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Inaccurate Overprinting Of Prints And Analysis Of Uneven Ink
- May 29, 2018 -

Offset printing is currently the main printing method for printing companies. The failure of the lithographic offset printing system due to the output system is due to inaccurate overprinting, uneven ink, and even printing failure, which is the main reason for delay in delivery. This is one of the more thorny issues that printers most often encounter.


One, two or more sheets of paper failure

1. The paper is uneven and the edges are curled up. When the airflow blown by the paper blowing nozzle touches the edge of the paper, the edge of the paper curls up into a circular arc angle, and the air blowing is divided into two parts. A portion of the airflow lifts the paper upwards, while the other portion of the airflow pushes the paper down so that the paper cannot be separated and the baffle brush loses its effect, resulting in double or multiple sheets, or even large stacks of paper feeds. The general approach is to remove the paper and perform an anti-knock paper process to make the paper flat and conducive to separation.

2. The loading height exceeds the allowable height of the paper teeth. According to previous printing experience, the height of general paper loading should be less than about 5mm. If the adjustment is not appropriate, it will exceed the allowable range, it will produce double or more into the paper transport. The usual method of handling is to use a cork wedge to control so that the paper stays on the same plane. If there is a regulator model, you can use the regulator to reduce the height of the paper to solve.

3. The blower foot and stop brush have too few paper edges. According to the previous operating experience, the general blocking paper brush extends into the paper edge approximately 8mm, and the air blowing foot extends into the paper edge approximately 10-15mm. If you do not adjust properly or the paper is miscut, you will see double or multiple sheets. Generally, the whole of the feeder will be adjusted to make the separation nozzle approximately 5mm away from the edge of the paper. Adjust the blocking brush and the presser foot to solve the problem.

4. When the paper is too thin, the air volume is adjusted improperly. When printing 70g/m2 thin paper, if the air volume is not properly adjusted, it is easy to produce double or multiple sheets. The general processing method has the following points: (1) Adjust the blowing foot downwards, increase the distance between the paper stack and the paper suction nozzle; (2) Adjust the air volume appropriately; (3) Reduce the paper suction nozzle The rubber band or transparent glue is used to reduce the wind force; (4) Press the baffle downwards and increase the pressure to control it.

5. The paper is not loose or sticky. Generally when the paper is cut, the cutting edge is bonded, or when the full version of the semi-finished product is printed on the ground, water stains and ink stains cause sticking, thereby forming two or more sheets. Therefore, before the paper is printed, that is, when the paper is loaded into the machine, it must be shaken and stacked. For the semi-finished products, they should be piled up and stacked loosely. When the machine is on the machine, it should be installed little to avoid adhesion due to heavy pressure. .

6. Blower foot wear. When the presser foot is used for a period of time, it will wear out. When it presses down on the paper, the paper will make an oblique angle, the force will change direction, the paper will move forward, and the paper brush will lose. The role of the paper suction nozzle down when the suction paper suction on the edge of the paper, and the formation of double or multiple. Generally, new hair pressure presser foot is used to deal with it. If the wear is not serious, it can be used to weld copper sheets, and then use a trowel to flatten the solution.


Second, the uneven ink product failure

The reasons for the uneven ink product are: (1) The cylindricity wear of the rubber rollers for water transfer, ink transfer, even ink, and ink printing depends on the abrasion of the cylindricity of the rubber rollers, the wear of the shaft head of the bearing, the aging of the rubber roller surface, and the decline in the ink transfer capacity. (2)Stripping water roller layer peeling, cylindricity wear is poor, the surface has a thin ink layer, the two heads have a part of the ink ribbon, causing uneven roller rolling ink; (3) support by version of the water The lock of the ink roller and the swinging bracket cause serious wear and tear, which causes the roller to oscillate, resulting in unstable plate pressure and unbalanced ink and ink. The uneven ink, dry plate, ink bar, and old printing plate are used. (4) The aging or poor quality of the water roller pile causes the water transfer ability to decline; (5) There is no regular check of the pressure of the ink roller and the platen roller and the plate-dependent ink roller; (6) If the roller The transmission gears wear out badly, and there are regular “gear rollers” with the same spacing as the gears.


Third, overprint failure

1. Effect of deformation of printing plate on overprinting. The metal sheet has certain extensibility. Under the action of tensile force, it will produce tensile deformation, and the zinc plate will have more pronounced tensile deformation. Under normal circumstances, we found that overprinting was not allowed during printing, and most of them were adjusted by stretching the printing plate. The greater the tensile force, the greater the tensile deformation of the layout, and the uneven distribution of the tensile deformation, the greatest force is exerted on the crotch edge and the crest side of the plate. The tensile deformation is also large, the internal force is small, and the tensile deformation is also small. small. Therefore try to use as little as possible to solve the problem of overprinting.

2. The effect of the increase or decrease of the cylinder liner on the overprint. In the printing process, due to the paper stretching caused by inaccurate overprint, the method of increasing or decreasing the thickness of the drum liner can be used to solve the problem of misalignment between the radial paths. The increase or decrease of the printing plate liner affects the plate's tensile deformation. The thicker the plate pad, the greater the dimensional elongation of the pattern. When the pad of the printing plate is reduced and the pad of the rubber roller is increased, the tensile deformation of the looming gray fabric increases, and the radial deformation of the pattern increases. Another is to reduce the thickness of the blanket cylinder, while increasing the thickness of the printing plate liner, the deformation of the blanket extrusion deformation is reduced, the latter commonly used.

3. The effect of blanket elongation on pattern overlay. The blanket is stretchable, and the tighter the blanket is stretched on the drum, the smaller the elongation is, and the pattern change is small. (1) Choose a blanket with a small elongation. (2) In the process of overprinting, try to keep the tightness of the blanket unchanged.

Overprinting is not allowed. In addition to the above-mentioned reasons, it is also necessary to pay attention to mechanical, adjustment, and paper issues. In short, overprinting is not always done at the instant of high-speed printing, and is caused by a slight accumulation of changes. It is difficult to observe directly. Therefore, as long as we pay adequate attention to printing, analyze the reasons, and take the most appropriate measures, we can avoid or reduce the occurrence of overprint errors.


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