Expert advice: paper failure and exclusion highlights in the printing process
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First, the paper stretch
Usually in a printing factory, the thread of the paper is determined based on the direction of the paper fibers and the axis of the drum.
For paper that is printed, the direction in which the fibers are arranged in parallel with the axis of the drum is called the longitudinal filament paper, and the paper whose fiber arrangement direction is perpendicular to the axis of the drum is called the paper sheet.
In the offset printing, vertical silk paper should be used for printing. This is because: from the aspect of paper feeding, the longitudinal silk paper is relatively hard in the direction of the direction (axial direction), and it is not easy to tear in the radial direction, which is convenient for blowing and conveying. From the aspect of overprinting, after drying The fiber, after complete wetting, may increase its diameter by 30% and the length by 1-2%. Therefore, the horizontal silk paper has more elongation (axial) than the longitudinal silk paper, which directly affects the overprint precision. However, the longitudinal thread 缕 paper is elongated more and more (radial), but the overprint can be made accurate by adjusting the lining or other methods.
Second, the corner of the paper
The corner refers to the phenomenon of inaccurate fitting on the side of the printed paper. In recent years, most corner failures have occurred mostly in multicolor offset printing. The main reasons are:
(1) The front gauge is not on a parallel straight line, causing the paper to reach the front gauge and less paper on one side.
(2) The difference in paper thickness is large
(3) The pressure of each color group is increasing
(4) blanket drum liner is not flat
(5) The difference between the two sides of the impression cylinder
(6) The impression pad is smooth or too low on both sides of the tooth on both sides of the tooth
(7) Due to the large blank area on both sides of the back of the printing plate, the amount of dampening liquid is large in this area, causing the paper to adhere to more dampening liquid, causing the tip to partially expand and contract, forming a corner.
(8) The amount of liquid used in the blank area on both sides of the printing plate is different, and the side will also form a corner.
Similar failures occur in monochrome offset presses.
Third, the paper's hair loss, powder off
Paper lint is mainly caused by two reasons. The first is the quality of the paper, followed by the printing conditions. The quality of the paper refers to the surface strength of the paper; the surface strength of the paper is the physical quantity that measures the bonding strength between the fibers, the filler and the rubber on the surface of the paper, and generally refers to the water-resistant layer perpendicular to the paper surface on the unit paper surface, and the resistance. The ability to tear. Paper with a high surface strength has less hair loss during printing. Therefore, in practical applications, the surface strength of paper is also called the paper's lint resistance. It reflects the paper's ability to resist lint during printing.
The printing conditions include more contents, such as the amount of dampening liquid supplied, the viscosity of the ink, the viscosity, the printing speed, and the like.
(1) Paper lint and pile of ink
The hair loss of paper is generally divided into dry hair and wet hair.
In a monochrome offset printing machine, the lint of the paper is dry hair, which occurs when the bonding force between the fibers or pigments is less than the adhesion of the ink. Wet hair loss occurs under the condition that water is added.
In the multicolor machine printing process, in addition to the first color ink of the first color group roller, the inks of the other color groups are printed on the surface of the printed ink or white paper, and the surface of the white paper is wetted. Liquid wetted. After the paper is wetted, its surface strength is inevitably lowered. Obviously, if the last color is printed on the white paper portion, the surface strength of the white paper is the weakest.
When the fibers detached from the paper, the paint particles and the paint particles are deposited on the blanket, the blanket is broken. Multi-color offset presses vary in the degree of ink stacking depending on the printed image. In general, however, the first color set of blanket cylinders does not easily occur, often in the second color or later color groups, which is the main reason for the paper to produce ink. The wetness caused by the increase in the viscosity due to the residual viscosity and the wetting of the dampening solution. Therefore, the more the roller is behind, the easier it is to stack the ink.
When the stacking ink begins to form, the image is first roughened due to the detached fibers, the coating particles are peeled off along with the ink, and a pile of ink is formed on the blanket. As a result, the ink of the convex portion is poorly transferred, causing the printing plate to break out.
(2) Processing method for paper falling and stacking ink
The most fundamental way to reduce paper lint is to choose high surface strength paper printing. However, in practice, it is not necessary to use such papers. From a process point of view, first, the dampening solution should be controlled to a minimum amount, and secondly, the printing speed is lowered, so as to reduce the viscosity of the ink during the printing process, and buffer the lint of the paper. . For more serious hair loss paper, in order to ensure the quality, before printing, press the water or print the fine product before printing the white oil or cleaning the blanket several times.
Fourth, paper static
Paper with static electricity affects paper feeding and printing. First, there is static electricity on the paper before the machine. Second, the static electricity is not obvious before the paper is printed. After the printing, the static electricity suddenly increases, making the paper delivery uneven. In the offset printing process, water is involved, and it is rare to increase the static electricity after printing.
Common methods for eliminating static electricity include: using a humidifier to increase the relative humidity of the interior and the workshop of the hanging paper to increase the water content of the paper; placing the paper in front of the machine in advance to adapt to the temperature and humidity of the printing workshop. If the paper feeder is still available, but the printed products are not in the delivery position, at this time, it is possible to slightly increase the moisture of the layout. In addition, a static eliminator or an antistatic agent can be used to eliminate the static electricity of the paper.
Five, paper bow
During the printing process, paper wrinkles (pleats) are one of the most common failures, and the economic loss is relatively large. There are two main reasons for such failures: one is caused by the paper itself (mostly in non-coating). Paper), the second is the improper use of the machine, the shape and further analysis of the reasons and treatment methods.
1, small hair-shaped pleats
Reasons for producing such pleats:
(1) “Floating edge” produced after the paper absorbs moisture (the paper itself has uneven water content or the relative humidity of the workshop is high, and the water content of the paper is low).
(2) The ink on the back side of the printed on both sides of the print "stack roller" (the ink deposited on the surface of the impression cylinder is too thick, causing partial pressure changes).
Approach:
Re-hang the paper; if it is printed on both sides, be sure to wash the impression cylinder.
2, straight and straight pleats (the pleats in the middle of the paper)
The main reason for the generation of such pleats is the deformation of the paper (tight side).
Approach:
Re-hang the paper, if some printing factory does not have the equipment to hang the paper, you can shake the uncut printing paper to be printed one week ahead of time, put it in the printing workshop to adapt to the temperature and humidity around the machine, and then cut it when printing. on board. 3, large pleats
Reasons for producing such pleats:
(1) The edge of the large paper is curled up, and the crepe paper is not good.
(2) The paper is too light at the side of the paper.
(3) The paper corner floats.
(4) The side is not properly positioned, and the paper corner touches the gap of the paper.
Approach:
After the paper is reversed, it is printed on the machine; for the looser crepe paper, it needs to be re-adjusted, and the paper angle floats mainly because the fangs cannot hold the paper. 4, circular pleats
The main reason for the generation of such pleats is that the middle portion of the gripper is too loose, which causes the paper to be lost, and is pulled out to produce pleats during printing.
Approach:
Correct the force of the gripper. If the pad is too smooth, replace it with a new one. 5, large hair-shaped pleats
Reasons for producing such pleats:
(1) The impression cylinder has more than two or more sheets of paper in one place, and even can not hold the paper.
(2) The height of the transfer paper pad is uneven, so that the edge of the paper is not in a straight line and becomes a wavy curve. When the embossing roller picks up the paper and presses the paper, the length of the paper is shortened and a large scatter is generated from the mouth. Pleats.
Treatment method: Check the pressure of the impression cylinder.