Like any four-color printing process, print registration is a key issue in four-color intaglio printing. To obtain accurate overprint quality, it is very important to fully grasp and control every factor that has a great influence on the pad printing process.
The variable factors in the gravure printing process include the color separation quality, the fit tolerance of the printing machine, the ink properties, the characteristics of the print head, the corrosion and positioning of the printing plate, and the like.
Pad printing machine fit tolerance
Gravure pad printing is a thin ink replication process. The thickness of a single ink layer after drying is approximately 5 Î¼m. Because the print head is able to transfer such a thin layer of ink onto the surface of the substrate, a gravure printing process is ideal for those substrates that suffer from problems due to over-printing of the ink layer when using four-color printing. Process.
For monochrome intaglio pad printing, the quality of the pad printer is not a critical issue. If color (four-color) pad printing is performed, the accuracy and stability of gravure printing machines are very important.
Nowadays, there are two types of overprinting methods commonly used by printing plants in color printing machines. One is that after the printing is completed, the printing machine automatically positions the substrate, and the printing head moves to the next color plate, ready to overprint the next color. There is a method of overprinting by a pad printing machine in which the substrate is fixed and moved accurately by moving the pad between the substrate and the pad printing intaglio printing plate. Positioning of the pad printing pad and the pad printing head determines four-color registration. Accuracy.
Production of plate-making film
Although the color gravure pad printing does not require high registration accuracy, it should also perform color separation operations in strict accordance with the manuscript. Examination of the color separation film should include examination of the number of screen lines, dot shapes, and the tone range of the positive picture.
In the color gravure printing process, the color separation film can have a maximum of 200 lines/inch. The number of recommended screens for color is 150 lines/inch. This number of screens can already reproduce high-resolution images, and the variables that need to be controlled in the process are not as demanding.
In the selection of dot shapes, elliptical dots can be used. Compared with circular dots, the dots in the middle of the image can be restored better. We have also experimented with the use of FM outlets (random outlets). The printing effect is very good. More importantly, with the use of FM network points, the accuracy of the magnetic alignment can be appropriately reduced.
Color gravure printing The color image of the color separation sheet has a 5% to 95% positive image. Since printing substrates often have no cutting area, it is not possible to place sets of standard notes, gray scales, color marks, etc. on the film as control strips for measuring register accuracy, dot gain, and tone reduction. However, individual prints can be used to measure the print characteristics under specific conditions, and some adjustments can be made before printing to compensate for the changing factors that appear on the press.
Ink performance has a great influence on four-color intaglio pad printing. First, consider the weight ratio of the pigment in the ink. The yellow, magenta, cyan, and black inks should have a uniform pigment density, and secondly, the weight ratio of solvents in the ink should be considered.
Because the solvent in gravure printing inks must have a specific weight ratio to ensure proper ink transfer from the print head to the print surface, it is critical to maintain the stability of the solvent weight ratio. If the pad printer uses a split ink tank for ink supply, a certain amount of solvent may be added periodically to ensure that the ink solvent content meets the printing requirements.
If the pad printer uses a closed ink tank, the weight ratio of the solvent controlling the ink is very simple. However, after a period of use, the ink must be replaced or adjusted. Because the ink consumption of the pad printer is relatively small, it is advisable to replace the ink every day, both in closed and split ink tanks.
Printing head performance
Use four-color printing with the same shape of the print head as much as possible. Because of the change in the shape of the pad, it may cause the image to be distorted. In addition, it is also necessary to ensure that the hardness of the print head is uniform. When ordering the print head, it is necessary to ensure that the print head is produced on the same batch of production lines. In order to extend the life of the print head, the print head should be maintained.
Plate orientation and corrosion accuracy
During gravure printing operation, the printing plate is the most problematic place in the pad printing process. Different multi-color pad printing machines have different plate making methods.
1) The four-color separation plate uses a plate base to etch the plates at the same time. Pad printers using this method offer a very limited range of adjustments, meaning that images on plates must be accurately etched and precisely positioned.
2) Four-color separation plate is plated separately. Each plate corresponds to a printing ink. Printing presses using this type of printing plate provide flexibility for printing register adjustment of four-color inks.
When debugging the printer, do not ignore the positioning accuracy adjustment of the plate. With no registration marks, overprinting accuracy becomes difficult, but it can be overcome. The method is to select three reference positions in the design draft. The color of the figures on these positions should have a certain representativeness as the overprint reference. The ideal printing effect is when the overprinting is accurate, the reference point edge is clear or the geometry is consistent, if the overprinting is not clear at a glance. As with other four-color prints, overprinting accurate prints sometimes has the visual effect of rose spots. There is no problem with the hue of printing and graphic arts, but overprinting is not allowed.
Although the depth of corrosion of the printing plate does not affect the overlay accuracy, the depth of corrosion of the printing plate has a great influence on the color accuracy of the printed matter. Corrosion plates generally use the following two methods. One method can control the depth of corrosion of each plate to be the same depth, and then adjust the brightness of the color by changing the ink supply; the other way is for each color plate. Adjust the depth of corrosion independently to achieve the desired color brightness. In the actual production, the first method is generally adopted because it is relatively easy to adjust the ink. However, with this method, the ink preparation requirements are very precise, and it is necessary to accurately record the amount of ink transferred each time.
Source: Printing Technology
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