The problem of rejection of prints and the view of "one line" (I)

The "one line" view is the author's many years of mechanical repair work, after a lot of practical and theoretical research summed up. The "one line" point of view emphasizes that in each transmission and handover process, the front edge of the paper should always be in a state of line, with no distortion, to ensure the accuracy of overprinting of the product.

The concept of rejection angle

The throwing angle is also called the paper-end scattered shape and generally shows the following two phenomena.

(1) The tail of the printed paper is inaccurate, the first color is large in the paper-feeding direction, and the back color gradually decreases. This type of overprint problem started from the position of the paper, and the more serious the situation was at the end of the paper.

(2) Observed with the naked eye, the registration of the rules is accurate, but the printed products appear discolored, the color of a batch of printed products is inconsistent, the color of the tail of the paper is severe, and there is no sense of layering in the line of the cable, and the hair is empty. The front of the sheet is light and heavy, and the front The text on the end of a few colors (especially the first color) is not clear. When examined with a 20x magnifier, it can be seen that there is a significant jump in the rule line on the tail of the paper, and the dots are deformed.

The problem of rejection angle is a typical manifestation of overprinting faults in printing products. Most overprinting faults are caused by the rejection angle or accompanied by the rejection of corners. There are both mechanical and operational reasons as well as paper problems. Strictly speaking, all multicolor printing equipment has the issue of throwing angles. High-quality new printers have a lighter rollover problem, and only lighter rollovers occur when printing thin or flounced paper. Some of the rollover problems are inherent to some models. Heidelberg multicolor offset printing press in the 1970s and 1980s, due to the limitations in the design and manufacture of equipment, there was a problem of rejection at the factory, but it was not serious. After a certain period of use, due to the influence of various factors, the machine's rollover problem gradually gives the operator a headache. Through maintenance, the problem of rejection angle can be minimized to a degree that cannot be observed with the naked eye. The domestic multi-color offset printing press is generally more serious when it comes to the rejection angle, and the national standard on the accuracy of overprinting is too loose, not suitable for printing high-quality prints.

One of the reasons for the rejection angle: When the paper is conveyed to the first group of impression cylinders, the front side is wavy. When printing, under the common rolling of the impression cylinder and the blanket cylinder, the waves are flattened but formed on the paper. Micro-crease, this micro-crease is partially or completely flattened during the subsequent imprinting of each unit. Therefore, in the paper-feeding direction, the first color print of the print is the largest, and the print of the rear unit gradually decreases. Changes in the rules line from the beginning of the paper, the more the change to the end of the paper. This is the most typical rejection problem.

The second reason for the rejection angle is that the paper is deformed under the peeling force of the blanket in the printing process, so that the front edge of the paper is not in an ideal straight line state. Under the effect of the printing pressure of the rear unit, this wave deformation is flattened and Micro-wrinkles form on the paper, causing a rejection angle.

The third reason for throwing angle: When printing, the longitudinal section of the paper cannot maintain the “one line” state, or the longitudinal section of the surface of the impression roller and the surface of the blanket after the blanket cylinder and the plate cylinder are pressed cannot maintain a “one line” In the “state,” the image on the plate cannot be accurately transferred to the paper, resulting in misprints and rejections.

The fourth reason of rejection angle: In the process of the paper after the completion of the first color or a certain color printing, due to unreasonable delivery time and other reasons, the paper is deformed when the paper is transferred to the following impression cylinder, and the wavy deformation occurs. It is flattened and forms a slight wrinkle on the paper, causing a rejection angle.

The premise of ensuring the accuracy of overprinting of printed products is to minimize the deformation of the paper during each transfer and imprinting process, especially the deformation of the front edge of the paper. Although there are compensators in Heidelberg multicolor offset presses, this compensation does not have any practical significance for the larger swing angles and the deformation of the dot ends of the paper tails. There are many reasons for the phenomenon of print rejection, and most failures are difficult for the compensator to compensate.

1. The “one line” problem of paper transfer to the first color impression cylinder

For example, in the phenomenon of one of the reasons for rejection, due to the difference in the state of transfer of each piece of paper, when the rules cannot be completely coincided, along with the beating of the cable, the handwriting of the previous colors (especially the first color) is blurred, Dots were distorted, and the color of prints was light and heavy.

To determine if the printer has a slanting angle, first check that the paper delivered to the first impression cylinder is flat. If the paper is wavy at the mouth, that is, the front edge of the paper is not in a "one line" state, the machine must have a problem of rejection. To ensure the "one line" state of the paper on the first-color impression cylinder, the following conditions must be met.

(1) "Line" status when paper is positioned at the front gauge

The paper on the front gauge is at the last link before all the dentition wipes. The ideal condition is that each sheet is the same and remains absolutely flat. If the front edge is not in a “line” state, after the paper passes through the transfer teeth to the impression cylinder row, the front edge of the paper is still not on a straight line. Under the effect of the printing pressure, the paper is deformed and converted into a print rejection angle. In order to maintain the “one line” state that the paper is intended to handle, the following conditions must be met.

1 The paperboard must have a strict flatness. The paperboard is not even and the front edge of the paper cannot be in a "one line" state. When the equipment is overhauled, the flatness of the paperboard should be corrected to within 0.05mm.

2 All the positioning planes of the front gauge must be on “one line”. This is especially important when printing thin paper. The deviation of the position of one of the front gauges will cause the paper to partially deviate from the straight line formed by the previously specified plane, thus destroying the “one line” state of the paper.

3 The paper can be correctly positioned at the front gauge If the front tongue of the front gauge is low, the paper will not be able to smoothly enter the front plane, causing the straightness of the front edge of the paper to fall. Therefore, the operator should adjust Feida's time for paper-cutting, the position and pressure of the paper-roller and brush wheel on the paper-feeder, and the tightness of the paper-feeding belt in accordance with the specifications, so as to ensure the accuracy of the former specification.

4 The paper is affixed to the paper feeder smoothly. Different models are installed with different devices to ensure the flatness of the paper in the previous specified position. In the old Heidelberg 102 series offset press, a suction device is installed below the paper feeding plate, and a flat paper spring plate is installed above. In the new type of machine, an air blowing device is installed at the front paper feeding board, and the paper is attached to the paper feeding board by the negative pressure formed by the air blowing process. Therefore, the operator should carefully adjust the air suction or blowing device on the paper feeding board, the gap between the flat paper spring and the paper. On the paperboard, paper must not be subject to warping problems, otherwise there will be a more serious rejection of angle failure.

For thin paper, no matter what kind of equipment, it can not completely solve the problem of flatness of the paper. Therefore, paper should be carefully knocked before printing to ensure the stiffness of the paper. The author suggests that when designing and printing, a device can be installed at the front predetermined position so that the paper can bear the pulling force in the direction of the paper in the front gauge to level the paper.

(2) Paper-feeding paper at the moment of paper-feeding paper The "one-line" state of the paper During the maintenance or manufacturing of the equipment, it is necessary to ensure that the paper-transmitting tooth is in a "one-line" state when it is pulled on the paper-feeding board. For this purpose, it must be satisfied. The following conditions.

1 The occluding plane of the transfer tooth pad is a standard plane used for a long period of time. The transfer dentition is deformed, and the spray layer at some occlusal planes is detached, making the occlusal plane of the transfer dentition not standard, and the delivery teeth catching the front edge of the paper. Not flat, so you must repair or replace the transfer pad.

2 The ideal gap between the paper handing pad and the paper feeding board is the thickness of the printed paper. In this way, when the paper handing paper is taken out, the deformation of the paper is minimal.

(3) The "Line" state after the paper feed paper 1 The time of the paper feed is accurate The ideal paper take position of the paper hand is the vertical position of the paper pad and the paper feed, and the angle reading on the dial It is 11.5 degrees. Due to the paper-feeding chip, the wear of the tooth pad and the adjustment reasons, the time for picking each tooth piece on the paper-feeding tooth row is not uniform. When the paper handing paper is taken at the front gauge, the paper handing mat is not absolutely stationary, but there is slight rotation, which is a problem that the rotary paper handing tooth cannot solve. Inconsistent pick-up time results in inconsistent paper conditions for each tooth chip during pick-up and deformation of the front edge of the paper. Some tooth chips only picked up after the paper teeth began to rotate, causing serious deformation of the front edge of the paper. Some tooth chips close the teeth too early, and then hit the paper at the moment when the dental pulp is rubbed. This causes the entire paper or the area of ​​the dental sheet to recede. After the other teeth are picked up, the front edge of the paper is deformed.

Due to the wear of the feed shaft, the shaft bearing, the copper socket and the open ball, the paper take time of the paper feed chip will change every time the paper is taken, making the paper feed time inaccurate. There is no guarantee of the rationality of each paper taking time and the consistency of the closing time of the dental piece, resulting in deformation of the paper.

2 Correctly adjust the transport position of the paper When printing, the operator should be accustomed to adjust the transport position of the paper according to the position of the front rule, and should not center the transport of the paper without thinking. When the paper is transported, if the edge of the paper is close to the front rule of the edge on the side of the pull tab (the nearest front edge of the edge of the paper), when the pull pulls the paper, the front gauge will hinder the pulling action of the pull gauge and cause the edge of the paper. Deformation. In this state, the paper feed state of the paper feed is not ideal, and the front side of the paper is not in a "one line" state.

(4) “One-line” problem of the paper front mouth during the movement of the paper feed teeth Sometimes, the tooth chips are removed because of the individual tooth chips. After the tooth chips are worn, they are not adjusted according to the standard; there are dead teeth on the paper handing teeth. Affected by the force, causing the release of the paper after the release of paper. These will cause the "line" status of the front edge of the paper to change.

Due to the small number of dental pieces of the rotary transfer paper, part of the paper is out of control, and it cannot withstand large external forces during the transfer of the paper. Therefore, the time required for the lateral stop to pull the paper must be reasonable (standard time is 10.5 degrees). Otherwise, the paper feed teeth will grab the paper with the side guide, causing the front edge of the paper to be deformed; the paper pressure on the paper press roller will stress the paper. Must be appropriate, otherwise, some part of the front edge of the paper will be pulled back.

(5) "One line" problem of the paper when the paper handing teeth and the impression cylinder are handed over. 1 The time of handing the paper teeth and the impression cylinder accurately. The ideal contact point between the paper handing teeth and the impression cylinder should be on the two rollers. Cut position. In order to ensure the stability of the transfer process, there must be a common transfer time of 1.0 to 1.6 degrees between the impression cylinder and the transfer teeth. The midpoint of this transfer time is the cut point of the two rollers. In this transfer interval, the distance between the transfer pad and the impression pad is the shortest and the change is minimal, which can minimize the deformation of the paper during the transfer. The opening time of the delivery tooth was (204.9+0.3) degrees, and the closing time of the impression cylinder was (203.9-0.3) degrees. If the transfer time is too long or the transfer point is not based on the tangent point of the roller, the gap between the pads will increase at the time of transfer, and the front edge of the paper will be deformed.

Factors influencing the handover time include: wear of the relevant parts in the paper transfer tooth row; inaccurate opening time of the paper feed teeth; inaccurate adjustment of the closed tooth time of the impression cylinder; the tooth axis and the tooth shaft in the impression cylinder row There is a "dead tooth" in the wear of copper sleeves, open teeth, or in the tooth row, which changes the time of closing the teeth of the impression cylinder or the closing time of a certain number of teeth, and the contact point is indeterminate.

2 Reasonable and reasonable gap between the paper-feeding pad and the embossing roller pad is the thickness of the paper, so as to prevent the paper from being deformed to the maximum extent. When adjusting the gap between the pads, you should pay attention to the adjustment of the height of the pad. When printing thin paper, it is recommended that the gap between the transfer tooth pad and the impression pad be adjusted to 0.15 to 0.20 mm.

There are two factors that affect the gap between the transfer paper pad and the impression pad. One is the embossing roller pad. The plane formed by the upper surface of a row of dental pads is the reference plane during the transfer process of the printed products. Therefore, the height of the dental pads must be carefully measured before installing the dental pads, and the height error of each row of dental pads is controlled within 0.02 mm. The second is a paper pad. After the paper pad is bent, it is difficult to ensure the flatness of the occlusal plane only by the adjustment of the operator. If the sprayed layer of an individual bite falls, the gap between the transfer pad and the impression pad increases. When replacing the transfer pad, the position of the pad must be recalibrated, calibrated against the impression of the roller pad or impression cylinder, or use special tools.