The premise of ensuring the accuracy of overprinting of printed products is to minimize the deformation of the paper during each handover and imprinting process, especially the deformation of the front edge of the paper. Although the Heidelberg multicolor offset press is equipped with a compensator, this compensation does not have any practical significance for the problem of large-scale fling angle and dot deformation of the paper tail. There are various reasons for the problem of the flick angle of the printed product, and most of the faults are difficult to compensate for by the compensator.
1. The "One Line" Problem of Paper Transfer to the First Color Impression Cylinder
As one of the reasons for the angle swing, due to the difference in the transmission state of each piece of paper, while the rule lines cannot completely overlap, along with the wire jump, the handwriting in the previous colors (especially the first color) are blurred, The dots are deformed, and the color of the printed product is lighter than heavy.
To determine whether the printing press has a flick angle, first check whether the paper transferred to the first color impression cylinder is flat. If the paper mouth is wavy, that is, the front edge of the paper is not in a "one line" state, then the machine must have a problem of flick angle. To ensure the "one line" status of the paper on the first color impression cylinder, the following conditions must be met.
(1) The state of "one line" when the paper is positioned at the front
The paper at the front is at the last link before all grippers. The ideal state is that each sheet is in the same state and remains absolutely flat. If the front edge is not in the "one line" state, after the paper passes through the delivery teeth and reaches the impression cylinder row, the front edge of the paper is still not in a straight line. Under the action of printing pressure, the paper deforms and transforms into a printed product corner. In order to keep the paper in the "one line" state of pre-regulation processing, the following conditions must be met.
â‘ The cardboard must have strict flatness
The paperboard is not flat, and the front edge of the paper cannot achieve the "one line" state. When the equipment is overhauled, the flatness of the cardboard should be corrected to within 0.05mm.
â‘¡ The positioning planes of all the front rules must be on "one line"
This is especially important when printing thin paper. A deviation of the position of a front rule will cause the paper to partially deviate from the straight line formed by the previous prescribed plane, which will destroy the "one line" state of the paper
â‘¢ The paper can be correctly positioned at the front
If the pressing force of the individual front gauge is low, the paper will not enter the positioning plane of the front gauge smoothly, so that the straightness of the front edge of the paper decreases. Therefore, the operator should adjust Feida's paper unloading time, the position and pressure of the paper pressing roller and brush wheel on the paperboard, and the tightness of the paper belt to ensure the accuracy of the previous position.
â‘£ The paper is attached to the cardboard
Different models are equipped with different devices to ensure the flatness of the paper in the previous position. In the old Heidelberg 102 series offset press, the suction board is equipped with a suction device under the cardboard, and a flat paper spring is installed above. In the new machine, a blower is installed at the front paperboard, and the negative pressure formed by the blowing process is used to attach the paper to the paperboard. Therefore, the operator should carefully adjust the suction or blowing device on the cardboard, the gap between the flat paper spring and the paper. On the cardboard, the paper must not have the problem of warp angle, otherwise, there will be more serious angle swing failure.
For thin paper, no matter what kind of equipment, can not completely solve the problem of paper flatness. Therefore, the paper should be carefully knocked before printing to ensure the stiffness of the paper. The author recommends that when designing and printing, a device can be installed at the front specified position, so that the paper can withstand the pulling force in the direction of the paper in the front regulation to flatten the paper.
(2) The state of "one line" of paper at the moment when the paper feeder picks up the paper
When servicing or manufacturing equipment, it should be ensured that the delivery teeth are in a "one line" state when they take paper on the cardboard. For this reason, the following conditions must be met.
â‘ The occlusal plane of the delivery tooth pad is the standard plane
After long-term use, the delivery pads are deformed, and the spray coating at some occlusal planes falls off, making the delivery pads' bite plane non-standard. The front edges of the delivery pads grip the paper and are uneven. Therefore, the delivery pad must be repaired or replaced. Tooth pad.
â‘¡ The ideal gap between the delivery pad and the cardboard is the thickness of the printed paper. In this way, when the delivery tooth picks up the paper, the deformation of the paper is minimal.
(3) The state of "one line" after handing paper
â‘ Accurate time for handing paper
The ideal paper picking position of the paper feeding tooth is the vertical position of the paper feeding tooth pad and the paper feeding board, and the angle reading on the dial is 11.5 degrees. Due to the wear and adjustment of the tooth delivery pads and dental pads, the gripping time of each tooth stick on the delivery row is not consistent. When the paper feeder picks up paper at the front gauge, the paper feeder pad is not absolutely stationary, but there is a slight rotation, which is a problem that cannot be solved by the rotary paper feeder. The gripping time is inconsistent, resulting in inconsistent paper state when each tooth is gripping the paper, and the front edge of the paper is deformed. Some dental tablets only hold the paper after the paper delivery teeth start to rotate, causing a severe deformation of the front edge of the paper. Some teeth are closed too early, and the paper is punched at the moment when the tooth is gripped, so that the whole paper or the edge of the tooth is retracted backwards. After other teeth are gripped, the front edge of the paper is deformed.
Due to the wear of the delivery shaft, the bearing of the shaft, the copper sleeve of the tooth holder, and the opening ball, each time the paper is taken, the paper picking time of the tooth picking sheet will change, making the paper picking time of the paper picking tooth inaccurate. The rationality of the time of taking paper each time and the consistency of the closing time of the dental slices cannot be guaranteed, causing the paper to deform.
â‘¡ Correctly adjust the paper transport position
During printing, the operator should be used to adjust the paper transport position according to the position of the previous rules, and should not transport the paper in the center without thinking. When the paper is conveyed, if the paper edge is closer to the front edge rule on the side of the pull gauge (the nearest front edge of the paper edge), when the pull gauge pulls the paper, the front gauge will hinder the pulling action of the pull gauge, causing the edge of the paper Deformed. In this state, the gripping state of the delivery tooth is not ideal, and the front edge of the paper is not in the "one line" state.
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