Printing machine ghost failure and troubleshooting
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Ghosting is one of the most headaches in printing. For small-format offset printing presses with short ink paths, ghosting is mainly manifested in the appearance of graphics in parts where there should be no graphics. There are several types of ghosts, usually appearing on large areas or on small areas.
1. Type of ghost
Ghosts can be divided into craft ghosts and mechanical ghosts.
(l) Craft ghost
Process ghosting is mainly caused by chemical changes in the drying process of the ink, mostly when printing glossy ink on matte paper or printing matt ink on glossy paper. The appearance of such ghosts is irregular and difficult to control and avoid. Sometimes it occurs after the job is printed and the ink is dry.
It is almost impossible to completely eliminate such process ghosts, but the printing plant can take corresponding measures to avoid ghosting, such as improving the ventilation of the delivery section and keeping the temperature of the processing workshop and the storage workshop constant. If ghosting still occurs, you can also illuminate the graphic part.
(2) Mechanical ghosting
Mechanical ghosting is the most common ghost, mainly caused by the mechanical causes of the printing press. Mechanical ghosting is affected by factors such as ink rollers, blankets, and plates on the press. Ghosts caused by blankets and plates need to be excluded by the printer; ghosts caused by ink rollers, also known as lack of ink ghosts, are mainly due to unreasonable prepress design.
The lack of ink ghosts is mostly due to the fact that a certain part of the printed image requires a large amount of ink, and the ink roller cannot be supplied in time. For example, a large bold type of the same color is designed over a large area of the ground, and the portion of the ink roller that is used to print the bold characters does not have sufficient time to ink before the actual field behind the printing, so the solid portion behind the bold type is The other parts of the field are slightly shallower.
2. Exclusion of ghosts
Ink ghosts have a greater chance of being printed with clear inks, and less often when using opaque inks and proofing. In cooperation with the plate-making process, some measures can be taken to reduce the lack of ink ghosts. First of all, care should be taken to avoid the use of designs that are prone to ink ghosting, such as large borders and large solids. If such a design is necessary, the following measures can be taken:
(l) Try to use pictures as much as possible in the design. For the screen printing without involving the field, there will be no ghosting.
(2) Change the design. Reduce the density of large borders or large solids, balance the difference between the density of the page and the real part of the page, try to balance the amount of ink on the entire page to reduce the appearance of ghosts. For example, if you put two pictures on the right side of the printed sheet, it is easy to produce ghosts when printing. If you change the design, put the two pictures separately, put one on each side, and balance the amount of ink used in the whole version. Shadow.
(3) When designing, design the layout reasonably. A suitable layout ensures that the ink roller has sufficient ink application time to avoid ghosting. For example, if a design is to place a large picture on the ground, if the printing machine is large enough for imposition, it can be placed after the two pictures on a white background.
(4) The excess ink is transferred on the ink roller by placing a color bar on the cut portion other than the graphic portion, and these color bars are called ink moving bars.
(5) Adjust the printing machine to increase the ink supply amount of the ink fountain part of the ink-free ghost. Moreover, some printing machines are equipped with a beveled roller, which can effectively solve the problem of ink-deficient ghosting, that is, using different parts of the ink roller to print different parts of the solid area, so that the ink roller does not lack ink.
(6) Rotate the design layout by 90°. Taking the printing of large bold characters on the ground as an example, if the ink roller first inks a large bold character, and then applies ink to the field, printing one by one, it is easy to cause ghosts. However, if the sheet is rotated by 90°, a part of the ink roller is inked on the font, and the other part is inked on the ground, the ink ghost can be effectively avoided.
The lack of ink ghosting in some places on the solid border is mainly caused by design reasons, that is, the ink roller can not quickly add enough ink to meet the needs of field printing. Due to the large amount of printing ink required for the solid border, the ink in the circumferential direction of the drum is prone to ink shortage, so that the solid border ink printed in the circumferential direction is lighter than the printed border in the axial direction.