Overprint inaccuracy is a relatively complicated technical problem in printing. It has four phenomena and three factors. Four phenomena, that is, the overprint of the printed sheet is inaccurate, the partial overprint of the printed sheet is inaccurate, the repeated overprint is inaccurate, and the front and back overprint is inaccurate; the three factors are the inaccurate original plate caused by plate making, film production, and imposition, and the overprint caused by paper expansion Inaccurate, overprint caused by malfunction of offset press or improper operation and debugging. Specifically, overprinting is divided into two types: vertical overprinting and horizontal overprinting. The following summarizes dozens of cases of overprinting failures in combination with production practice for analysis and proposes solutions.
Vertical overprint is not allowed
1. Overprinting caused by the paper not getting in one side
Inaccurate overprinting on one side means that the crosshairs on the vertical single side are not overprinted, and the crosshairs on which the overprinting is inaccurate are in the gripping direction of the standard line. The following 7 types of fault analysis and solutions are common.
(1) The side rule falls too early
It means that when the normally conveyed paper has not reached the front gauge, the side gauge pulls the paper pressure ball to drop the pressure paper too early, so that the paper is not in place, and the overprint is not accurate.
Solution: The time for the side gauge to fall normally should be after the paper is normally transported to the front gauge; the time for picking up should be while swinging the paper gripper to grip the paper, that is, while swinging the opening and closing tooth roller away from the cam. The two positioning screws on the side gauge cam can be loosened to reverse the cam until it meets the above time for falling and lifting.
(2) The front gauge paper tongue is too low
When the front gauge blocking tongue is less than 3 times the thickness of the printed paper, the paper will be blocked from entering the front gauge and will not go into place.
Solution: Loosen the adjustment screw of the front gauge, lift the front gauge and place 3 printed sheets under the paper stopper tongue, and tighten the adjustment screw. The idling machine stops at the position where the front rule is dropped for several weeks, and then inserts 3 printed sheets under the paper stopper tongue for retest, so that 3 sheets of paper can come in and out freely, and 4 sheets of paper can not be inserted.
(3) The paper positioning time is too late
Sometimes the paper arrives at the front gauge too slowly, especially the side gauge can't advance while the paper is pressed by the side gauge, and the other side continues to advance to the front gauge, which will also cause the failure of one side not to go in place.
Solution: Loosen the fine adjustment screw that adjusts the speed of the paper feed, turn the feeder handwheel in the direction of the paper feed, and then tighten the screw to retry. After the front rule is dropped into place, the front edge of the paper should be 6-8mm away from the paper blocking tongue.
(4) The side gauge pulls the paper and presses the ball too slowly
The reason is: when the swinging gripper grips the paper and starts to move forward, the paper pressure ball still presses the paper, pulling the paper out of the swing gripper, the printed product and the overprint that is not in one side are not accurate The phenomenon is the same.
Solution: The side gauge pulls the paper and presses the ball too slowly, indicating that the cam working point has not been adjusted. At this time, the two positioning screws of the side gauge cam can be loosened, then the cam is dialed a little, the screw is tightened, and the retest is performed until Meet the requirements.
(5) The grip of paper teeth is too light
Impression cylinder gripper and swing gripper gripping force is too light, too little gripping paper or uneven gripping force is also easy to cause inaccurate overprint, which is reflected on the printed matter and the inaccurate overprint phenomenon that is not in place.
Solution: If it belongs to the fatigue problem of the support spring or the tension spring, it should be replaced; if the swing gripper tooth pad is too low and not flat, the whole set should be replaced. After installation, the method of general adjustment can be used to jog the printing machine to the junction of the impression cylinder and the swing gripper, with the swing tooth pad being 0.3mm away from the plane of the impression cylinder. During the overall adjustment, loosen the overall iron plate of the dental pad, adjust the nut on the swing shaft, turn the dental pad clockwise to raise, and turn the dental pad counterclockwise to lower until it reaches a distance of 0.3mm. As for too little gripping paper, you must ensure that the gripping teeth are swayed and imprinted. The gripping paper is between 5 and 6 mm. Generally, you don't need to move the front gauge when changing plates and adjusting rules.
(6) The trailing tip positioning brush fails
The positioning brush with a slight drag is mainly to prevent the paper from entering the front gauge and then ejecting. There may be the following reasons for its failure: â‘ When the paper feed is stopped due to the skew of the paper feed, in order to remove the paper on the paper feed, the frame is turned up and down too heavy, causing the brush to tilt or flip. â‘¡ The brush is fixed in a position for a long time. If it is used for too long, the part where the brush rubs against the edge of the paper is damaged and becomes useless. â‘¢The position of the brush against the paper is not correct, the paper is on the tip or root of the brush.
Solution: The first â‘ , â‘¢ problems need to re-adjust the brush; the second â‘¡ problem can be removed brush, hot on the fire. The method is: first, a flat small iron plate is burnt red in the fire, and after being picked out, it is laid flat on the ground, and then the brush is gently tilted on the burnt iron plate according to the original slope, high It is convenient to be ironed. If the wear is large, it can be ironed several times until it is ironed (please note that this brush should not be cut).
(7) The brush wheel is too light or the two pinch rollers at the trailing tip do not bear against the rear edge of the paper. The brush wheel and the two paper pressure wheels at the trailing edge can ensure the stability and accuracy of the paper when it enters the front regulation. Their pressure on the paper can neither be too heavy nor too light. Too heavy can easily cause the back to be smudged; too light can lose its proper role on the paper and can easily cause the paper to go out of place. Sometimes, of the 2 brush wheels or 2 paper press wheels, one is suitable for pressure, and the other is light. The light end will cause the unilateral to go out of position.
Solution: Two brush wheels that play an important role in overprinting and two paper pressing balls at the trailing tip, the pressure must be consistent and in place, that is, the brush wheel must be pressed on the paper surface of the conveyor belt; The two pressure balls at the trailing tip should tightly bear against the rear edge of the paper.
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