Ghost image is one of the most troublesome problems in printing. For small format offset presses with short ink paths, ghost images are mainly manifested in areas where there should be no images. There are several types of ghosts, which usually appear on a large area on the ground or a small area of ​​blank space.
1. Types of ghosts
Ghosts can be divided into craft ghosts and mechanical ghosts.
(L) Craft ghost
Process ghosting is mainly caused by the chemical changes of the ink during the drying process, which mostly occurs when printing glossy ink on matte paper or printing matte 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 dried.
It is almost impossible to eliminate ghosts of this type of process completely, but the printing house can take corresponding measures to avoid ghosts, such as improving the ventilation conditions of the paper collection department, keeping the temperature of the processing workshop and storage workshop constant. If ghosts still appear, you can also glaze in the graphic part.
(2) Mechanical ghosts Mechanical ghosts are the most common ghosts, mainly caused by the mechanical reasons of the printing press. Mechanical ghosts are affected by factors such as ink rollers, blankets and printing plates on printing presses. Ghosts caused by blankets and printing plates need to be eliminated by printers; ghosts caused by ink rollers, also known as ink-out ghosts, are mainly due to unreasonable pre-press design.
Insufficient ghost ghosts are mostly caused by the fact that a certain part of the printed image requires a lot of ink, and the ink roller cannot supply ink in time. For example, a large bold font of the same color is designed over a large area of ​​solid ground. The part on the ink roller used to print bold font has insufficient time to ink before printing the solid ground behind, so the solid portion behind the bold font is The rest of the field is slightly shallower.
2. Elimination of ghosts
Insufficient ghost ghosts are more likely to occur when printing with transparent ink, but rarely occur when using opaque ink and proofing. In cooperation with the plate-making and printing process, some measures can be taken to reduce ink-out ghosts. First, attention should be paid to avoid designs that are prone to ink-out ghosts, such as large frames and large fields. If such a design must be adopted, the following measures can be taken:
(L) Use pictures as much as possible in the design. For screen printing that does not involve the field, there will generally be no ghosting.
(2) Change the design. Reduce the density of large borders or large fields, balance the density difference between the image on the web page and the part of the field, and try to balance the ink volume of the entire page to reduce the appearance of ghosts. For example, if two pictures are placed on the right side of the printed sheet, ghosts are likely to occur during printing; if the design is changed so that the two pictures are placed separately, one is placed on each side to balance the ink consumption of the whole version, the rabbit ghost can be avoided Shadow.
(3) When typesetting, design the layout reasonably. Appropriate layout arrangement can ensure that the ink roller has sufficient time for inking to avoid ghosts. For example, if a design is to place a large picture on the ground, when imposing, if the printing press is large enough, it can be placed after two pictures on a white background.
(4) Use the color bar in the cut part other than the graphic part to transfer the excess ink on the ink roller. These color bars are called ink transfer bars.
(5) Adjust the printing machine to increase the amount of ink supplied to the ink fountain in the ink-deficient ghost. Moreover, some printing machines are equipped with inclined ink rollers, which can effectively solve the problem of ink shortage ghosts, that is, use different parts of the ink roller to print different parts of a large area on the ground, so that the ink roller will not be short of ink.
(6) Rotate the design layout by 90 °. Take the example of printing large bold characters on the ground. If the ink roller first inked the large bold characters, and then inked the solid, printing one by one, it is easy to cause ink shortage ghosts. But if the printed sheet is turned 90 °, so that part of the ink roller inked the font and the other part is inked on the ground, it can effectively avoid the lack of ink ghosts.
The phenomenon of lack of ink ghosting in the light spots in the individual parts of the solid border is mainly caused by design reasons, that is, the ink roller cannot quickly replenish enough ink to meet the needs of solid printing. Due to the large amount of printing ink required for the solid frame, the lack of ink in the circumferential direction of the cylinder is prone to occur, resulting in the ink color of the solid frame printed along the circumferential direction being lighter than the frame printed along the axial direction.
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