally irrelevant to the pressure of various color groups of the paper feeding printing plate and the rubber cloth lining. It may be related to the holding of the teeth, but when checking the holding of the teeth, the holding force of the teeth is normal, and then observe the holding of the teeth of the double diameter imprinting cylinder. When holding the paper, the holding teeth did not hold the paper within a length of about 4mm in the lower right corner, and the paper was slightly deformed during pressing, resulting in angle swing

treatment method:

reset the position of the paper side pull gauge, adjust the relevant parts, and then start the machine for trial printing, and troubleshoot

[expert tips]:

whether it is a multi-color offset press or a monochrome Offset Press, do not change some key parts recklessly, and pay more attention to the position and range of the paper teeth. For paper with large hardness, such failures are not easy to occur

III. paper hair loss and powder loss

paper hair loss is mainly caused by two reasons. The first is the quality of the paper, followed by the printing conditions. The quality of paper refers to the surface strength of paper; The surface strength of paper is a physical quantity that measures the binding force between fibers, fillers and glues on the surface of paper. It generally refers to the water resistance and tear resistance of a unit of paper perpendicular to the paper. Paper with high surface strength will lose less hair in printing. Therefore, in practical applications, the surface strength of paper is also called the resistance of paper to hair loss. It reflects the ability of the paper to resist hair loss during the printing process

printing conditions include many contents, such as the supply of wetting liquid, the viscosity of ink, viscosity, printing speed, etc

⑴ paper depilation and ink stacking

paper depilation is generally divided into dry depilation and wet depilation

in a monochrome Offset Press, the hair loss of the paper is dry hair loss, which occurs when the adhesion between fibers or pigments is less than that of the ink. Wet hair loss occurs under this condition with the participation of water

in the printing process of multi-color machine, except for the first color ink of the first color group cylinder, the inks of the other color groups should be printed on the surface of the already printed ink or white paper, and the surface of the white paper is wetted by wetting liquid. After wetting, the surface strength of the paper will inevitably decline. Obviously, if the last color is printed on the white part, the surface strength of the white paper is the weakest

the fibers falling off the paper and the coating particles are accumulated on the blanket, and the blanket ink stacking fault occurs. According to the different printing pictures and texts, the ink stacking degree of multi-color offset press is also different. However, generally speaking, it is not easy to happen on the blanket roller of the first color group, which often happens in the second color or later color groups, which is the main reason for the ink pile on the paper. The increase of viscosity caused by residual viscosity and wet hair loss caused by the wetting of wetting liquid. Therefore, the more the drum is behind, the more likely it is to pile ink

when the ink pile begins to form, the image becomes rough first. This is due to the falling off fibers, the coating particles peel off with the ink, and the ink pile is formed on the blanket. As a result, the ink in the convex part is transferred poorly, causing the printing plate flower to paste the plate

(2) treatment method of paper hair falling and ink piling

the most fundamental way to reduce paper hair falling is to choose paper printing with high surface strength. However, this kind of paper may not be used in practice, so from the perspective of technology, firstly, the wetting liquid should be controlled in the minimum range, and secondly, the printing speed should be reduced, so as to reduce the viscosity of the ink in the printing process and buffer the hair loss of the paper. For the paper with serious hair loss, in order to ensure the quality, press it with clean water before formal printing or overprint it with white oil or clean the blanket for many times before printing fine products

[case]:

a printing factory printed a batch of "human body art appreciation" albums with four-color offset printing. The printing machine is an imported four-color offset printing machine, and the paper used is 128G/m2 double-sided coated paper. When the operator finished the preparation work, and less than 500 copies were officially printed, he found that many spots of different sizes appeared on the original smooth and delicate human skin. At the same time, the British government took control of the economic opportunity. At this time, he shut down the machine immediately. The captain suspected that the color sequence was improper, and then changed the color sequence before printing, but there were still spots in different positions

[case analysis]:

according to the lighting results of the paper, it is found that the dust degree of the paper is too high, and then due to the unique wet powder dropping phenomenon after four-color machine printing, the above fault occurs

treatment method: replace the paper

[expert tips]:

there are quite a few examples like this in actual work. Some papers are not suitable for printing high-end color live parts due to paper problems. Never make do with it

[case]:

a J2108 machine in a factory used 128G/㎡ double-sided coated paper to print color live parts, and there was no abnormal phenomenon during printing. Later, 80g/㎡ single-sided coated paper was used to print the same pictures and texts, and it was found that the words were hairy and dotted. In addition, there was ink accumulation on the surface of the blanket, which was later analyzed to be caused by insufficient lining of the blanket roller

[expert tips]:

insufficient rubber cloth lining is often the main reason for ink piling. This is because the ink cannot be transferred well due to insufficient lining, resulting in the thicker and thicker accumulation on the blanket cylinder, forming a pile of ink. Therefore, the thickness of the blanket lining must be increased accordingly. It can be seen that the ink stacking failure is not necessarily caused by poor paper

IV. paper static electricity

paper has static electricity, which affects paper feeding and printing. First, there is static electricity on the paper before it is put on the machine. Second, the static electricity is not obvious before paper printing. After embossing, the static electricity suddenly increases, causing uneven paper delivery. It is rare that water is involved in the offset printing process, which generally increases static electricity after printing

the common methods to eliminate static electricity are: using humidifier to increase the relative humidity in the room and workshop where the paper is hung to improve the water content of the paper; Put the paper next to the machine in advance to adapt to the temperature and humidity of the printing workshop. If the paper fed by the paper feeder is OK, but the printed products are uneven in the paper receiving part, at this time, it is effective to slightly increase the moisture content of the layout. In addition, Static Eliminators or antistatic agents can also be used to eliminate the phenomenon of static electricity on paper

[case]:

a factory used offset paper to print a batch of color live parts. During the paper feeding process, it was either double sheets or skewed, which could not be printed normally. It was judged that the paper had static electricity, but the factory had no equipment to hang the paper. At this time, the captain sprinkled some water on the ground, shook and piled up all the papers. The next day, the paper was reloaded, and the paper feeding was normal. The above situation would not happen again

[expert tips]:

paper static electricity has a great relationship with the temperature and humidity of the printing environment, which must be paid attention to

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