Monochrome multi-color overprinting failure analysis
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Overprinting accuracy is an important indicator for evaluating the quality of printed products. Among them, the overprinting accuracy is affected by various 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 beautiful prints, many printing companies have introduced multi-color printing machines of four or more colors. However, due to its own strength and other reasons, the main equipment used in our factory is still the domestic J2108 monochrome offset printing machine. Then how to play and utilize the existing equipment, print each batch of color printing products, reward customers with qualified printing products, and win credibility, which is the pursuit of our factory printing workshop.
Overprinting is not one of the common faults in offset printing, and most offset printing products are multi-colored overprinted. In the multi-color overprinting process, all parts of the paper feeding system and printing system have precise time control and strict positional restrictions, so that the printed sheets can be printed from the paper feeding to the end of the delivery, and finally the printed products with uniform graphic specifications are obtained. In the process of being conveyed and imprinted, if any control component fails or is improperly adjusted, the overprint may be inaccurate. Insufficient overprinting is generally not allowed for vertical overprinting, horizontal overprinting is not allowed, and vertical and horizontal simultaneous overprinting is not allowed. Therefore, once the overprinting is found to be inaccurate during printing, it should be decisively shut down, promptly checked and take corresponding measures to eliminate. The author combines years of production practice, carefully analyzes and summarizes, and now summarizes the causes and solutions of some faults as follows for reference by peers.
Overall overprint is not allowed
1. Inaccurate positioning of the rules results in inaccurate overprinting
Solution: Carefully check the working conditions of the front and side gauges, such as the front gauge, when the side gauges position the paper, adjust it according to the requirements of the manual, and pay attention to the height of the front gauge and side gauges.
2. Improper picking of the paper is not correct
(1) The paper is dithered when the paper is picked up, causing the paper to be unsettled
Solution: Adjust the retaining pad of the paper feeder so that it remains stationary on the paperboard and the transition is gentle.
(2) Axial movement when the paper feed paper is taken
Solution: Adjust the axial lock nut to ensure that it has a 0.03mm turbulence.
(3) The paper has insufficient force and the paper has sliding
Solution: Increase the force of the transfer paper, especially when printing at high speed (mainly the inertia of the paper).
(4) The opening and closing time of the paper feeding teeth is not accurate
Solution: Re-adjust the opening and closing time of the paper.
3. Improper coordination between the paper teeth and the rules, resulting in inaccurate overprinting
(1) When the paper feed has not been taken, the rules have started to release paper.
Solution: Adjust the rules so that the paper and the rule have a common paper holding time.
(2) The rules have not been completed, and the paper is started to pick up the paper.
Solution: Same as 3(1).
(3) When the paper feed paper is removed from the paperboard, the rules have not been released.
Solution: Adjust the rules to release paper in advance.
(4) The height of the upper pendulum transfer pad 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 paper is closed, and the position of the paper will change. The height of the general paper pad should be the same as the previous rule, adding 0.2mm to the paper thickness.
4. Improper cooperation between the transfer paper and the impression cylinder, resulting in inaccurate overprinting
(1) The handover time is too short
Solution: Adjust the position of the corresponding open and closed cams.
(2) Improper distance between the pads
Solution: Re-adjust the height of their respective pads to a distance of 0.2 mm from the paper thickness.
5. The nip of the embossing cylinder is insufficient, and the paper slides in the fangs, resulting in inaccurate overprinting.
(1) The pressure of the tooth piece is too small
Solution: Increase the pressure on the teeth.
(2) The friction coefficient of the tooth pad is too small
Solution: Replace the dental pad.
6. The backing paper roller has insufficient force and the paper slides in the molars
Solution: Same as 5.
7. The axial movement of the drum causes the overprint to be inaccurate
Solution: Replace the dental pad.
8. Other reasons for overprinting
(1) The viscosity of the ink is too large
Solution: Add varnish or remover to reduce the viscosity of the ink.
(2) The surface of the blanket is sticky
Solution: Replace the blanket.
(3) Printing pressure is too large
Solution: Adjust the printing pressure.
Partial overprinting is not allowed
Partial overprinting means that the local position of the paper is relatively slippery 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 overprint is.
Solution: Reduce the paper feed speed or increase the number of front gauges so that the front of the paper is accurately positioned at the rule.
2. Insufficient force on the individual fangs on the transfer paper, so that the position of the front opening of the paper changes there, resulting in inaccurate overprinting
Solution: Replace the dental pad or increase the force of the individual tooth decay.
3. The amount of crepe paper in the transfer paper is uneven, and there are fewer sides on one side, and the relative amount of the crepe paper is relatively slipped, resulting in inaccurate overprinting.
Solution: Adjust the front gauge to make it evenly.
4. Insufficient force of the individual impressions of the impression cylinder, resulting in inaccurate overprinting
Solution: Replace the dental pad or increase the force of the individual tooth decay.
5. Excessive local force, causing the resistance of the paper to advance, so that the paper slides there, resulting in inaccurate overprinting
Solution: Check the blanket and pad and recalibrate the print pressure.
6. The paper swells with water, causing the second half of the paper
Solution: Condition the paper before printing.
Imprinting caused by plate making is not allowed
1. Imposition accuracy is not high, resulting in inaccurate overprinting
Solution: The following word lines or corner lines are used as the reference line during printing, and the same is true for imposition. If the position between the graphic or the rule line is not properly matched, it is very difficult to achieve accurate overprinting during printing. Therefore, to maximize the accuracy of imposition, in particular, the imposition of the use of high magnification magnifier.
2. More times of printing, resulting in inaccurate overprinting
Only once, there is very little overprinting, and multiple prints are easy to appear. If you must print the plate multiple times, you must make a platen for partial exposure.
3. The plate is deformed when the plate is pulled, resulting in inaccurate overprinting
Solution: When the plate is subjected to a large force, it is prone to deformation or cracking. It is not advisable to use too much force when pulling the plate. If you can't pull it, you should look for the reason carefully.
4. The problem of the process of editing 100, resulting in overprinting
(1) Multicolor jobs appear overprinted
Solution: The more the number of overprints, the more likely the error will occur, so the number of overprints should be minimized.
(2) Anti-white characters or lines of multi-color overprint appear inaccurate overprint
Solution: Try to replace the overprint with a spot color. A reasonable process can also be used to reduce the number of overprints and minimize the possibility of errors.
Inspection of overprint during printing
Overprinting accuracy is a basic requirement for the quality of printed products, so this content is essential for each inspection.
(1) Check the registration of the rules and check the registration of the internal map. Some small-format impositions of printed products, with hollow-stitched prints, or when the paper is partially deformed, there will be cases where the ruled line has been registered and the inner picture is inaccurate. At this time, it is necessary to re-adjust the rule line according to the internal picture.
(2) When printing the first color, according to the requirements of printing and cutting, check whether the position of the whole picture on the printed sheet is correct, and often take a stack of printed sheets, and then open them to check whether the lines are in a straight line. .
(3) For printed products printed on the front and back sides, the registration of both sides should also be checked frequently.
(4) In normal printing, high-speed machines can generally be sampled and tested every 500 or so. When sampling, pay attention to the finger to be pinched in the blank of the printed sheet, and take it out quickly and smoothly. The inspection is carried out in two parts. Initial inspection: Take out the sample, place it on the sample platform, and carry out a comprehensive inspection in a rough manner. If there is any big difference in the gauge line and see if it is on a straight line, whether the paper has wrinkles or the like. Fine inspection: With the aid of a magnifying glass, you can check the accuracy of the gauge line and the overprint status of the inner map.
(5) Ensure that the machine runs at a constant speed during the printing process. Otherwise, it is easy to cause the impact of the paper on the front gauge to be different, which makes the paper misaligned.