At present, there are many small and medium-sized printing companies that use monochromatic offset presses to overprint color jobs. Simple printing products are overprinted in two colors. Generally speaking, overprinting a single side with four colors is common, but some printing companies overprint a total of eight colors. This requires high-quality and high-precision printing presses, but also requires more exquisite skills of printing operators, and should pay attention to in the process of overprinting:
1. Paper
Paper is one of the six elements in the printing process. The quality of paper directly affects the speed of printing and the quality of printed matter. First of all, the paper should be flat, the cutting size should be consistent, and the errors of straight angle and straightness are relatively small. For single sheets of paper used for printing books and periodicals, the format size (width × length) error does not exceed ± 1 mm. For printed covers and more elaborate albums, the format size (width × length) error should not exceed ± 0.5 mm, and the positioning is indicated. Edge, that is, regular edge, the smoothness of the paper refers to the degree of smoothness of the surface of the paper, the smoothness determines the closeness of the paper and the blanket contact, obviously under the same printing pressure, printed on the paper with low smoothness The image definition of the product is poor, for example, under the same printing conditions, the use of coated paper is better than the use of coated pattern paper; the paper moisture content is required to be between 6% and 8%, and the content is not too high. The surface tension and surface strength of the paper are reduced, the plasticity is enhanced, and the curing of the imprinted ink film is delayed. If the water content is too low, the paper is prone to static electricity, and tends to be brittle and hard, which is not conducive to printing. Uneven moisture content will result in uneven deformation of tight edges, ruffles, curling or unevenness, and folds and other failures will occur after printing. This requires a suitable temperature and humidity in the printing workshop. Generally, the temperature is controlled at 18 ℃ -25 ℃, and the relative humidity is maintained at 50% -60%, which is conducive to the preservation of paper and the smooth progress of printing. In addition, the fiber type and pH of the paper itself will have a greater impact on printing.
2. Ink
The viscosity and fluidity of the ink play an important role in printing quality. The fluidity and viscosity of the ink have a close relationship. The increase of the viscosity of the ink will reduce its fluidity, otherwise the fluidity will be improved. The thick ink does not mean that the viscosity is high, and the ink with high viscosity is not necessarily thick, only the same viscosity Ink, the greater the consistency, the worse the fluidity. If the fluidity is too small, the transfer of ink is not smooth and uneven during the printing process, resulting in shallow front and back depth and other ink failures on the same printing surface; the fluidity is too large, the imprint cannot be accurately transferred, and the picture and text levels are not clear, The ink color is not full, the color is not bright and colorful, at the same time, the fluidity of the ink is too large will also lead to frequent dirty plates and ink bars. In the printing process, the viscosity and fluidity of the ink can be adjusted by using ink adjustment, but avoid adding corn flour to the ink to make the printed matter dry quickly. It can also be achieved by using quick-drying inks. Therefore, in the printing process, it is necessary to adjust the viscosity and fluidity of the ink. The preservation of ink has always been a headache for printing plants, because the ink is placed in the air for a long time, it is easily oxidized by the air and forms a thick layer of ink skin. The ink skin is scraped off, which inevitably wastes part of the ink. The measures generally adopted by printing plants are to add a certain amount of water to the ink cartridge to avoid the ink from contacting the air. But in fact, the oxygen contained in the water will also oxidize the ink, and it is easy to emulsify the ink when the water is in contact with the ink for a long time. When printing on the machine, there will be no ink, although the amount of ink is quite large, but the The ink roller (mainly the hard roller) is still not inking. It is recommended to use wax paper to isolate the ink from contacting the air when storing the ink, or to seal it with plastic paper, that is, stick the wax paper or plastic paper to the surface of the ink layer, and remove the wax paper or plastic paper before the next printing, but it is best Do not use plain paper for sealing, to avoid the trouble of tearing the paper when uncovering the paper.
3. Fountain solution
The function of the fountain solution is to wet the blank part of the printing plate and form a metal oxide or hydrophilic salt layer. Using the principle of ink repellence, a barrier is established between the blank part and the ink to make the ink not stick to the blank of the printing plate. section. Generally, the fountain solution must be a weak acid medium, and the PH value of the fountain solution should be adjusted between 5.0-6.0. If the PH value is too low, that is, the acidity of the fountain solution will cause serious corrosion of the base, and the ink will dry slowly; too high will dissolve the diazo compound in the graphic part of the photodegradable PS plate, resulting in the incompleteness of the plate image. Causes emulsification of the ink, resulting in a serious decline in print quality. At present, the wetting methods used in small offset presses are mainly water-wet sleeve-based dampening devices, Dagren wetting mechanism under water and ink, and Kunpak direct water supply device. According to different dampening methods in the printing process While controlling the amount of water, no matter what kind of water supply method, too much water will appear on the printed matter, small dots are easily washed away by the water, and the printing paper absorbs water and expands and deforms, causing trouble for overprinting. If the amount of water is too small, it will obviously dirty the version, but care should be taken to avoid the phenomenon of water and ink. The amount of water can be determined according to the area and distribution of the layout, the type of paper used, the speed of the machine and the surrounding environment. The water-ink balance of offset printing is only a dynamic relative balance, and there is no ideal absolute balance. It is also an important aspect to master the water-ink balance. During the printing process, the operator must always observe the printing effect to ensure the printing quality.
4. The inspection machine is good and the overprint is accurate
Before printing, it is necessary to check whether the printing machine can overprint accurately. After printing 10-20 printed products, check the world, the left and right rule lines separately. If the rule is not on the paper (the smaller rule line is farther away from the paper), you can manually draw a rule line on the layout corresponding to the edge of the paper, and apply some ink on the rule line and then print to check the overprint ; You can also print the printer again and check whether it can be accurately overprinted after two printings.
During the printing process, we must first ensure that the paper is stable, the paper can reach the front gauge in parallel and flat, and the distance between the paper and the side fence of the pull gauge is required to be 3mm-5mm, to ensure that the pull gauge can pull the paper to the desired position, and Do not pull the paper over the head for too long. Furthermore, the pressure of the tension gauge compression spring should be moderate. When printing thick paper, use a thick compression spring. When printing thin paper, use a thin compression spring, and the spring force of the spring should be adjusted by the spiral on it. Excessive pressure of the compression spring will cause the paper to be pulled for too long, it is easy to deform the paper, and it will affect the coordination of the pull gauge with the actions of other mechanisms. If the pressure is too low, the paper will not be easily pulled to the side fence, and it is obvious that the left and right overprints will be inaccurate. Only when the distance between the paper and the side fence of the pull gauge is set correctly, and the spring pressure is appropriate, can the paper pulling force be proper and the overprint be accurate. When using the small offset printing of the push gauge, you should pay attention to the proper distance between the paper and the regulation surface baffle, and the other side of the reed has a certain amount of compression after the paper is pushed.
5. Paper position
During the printing process, generally speaking, the left and right position requirements of the small offset press on the paper, that is, the left and right position requirements of the pull gauge are not very strict, but in some cases, whether the printed matter overprint is accurate depends on the position of the paper. Due to its special characteristics, small offset printing presses are different from large offset printing presses. The printing methods used by large-format printing machines of more than four format are usually direct or indirect, while small offset printing machines generally use more than continuous paper. During paper feeding, the continuous entry of the paper is handed over to the impression cylinder by the friction force between the upper and lower paper feed rollers. The paper is positioned once at the front gauge and is positioned twice at the impression cylinder. There are two types of overprinting that are not allowed due to the poor position of the paper: One is that after the paper reaches the front gauge, during the process of pulling the paper, the front end of the paper exactly touches the side end surface of a front gauge hook In this way, the paper is an unstable movement during the process of being pulled, or the paper is not pulled into place because it hits the front end of the front side, resulting in inaccurate overprinting. Another situation is that the paper does not touch the front end of the front side at the front, but during the second positioning, the edge of the paper just touches the end of the small stop on the impression cylinder. Similarly, in this case, too There will be inaccurate overprinting caused by poor paper position. Therefore, we must pay attention to the influence of the left and right position of the paper on the overprint in the small offset press.
6. Reasonable arrangement of printing color sequence
The reasonable arrangement of the color sequence of the general four-color machine printing is: black, blue, red, yellow, and some are printed in the reverse color sequence of yellow, red, blue, and black. When printing with a four-color machine, the printing interval between colors is short, and the ink has no drying time, which is a superimposed method of warm stacking and wet printing. When printing with a single-color machine, the printing interval between colors is longer, and the second printing At the time of coloring, the first color is already dried, so it belongs to the wet-overprinting method. Because of the different overprinting methods, the color sequence arrangement is also different. There are many color sequence arrangements for monochrome machines, but the specific choice of color sequence can be considered from the following aspects: 1. To arrange the color sequence according to the brightness of the three primary colors, dark colors should be printed first, and then bright colors, that is, blue and black first. Sealed, red and yellow backprint. 2. Arrange the color sequence according to the transparency and hiding power of the three primary colors ink, blue and black with poor transparency are printed first, and yellow and red ink with good transparency are printed after. 3. To arrange the color sequence according to the size of the graphic area, print the small image first, then print the full version or large format image. 4. According to the characteristics of the original, the color pictures with warm colors should be printed with blue or black first, and the color pictures with cold colors should be printed with red or yellow first. 5. Considered by overprinting, print the color with more difficult overprinting first. Generally, landscapes are printed in the order of blue and yellow, while person images are printed in the order of red and yellow. The recommended color sequence when printing four colors using a monochrome machine is: blue, red, yellow, black obvious. Operators should pay attention when changing ink. In order to maintain the purity of the ink color, it is best to add a little ink of the next color to be printed and clean it again when the cleaning of the original color ink is about to be completed or when it is completed. Especially when dark ink is changed to light ink, it is best to clean the machine twice.
7. Other
In the process of overprinting, if you are not printing full-page solids or prints with more color blocks, it is best not to use a powder spray device. Although the powder spray can make the ink dry quickly and prevent the back of the print from being dirty, the powder spraying is not good. It directly affects the finish and brightness of the printed matter. Generally, powder spraying is used in the last color printing, and then in the four-color overprinting process, it is best to use the same speed printing, do not change the printing speed. Because the change of speed will more or less affect the overprint accuracy of the printed matter. In addition, if the accuracy requirements of color printing are very high, the qualified printing house can also run the water before the official printing, that is, install the printing plate, fold down the plate ink roller or make the ink supply system not supply ink, and only supply water "Printing" without ink supply can reduce the phenomenon of overprinting caused by paper deformation.
To sum up, the color printing of monochromatic offset presses has high requirements for both machines and operators. Only by mastering the overprinting technology, and with the excellent equipment and printing consumables, can we print fine and high-quality prints.
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