Overprint accuracy is an important indicator for evaluating the quality of printing. Among them, the accuracy of overprinting is affected by many factors such as the accuracy of the machine, the performance of the raw materials and the technical level of the operator. In order to control the precision of overprinting and produce exquisite printed products, many printing companies have introduced multi-color printing machines with four colors or more. However, due to its own strength and other reasons, the main equipment used in our factory is still the domestic J2108 monochrome offset press. So how to make good use of existing equipment, print each batch of color prints well, give back to customers with qualified prints, and win credibility is what our printing workshop pursues. Overprinting is not one of the common faults of offset printing, and most offset printings are made by multicolor overprinting. In the process of multi-color overprinting, the paper feeding system and all components of the printing system have their precise time control and strict position restrictions, so that the printed sheets are fed from the paper to the end of the paper collection, and finally the printed products with uniform graphics and text specifications are obtained. In the process of paper being transported and imprinted, any control component failure or improper adjustment will cause inaccurate overprinting. Overprinting is generally not allowed in vertical overprinting, horizontal overprinting and vertical and horizontal overprinting at the same time. Therefore, once printing is found to be inaccurate, it should stop decisively, check it in time and take corresponding measures to eliminate it. The author has carefully analyzed and summarized the production practices for many years, and now summarizes the causes and solutions of some faults as follows, for peer reference. The overall overprint is not allowed 1. Inaccurate rule positioning causes inaccurate overprint Solution: Carefully check the working conditions of the front and side rules, such as the front and side rules, when to position the paper, adjust it according to the instructions, and pay attention to the height of the front and side rules should meet the requirements. 2. Improper delivery and picking of paper cause incorrect registration (1) The paper feeder shakes when picking up paper, resulting in unstable paper handover Solution: Adjust the stopper of the delivery tooth to keep it still on the cardboard, and the transition is smooth. (2) Axial movement when feeding paper Solution: Adjust the axial lock nut to ensure that it has a movement of 0.03mm. (3) The gripping force of the paper feeding tooth is insufficient, and the paper slips Solution: Increase the gripping force of the paper delivery teeth, especially when printing at high speed (mainly due to the inertial effect of paper). (4) The opening and closing time of the paper tooth is not accurate Solution: readjust the opening and closing time of the delivery tooth. 3. Improper coordination between the delivery tooth and the rules, resulting in inaccurate overprinting (1) When the paper delivery teeth have not yet taken the paper, the rules have begun to put the paper Solution: Adjust the rules so that the delivery tooth and the rules have a common holding time. (2) The positioning has not been completed yet. Solution: Same as 3 (1). (3) When handing paper and holding paper to leave the paperboard, the rules have not been put Solution: Adjust the rules to put paper in advance. (4) The height of the upswing paper tooth pads is not suitable Consequences and solutions: too low, it is easy to impact the paper and make it shift; too high, the paper will be pulled up when the tooth is closed, and the position of the paper will change. Generally, the height of the tooth pad for paper delivery should be the same as the previous rule, adding 0.2mm to the paper thickness. 4. Improper cooperation between the delivery tooth and the impression cylinder, resulting in inaccurate overprint (1) The handover time is too short Solution: adjust the position of the corresponding opening and closing teeth cam. (2) Improper distance between dental pads Solution: readjust the height of their respective tooth pads so that the distance is 0.2mm from the paper thickness. 5. The gripping force of the impression cylinder is insufficient, and the paper slides inside the gripper, resulting in inaccurate overprint (1) Tooth pressure is too low Solution: Increase the pressure of dental tablets. (2) The friction coefficient of the dental pad is too small Solution: replace the dental pad. 6. The gripping force of the gripper of the rear transfer roller is insufficient, and the paper slides inside the gripper Solution: Same as 5. 7. The axial movement of the cylinder causes overprinting to be inaccurate Solution: replace the dental pad. 8. Overprinting is not allowed due to other reasons (1) The viscosity of the ink is too large Solution: Add ink or viscosity remover to reduce the viscosity of the ink. (2) The surface of the blanket is sticky Solution: replace the blanket. (3) The printing pressure is too high Solution: Adjust the printing pressure. Partial overprint is not allowed Inaccurate partial overprinting refers to the relative slippage of the local position of the paper during the printing process. 1. When there are only two front gauges, when the paper reaches the front gauge, the middle part protrudes forward. The softer the paper, the more serious the overprinting fault. Solution: Reduce the paper feed speed or increase the number of front gauges, so that the front edge of the paper is accurately positioned at the rules. 2. The gripping force of individual grippers on the delivery tooth is insufficient, so that the position of the front opening of the paper changes at this place, resulting in inaccurate overprinting Solution: replace the dental pad or increase the gripping force of individual teeth. 3. The amount of gripping paper of the delivery paper tooth is uneven, and there is more on one side and less on the side. The part with less gripping paper has a relative slippage, which causes overprinting Solution: Adjust the front gauge to make the amount of paper grip even. 4. Insufficient gripping force of individual grippers on impression cylinder, resulting in inaccurate overprinting Solution: replace the dental pad or increase the gripping force of individual teeth. 5. Partial gripping force is too large, causing the resistance of the paper to increase, thereby causing the paper to slide there, resulting in inaccurate overprint Solution: Check the blanket and pad, and recalibrate the printing pressure. 6. The paper swells with water, causing the back half of the paper to flick Solution: Humidify the paper before printing. Overprinting caused by plate making is not allowed 1. The imposition precision is not high, resulting in inaccurate overprint Solution: The following word line or corner line is used as the reference line when printing, and it is also the same when imposition. If the position of the graphic or the rule line is not properly matched, it is very difficult to achieve accurate overprinting during printing. Therefore, it is necessary to maximize the precision of the imposition, and the imposition staff is particularly required to use a high magnifying glass. 2. There are many printing times, resulting in inaccurate overprinting Exposure only once, inaccurate overprinting rarely occurs, and it is easy to appear after multiple exposures. If it is necessary to print the plate multiple times, you must make plate printing paper for partition exposure. 3. The printing plate is deformed when pulling the plate, resulting in inaccurate overprint Solution: The printing plate is prone to deformation or cracks when subjected to a large force. It is not advisable to use too much force when pulling the plate, and you should look for the cause carefully when you cannot pull the plate. 4. The problem of craftsmanship, resulting in inaccurate overprinting (1) Overprint is not allowed for multi-color jobs Solution: The more the number of overprints, the greater the possibility of errors. Therefore, the number of overprints should be reduced as much as possible. (2) Overprinted characters or lines of multi-color overprinting are not allowed to be overprinted Solution: Try to replace spot printing with spot color. A reasonable process can also be used to reduce the number of overprints and minimize the possibility of errors. Overprint inspection during printing Accurate overprinting is a basic requirement for print quality, so this item is essential every time you check. (1) Check the registration of the gauge line, and then check the registration of the internal map. Some small-scale imposition combinations of printed products, printed products with hollow characters overprint, or partial deformation of the paper, there will be situations where the rules have been registered and the inner images are not. At this time, it is necessary to readjust the rules according to the situation in the internal map. (2) When printing the first color, according to the requirements of printing and cutting, check whether the position of the entire graphic on the printed sheet is correct, and often extract a stack of printed sheets, straighten and twist them, check whether the gauge line is on a straight line . (3) For the printed products printed on the front and back sides, the registration on both sides should also be checked frequently. (4) During normal printing, the high-speed machine can generally sample and check every 500 sheets or so. When sampling, pay attention to the fingers should be pinched in the blank space of the printed sheet, and take out quickly and smoothly. The inspection is carried out in two parts. Initial inspection: take out the sample, put it on the sample stand, and perform a comprehensive inspection in a rough way, such as whether there is a large gap between the gauge line and whether it is on a straight line, whether the paper is wrinkled, etc. Detailed inspection: You can use the magnifying glass to check the accuracy of the gauge line and the overprinting status of the internal map. (5) During the printing process, ensure that the machine runs at a uniform speed. Otherwise, it is easy to cause the paper to produce different impulses at the front gauge, making the paper inaccurate.
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Failure Analysis of Multicolor Overprinting of Monochrome Machine