The effect of printing speed on printing quality in offset printing (two)
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The premise of ensuring accurate overprinting of printed products is to minimize the deformation of the paper during each transfer and embossing process, especially the deformation of the front edge of the paper. Although the Heidelberg multi-color offset press has a compensator, this compensation has no practical significance for the large angle of the corner and the deformation of the dot at the end of the paper. There are many reasons for the problem of the corner of the print, and most of the faults are difficult to compensate for the compensator.
1. The "one line" problem that the paper passes to the first color impression cylinder
For example, in the case of one of the causes of the corner, due to the difference in the transfer state of each sheet of paper, the rule lines of the preceding colors (especially the first color) are blurred, along with the jump of the line, while the rule lines are not completely coincident. The dot is deformed, and the color of the printed product is light and heavy.
To determine if the press has a corner, first check that the paper delivered to the first color impression cylinder is flat. If the paper mouth is wavy, that is, the front edge of the paper is not "one line", the machine must have a corner problem. To ensure the "one line" status of the paper on the first color impression cylinder, the following conditions must be met.
(1) "One line" status when the paper is positioned at the front gauge
The paper at the front gauge is in the last step before all the crepe papers. Ideally, each paper is in the same state and remains absolutely flat. If the front edge is not in the "one line" state, after the paper passes the transfer paper to the impression cylinder row, the front edge of the paper is still not in a straight line. Under the printing pressure, the paper is deformed and converted into a printed corner. In order to keep the paper in the "one line" state of the pre-processing, the following conditions must be met.
1 The paperboard should 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 paperboard should be corrected to within 0.05mm.
2 The positioning plane of all front gauges must be on “one line”
This is especially important when printing thin paper. The positional deviation of a front gauge will cause the paper to partially deviate from the straight line formed by the pre-positioning plane, which will destroy the "one line" state of the paper.
3 Paper can be correctly positioned at the front gauge
If the tongue of the individual front gauge is low, the paper will not smoothly enter the positioning plane of the front gauge, causing the straightness of the front edge of the paper to decrease. Therefore, the operator should adjust the feeder's paper-cutting time and the position and pressure of the platen roller and brush wheel on the paperboard and the tightness of the paper-feeding tape according to the specifications to ensure the accuracy of the pre-defined position.
4 The paper is slid smoothly attached to the cardboard
Different models are equipped with different devices to ensure the flatness of the paper when it is in the front position. In the old Heidelberg 102 series offset press, a suction device is arranged below the paperboard and a flat spring leaf is placed on top. In the new machine, the front paperboard is equipped with an air blowing device, and the negative pressure formed by the air blowing process is used to attach the paper to the paperboard. Therefore, the operator should carefully adjust the suction or blowing device on the paperboard, the gap between the flat spring piece and the paper. On the paperboard, the paper must not have the problem of warpage, otherwise there will be more serious corner failure.
For thinner papers, no matter which device, the flatness of the paper cannot be completely solved. Therefore, the paper should be carefully knocked before printing to ensure the stiffness of the paper. The author suggests that when designing the printing, a device can be installed at the pre-specified position to allow the paper to withstand the pulling force in the paper-feeding direction at the front gauge, and to flatten the paper.
(2) The “one line” state of the paper at the instant of paper feeding on the paperboard
When repairing or manufacturing equipment, it should be ensured that the paper feeder is in a "line" state when the paper is taken on the paperboard. To this end, the following conditions must be met.
1 The occlusal plane of the transfer pad is a standard plane
After long-term use, the transfer paper pad is deformed, and the sprayed layer at part of the occlusal plane is detached, so that the occlusal plane of the transfer paper pad is not standard, and the front edge of the transfer paper is uneven after the paper is jammed, therefore, the transfer paper must be repaired or replaced. Dental pad.
2 The ideal gap between the transfer pad and the paperboard is the thickness of the printed paper, so that the paper has the least deformation when the paper is taken.
(3) "One line" status after the paper is fed
1 hand paper picking paper time accurate
The ideal paper take-up position for the transfer paper is the vertical position of the transfer paper pad and the paperboard, and the angle reading on the dial is 11.5 degrees. Due to the wear of the transfer paper, the tooth pad and the adjustment, the paper pick-up time of each tooth on the transfer paper is not consistent. When the transfer paper is taken at the front gauge, the transfer paper pad is not absolutely stationary, but there is a slight rotation, which is a problem that the rotary transfer paper cannot solve. The paper jamming time is inconsistent, resulting in inconsistent paper status of each tooth when the paper is picked up, and the front edge of the paper is deformed. Some teeth only pick up the paper after the transfer paper starts to rotate, causing the front edge of the paper to be severely deformed. Some teeth have too early closing time, and the paper is cut at the moment of the paper smashing, so that the paper edge or the paper edge of the tooth portion is backward. After the other teeth are creped, the front edge of the paper is deformed.
Due to the wear of the transfer shaft, the dental shaft bearing, the copper sleeve of the socket, and the open ball, the paper take-up time of the transfer paper will change each time the paper is taken, which makes the paper pick-up time of the transfer paper inaccurate. The reasonableness of the paper take-up time and the consistency of the closing time of the teeth cannot be guaranteed, resulting in deformation of the paper.
2 Correctly adjust the paper transfer position
During printing, the operator should be accustomed to adjusting the paper transport position according to the position of the front gauge, and should not center the paper without thinking. When the paper is transported, if the edge of the paper is close to the side of the pull gauge (the front gauge of the paper edge), the front gauge will hinder the pulling action of the pull gauge when the paper is pulled by the pull gauge, so that the paper edge Deformation. In this state, the paper feed state of the transfer paper is not ideal, and the front edge of the paper is not in a "one line" state.