At present, many small and medium-sized printing companies use monochrome offset printing machines to overprint color printing, simple printing and overprinting of two-color printing. Generally speaking, overprinting of single-color and four-color printing is the most common, but there are also printers printing over eight colors on both sides. This requires high-quality, high-precision printing presses, but also requires the printing operator more sophisticated technology, and in the process of overprinting should pay attention to:
First, 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, the paper needs to be flat, the cutting size must be the same, and the straightness and straightness errors are relatively small. For printed sheets, the error of the web size (width x length) shall not exceed ±1 mm. For printed covers and finer pictures, the error of the web size (width × length) shall not exceed ±0.5 mm, and the positioning Edge, the edge of the rule, the smoothness of the paper refers to the level of the paper surface smooth, smoothness determines the degree of contact between the paper and the blanket, apparently under the same printing pressure, printed with a low smoothness of the paper Product graphics are poorly defined, for example, in the same printing conditions, the use of coated paper is better than the use of coated paper; the moisture content of the paper is between 6% and 8%, but not too high. The surface tension and surface strength of the paper are reduced, plasticity is enhanced, and the imprinted ink film is cured. If the water content is too low, the paper is prone to static electricity and tends to be brittle, which is not conducive to printing. If the moisture content is not uniform, tight edges, scallops, curls, or unequal deformation will occur. After printing, pleats and other defects will occur. This requires the printing shop to have the appropriate temperature and humidity, the general requirements of the temperature control at 18 °C -25 °C, relative humidity maintained at 50% -60%, which are conducive to the preservation of the paper and printing the smooth progress. In addition, the paper's own fiber type and pH will also have a greater impact on printing.
Second, ink
The viscosity and fluidity of the ink play an important role in print quality. The fluidity and viscosity of the ink are closely related. The ink will increase its viscosity and at the same time reduce its fluidity. On the contrary, it will increase the fluidity. An ink with a large consistency does not necessarily have a large viscosity, and an ink with a large viscosity may not necessarily be thick, but only has the same viscosity. Ink, the greater the consistency, the worse the fluidity. If the fluidity is too small, the transfer of the ink during the printing process is not smooth and uneven, resulting in a shallow front and back depth on the same printing surface and other ink failures; the fluidity is too large, the imprinting cannot be transferred accurately, and the graphic level is not clear. Ink is not full, the color is not bright, at the same time, the ink flow is too large can also lead to the production of dirty plates and ink bars. In the printing process, the viscosity and fluidity of the ink can be adjusted by using ink transfer, but it is not recommended to add corn powder to the ink in order to make the print dry quickly. It can also be achieved by using fast-drying inks. Therefore, the viscosity and fluidity of the ink must be adjusted during the printing process. The preservation of ink has always been a headache for printing companies. Because the ink is left in the air for a little longer, it is easily oxidized by the air and forms a thick layer of ink. When the ink is taken for the next printing, it must be Scratching the ink, it is necessary to waste part of the ink. The general measure taken by the printing factory is to add a certain amount of water to the ink cartridge to prevent the ink from contacting the air. However, in fact, the oxygen contained in the water also oxidizes the ink, and the contact time between the water and the ink is easy to emulsify the ink. When printing on the machine, there is a phenomenon that no ink is applied. Although the amount of ink is large, it is uniform. Ink rollers (mainly hard rollers) are still not inked. It is advisable to use wax paper to isolate ink from air when it is stored, or to seal it with plastic paper. That is, wax paper or plastic paper is applied to the surface of the ink layer, and the wax paper or plastic paper can be peeled off before the next printing, but it is best. Do not use plain paper for storage. Avoid tearing the paper and causing trouble.
Third, fountain solution
The function of the fountain solution is to wet the blank part of the printing plate and form a layer of metal oxide or hydrophilic salt. Using the principle of ink repulsion, a barrier is established between the blank part and the ink so that the ink does not adhere to the blank of the printing plate. section. General requirements The dampening solution must be a weak acid medium. It is advisable to adjust the pH of the dampening solution between 5.0-6.0. PH value is too low, that is, the dampener solution is too acidic to cause serious corrosion of the printing plate, and the ink dries slowly. If the pH value is too high, the diazo compound in the graphic part of the photo-decomposition type PS is dissolved and the printing plate image is incomplete. Cause ink emulsification, so that the quality of the print product is seriously reduced. At present, the wetting methods adopted by small offset printing presses are mainly water fleece-set printing plate wetting devices, ink-jet Dachrun moistening mechanism and Kunpak direct water supply device, and they must be based on different modes of dampening in the printing process. The control of the amount of water, no matter what kind of water supply method, the amount of water is too large in the printed matter appears on the water, the small outlets are easily washed away by water, and the printing paper absorbs water swelling deformation, causing overprint trouble. If the amount of water is too small, it will obviously be dirty, but care should be taken to avoid water and large ink. The amount of water can be determined according to the area and distribution of the layout part, the type of paper used, the speed of the machine, and the surrounding environment. The ink-jet offset of offset printing has only dynamic relative balance, and there is no ideal absolute balance. Mastering ink-water balance is also an important aspect. In the printing process, operators must constantly observe the effects of printing to ensure the printing quality.
Fourth, the test machine is good, overprint accuracy
Before printing, we must check whether the printing press can accurately register. After printing 10 to 20 prints, we will test the world and the rules. If the rules are not on the paper (the paper is slightly farther away from the rule line), a ruled line can be manually drawn on the plate corresponding to the edge of the paper, and the ink can be coated on the rule line and then printed to check the overprint. It is also possible to print the printer once more and check whether the two can be overprinted together exactly after printing.
In 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, so that the pull gauge can pull the paper to the required position, and The paper will not be pulled too far when the paper is pulled too long. Furthermore, the pressure of the pull gauge pressure spring should be moderate, and a thick compression spring should be used when printing thick paper. When printing thin paper, a thin compression spring should be used, and the spring force of the spring should be moderated by the spiral adjustment on the spring. The excessive pressure of the pressure spring leads to a long time for pulling the paper, it is easy to pull the paper to deform, and it affects the action of the pull gauge and other mechanisms. If the pressure is too low, the paper is not easily pulled to the side shutter. Obviously, overprinting will not occur. Only properly set the distance between the paper and the side cover of the pull gauge, and the spring pressure is appropriate, in order to ensure proper pull force and accurate overprinting. The use of small offset printing rules should pay attention to the distance between the paper and the regular baffle plate, and push the paper on the other side of the reed has a certain amount of compression.
Fifth, the paper position
In the printing process, generally speaking, a small offset press requires the left and right position of the paper, that is, the position of the left and right position of the pull gauge is not very strict, but under certain circumstances, whether the overprint of the print has a great relationship with the position of the paper. Since small offset presses have their own particularities, unlike large offset presses, the continuous paper feed method used in large-scale four-up or more format presses is usually direct feed or indirect feed, while small offset presses generally use transcendental feeds. When the paper is fed, the paper continues to be fed to the impression cylinder by the friction force between the two upper and lower transfer rollers, the paper is positioned once at the front gauge, and the secondary positioning is performed at the impression cylinder. Because the position of the paper is not good, overprinting is not allowed in two situations. One is that after the paper reaches the front gauge, the front end of the paper just touches the side end surface of a front gauge hook during pulling the paper. In this way, the paper is an unstable movement in the process of being pulled, or the paper is not pulled into position due to the collision with the front gauge side end face, resulting in inaccurate overprinting. In another case, the front side of the paper does not touch the face of the front face, but in the secondary positioning, the edge of the paper just touches the face of the small stop on the impression cylinder, and in this case also There may be inaccurate overprinting caused by poor paper position. Therefore, in the offset printing machine overprinting should also pay attention to the left and right position of the paper on the overprint caused by the impact.
Sixth, reasonable arrangement of printing color sequence
In general, the four-color machine printing color sequence is a reasonable arrangement: black, blue, red, yellow, but also according to the order of color yellow, red, blue, black printing. When the four-color printing machine prints, the printing interval between color and color is short, the ink does not have drying time, it is a superposition and printing mode of warm and wet, and the printing interval between color and color is longer when the monochrome machine is printing, and printing is second. In the case of color, the first color is already dry, so it is a wet-stacked overprinting method. Due to different overprinting methods, the color arrangement is also different. There are many color sequential arrangements in a monochrome machine, but the specific choice of color sequence can be considered from various aspects as follows:
1. According to the brightness of the three primary colors to arrange the color sequence must first print dark, bright color after printing, that is, blue, black and India, red, yellow after India.
2. According to the three primary colors ink transparency and hiding power arranged color sequence, poor transparency of the blue, black print, good transparency, yellow and red ink after printing.
3. According to the size of the text area to arrange the color sequence is to print a small picture, after printing full version or large format graphics.
4. According to the characteristics of the manuscript, a color picture based on warm colors should be printed with blue or black first, while a color picture with cold tone should be printed with red or yellow first.
5. Considering overprinting, printing is more difficult to print. In general, landscapes are printed in blue and yellow, while portraits are printed in red and yellow.
The printing color sequence recommended when printing four colors using a monochrome machine is: blue, red, yellow, and black. In this way, as long as the first two colors are overprinted accurately and the color tone is good, the two-color overprint behind them will not be too inconvenient. obvious. The operator should pay attention when changing the ink. In order to maintain the purity of the ink, it is best to add a bit of the next color ink to be printed soon after the cleaning of the natural ink is completed or completed, and then clean it again. Especially when dark ink is used for light color ink, it is better to clean the machine twice.
Seventh, other
In the process of overprinting, if it is not a full-print version of the printed matter in the field or more color, it is best not to use a dusting device, although the powder can make the ink dry faster, to prevent the back of the printed material from being dirty, but the powder is not well mastered. Directly affect the print finish and brightness, generally in the last color printing when using powder, and then in the four-color overprint process, it is best to use the same speed printing, do not change the printing speed. Because the speed change will more or less affect the overprint accuracy of the print. In addition, if the accuracy of color printing is very demanding, conditional printers can also run the water once before the official printing, that is, install the printing plate, fold down the printing roller, or make the ink supply system not supply ink, and only supply water. The “printing†is not performed for the ink, so that overprinting due to paper deformation can be reduced.
In summary, the monochrome offset press overprints color prints, both for the machine and for the operator. Only by mastering the overprinting technology, can we print fine and high-quality printed matter under the conditions of sophisticated equipment and printing consumables.