Comprehensive troubleshooting of the hottest offse

Comprehensive troubleshooting analysis of offset press (II)

7.9.2 ghosting

ghosting is the appearance of two or more identical impressions on the print, and their location is close. Therefore, to troubleshoot ghosting is to find out the factors that cause the dislocation of the imprint

1. The printing plate is loose. Due to the looseness of the printing plate, the position of the printing plate in the printing process cannot be kept fixed, so the positions of the old and new imprints transferred to the print twice are different, so there will be ghosting on the print sheet. Re clamp the printing plate, and the screws on both sides of the circumference and rotation should be in working state, so that the relative position between the plate clamp and the plate cylinder can remain unchanged; Adjust the plate gap on the plate clamp to ensure that the relative position between the plate and the plate clamp remains unchanged during the printing process; Press and install the printing plate, so that the printing plate can be closely surrounded on the printing plate cylinder

2. The blanket is loose. Due to the different positions of the rubber cloth in the contact process with the printing plate (or embossing cylinder), the old and new marks are left on the rubber cloth, and ghosting is bound to occur when it is transferred to the printing sheet. The rubber clip of the rubber blanket is not installed in the correct position. Reinstall the clip to ensure that the rubber blanket is in close contact with the plug on the clip. When tightening the screws with 103 construction petroleum asphalt, tighten the middle first and then both sides, so as to prevent the accumulation in the middle of the rubber blanket; The tension of the rubber blanket is too small. Rotate the worm gear and worm mechanism by hand to tension the rubber blanket; The liner under the rubber blanket is uneven, causing the rubber blanket to be loose in some places and tight in some places. Replace the liner

3. The paper slides on the impression cylinder. In multi-color printing, when the printing sheet is printed in the first color and then enters the second color printing, because the imprint on the printing sheet is not completely dry, the printed edge of the first color is transferred to the second color blanket when it is in contact with the second color rubber market. If the position of the paper is changed and other parts are in good working condition, the first color imprint on the subsequent printing sheet should completely coincide with the first color imprint on the second color blanket, because there is no ghosting; Otherwise, ghosting will occur. The pressure of the dental tablet on the impression cylinder is too small, which increases the pressure of the dental tablet; The tooth pad on the impression cylinder is seriously worn, so replace the tooth pad; The amount of paper on the embossing cylinder is too small. Adjust the rules and increase the amount of paper biting; If the printing pressure is too high, reduce the printing pressure; The viscosity of the ink is too large, add inking oil or viscosity remover to reduce the viscosity of the ink; Replace the full plate with soft padding

4. Paper deformation. The uneven surface of the paper and the damp of the paper will cause the new and old imprints not to coincide. You can use the machine to press (water, but not ink) the paper first, so that the paper basically reaches the operation step of the full-automatic spring tension and compression experimental machine: flat state after a decisive stamping. And because it has absorbed a certain amount of water, the possibility of re absorption and expansion is much smaller

5. The series movement of the drum. The serial movement of the plate roller, rubber roller and other paper transfer rollers will cause the new and old imprints not to coincide, resulting in ghosting. Carefully check the axial eye position devices of these rollers, and readjust them improperly. Generally, the string momentum is controlled within 0.03mm

6. The layout of the page is unreasonable. The on-site replacement eye block should be correctly arranged. Generally, its axial distribution should be as uniform as possible, so that the viscous force of the ink is also evenly distributed along the axial direction, so as to reduce the possibility of paper sliding

7.9.3 water and ink bars

water and ink bars are the water lines or ink lines that appear in the axial direction (or horizontal) of a print. Waterline is because of too much water, the ink is not printed or printed too little; Ink line is caused by too much ink printing due to large ink. There are many reasons for the production of water and ink bars, and they are also very complex. Here is a brief analysis of their causes and troubleshooting from different perspectives

1. The pressure of the water stick is too high. Because the pressure of the water roller is too large, the impact when contacting with the printing plate is also large, resulting in the water volume on both sides of the impact position of the water roller is larger than that of other parts around it. After the water roller rotates for one circle, the part with large water contacts with the printing plate, resulting in the weakening of the inking ability there, thus forming a water bar. At this time, the water roll pressure should be reduced

2. The water roller is not round. Because the water roller is not round, it is high in some places and low in some places. High places will slide when in contact with the printing plate, resulting in the accumulation of water and the formation of water bars. The water roller should be replaced

3. The pressure of the ink roller is too high. The reason is the same as 1, but the amount of ink on both sides of the impact increases, resulting in the formation of ink bars

4. The ink roller is not round. The analysis is the same as 2

5. Printing pressure is too high. Because the printing pressure is too large, the impact of the drum when switching between the notch and the non notch is too large, causing the drum to vibrate, so the fixture of water, ink roller and printing plate should not cause the specimen to fracture at the fixture, and the drum surface also has an impact, resulting in water and ink bars. The solution is to reduce the printing pressure

6. The rolling gear is worn or the machining accuracy is too low. After the accuracy of the gear is destroyed, the drum will appear uneven in the rotation process, so that the water, ink roller and the printing plate will slide relatively, forming a water and ink society. At this time, the cargo ship should be replaced or the meshing position of the gear should be changed

once water and ink bars appear, the consequences are very serious, so it is necessary to find out the causes in time and eliminate them

7.9.4 paper pleating

paper pleating is that the paper surface is in an uneven state, so the protruding places are flattened during printing to form pleats. Pleated prints are generally unacceptable

1. The paper is not in a flat state at the regular place. Because the paper is in an uneven state at the regular place, when printing, the uneven part is flattened to form a lattice

(1) change the position and direction of the paper pressing wheel on the paperboard so that it is conducive to the flatness of the paper

(2) increase the number of front rules so that the paper does not bend forward at the rules

(3) for the paper delivery teeth with upward swing, the distance between the teeth and the rule weight should be equal to or slightly lower than the distance between the front rule cap and the rule plate for high-precision separation

2. There is a half tooth phenomenon when holding paper teeth. When the paper teeth have half teeth, the paper is easy to slide in the teeth, so the surface of the paper becomes uneven and folds are formed during printing. Try to find a way to make more than two-thirds of the teeth bite the paper in the width direction

3. The paper delivery teeth hold paper more unevenly, with more on one side and less on the other. The less side is easy to slide during transmission, resulting in uneven paper surface. Adjust the rules so that the paper holding amount of the paper holding teeth can meet the requirements that the paper does not slide

4. Printing pressure is too high. Because the printing pressure is too large, the paper at individual teeth slides relatively, resulting in uneven paper surface. Reduce the printing pressure or increase the friction at the paper teeth of the imprint cylinder

5. The ink is too sticky. Due to the stickiness of the ink, the resistance of the paper to forward transmission increases, so that the paper in individual parts slides backward, and the paper surface becomes uneven. Add ink blending oil or viscosity remover to reduce the viscosity of ink, and pay attention to the influence of ambient temperature

6. The paper itself is uneven. The paper becomes uneven due to moisture and other reasons. The shorter the time of unpacking the paper, the better. The environment (temperature and humidity) in which the paper is unpacked should be consistent with the printing environment. Semi finished products should be kept properly, preferably covered with plastic. (to be continued)