3. The overprint generated in the transfer process is not allowed to fail
Cause 1: The paper-feeding position of the paper-feeding tooth is too high, which may result in uneven squeezing and overprinting.
Solution: The correct position is the thickness of the 3 sheets of paper or the thickness of the thick paper plus 0.2 mm.
Cause 2: The force of the paper-feeding teeth and the roller teeth is too small, which can easily cause problems during the transfer of the paper, resulting in failure of overprinting.
Solution: On the machine where the cam is closed and the cam controlled high point is closed, it may be due to the wear of the open roller. To re-adjust, the size of the force should be based on the piece of paper that can be pulled tightly, but do not blindly increase the force of tooth decay. Otherwise it will lead to serious wear of the relevant parts.
Cause 3: The running path of the tilt-type delivery paper is not stable.
Solution: The running path of the pendulum-type feed paper teeth is an inverted drop-shaped curve. It should be a smooth curve. However, if a zigzag pattern is found during the inspection, overprinting will result in inaccurate overprinting. malfunction. You can draw this trajectory with a pencil on the side of the paper feed row. In general, it is recommended that the machine be overhauled. Because once the machine's cam has such a failure, it proves that many parts of the machine have been severely worn.
Reason 4: tooth pads. Denture wear is severe, causing the paper to slip backwards during transfer or during embossing.
Solution: It is recommended to replace it, and replace as much as possible in order to prevent the uneven force of cavities from being overprinted.
Cause 5: The unevenness of the tooth pad on the roller dents or the paper feed teeth.
Solution: Adjust each pad to the same level.
Cause 6: The distance between the drum dentition and the dentition of the delivery tooth is not suitable (the relative position of the occlusal tooth transfer between the multicolor drums will also be inappropriate). Local overprints are not allowed to malfunction.
Solution: The height of the paper-feeding tooth pad is adjusted according to the first impression roller or the first transfer roller, and the multi-color machine starts from the first transfer roller and sequentially adjusts according to the paper-feeding direction. of. Thickness of 3 sheets or thick paper thickness plus 0.2mm. It can be used as a machine width when adjusting. Elastic steel strip with a width of about 10mm and a thickness of about 0.2mm is allowed to pinch the delivery teeth after the front positioning position, release the paper feed teeth after being moved to the delivery position, and then adjust the delivery according to the bending of the steel strip. The height of the paper pad.
Cause 7: The impression force of the impression cylinder is uneven.
Solution: It may be caused by machine wear or improper adjustment. Sometimes it is caused by the uneven force of the large spring on both sides of the jaw axis. The wear of many parts during the transfer process may cause overprinting problems. During the operation process, it is necessary to pay attention to lubrication maintenance and timely repair.
4. Improper overprint due to poor operation of the drum
Cause 1: The three rollers generate axial turbulence during operation.
Solution: Some may be caused by the loosening of the screw at the end of the shaft, and some may be caused by wear of the thrust bearing on both sides of the shaft.
Cause 2: The axial and circumferential directions of the plate cylinder are shaken during operation.
Solution: In addition to the above reasons. Due to the fact that there is usually an adjustment mechanism on both sides of the plate cylinder of a modern printing press, it may sometimes be caused by circuit problems of the plate adjustment mechanism.
Reasons for printing process
1. Printing plate
Cause 1: The plate is deformed due to improper drawing.
Solution: It is recommended to remove the plate tension before drawing. Resolutely put an end to deadlifts and hardtops.
Cause 2: The thickness or pad of the plates varies from color to color.
Solution: Take a 270mm roller print 600mm graphic for example. For every 0.1mm increase in plate thickness or pad thickness, the print will be shortened by 0.25mm. Sometimes operators also use this feature of the printing plate to change the length of the print.
2. Water volume
Cause: Too much water can cause paper distortion.
Solution: no matter from which angle (prints dry, overprint, ink, etc.). All should encourage the reduction of water volume within a reasonable range.
3. Silk direction of paper
It is generally advisable to use a longitudinal (and printer's paper-feeding direction) filaments, and print them with a paper that is oriented perpendicular to the wide sides of the paper.
Experiments show that when a fiber absorbs water, its diameter will increase by 25% to 30%, while the length will only increase by 1%. When printing with this type of paper, it is possible to largely avoid the paper deformation caused by the backward tension and the reduced strength of the paper after water absorption during the imprinting process of the paper.
4. The temperature and humidity of the workshop
When the ambient temperature changes by Â±5Â°C, the moisture content of the paper changes by 0.15%. When the moisture content of the paper changes by 0.1% in multi-color printing, overprinting is affected. Under normal circumstances, the ambient temperature of the workshop should not exceed 3Â°C.
When the humidity of the environment changes by 10% and the relative humidity is below 80%, the moisture content of the paper changes by 1%; when the relative humidity of the environment exceeds 80%, the moisture content of the paper changes by 1.5%-2.0%. So we must strictly control the ambient humidity below 80%.
It is best for qualified companies to ensure that the workshop temperature is constant at 20Â°C and the humidity is maintained at 55%-65%.
Whether it is a malfunction of the machine or improper use of the process, it must be mentioned on the premise that the printing plate is correct. From the aspects of plate making, materials, and printing, the author briefly discussed some reasons for the failure of overprinting of printed products, and may encounter other problems in actual production. This requires detailed analysis of specific issues.