At present, many small and medium-sized printing companies use monochrome offset printing machines to overprint color printing, simple printing and overprinting of two colors. Generally speaking, overprinting of single-color and four-colors is the most common, but there are also printing factories overprinting the front and back of a total of eight colors. This requires high-quality, high-precision printing presses, but also requires the printing operator to have a 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) does not exceed ±1 mm. For printed covers and finer pictures, the error of the web size (width × length) does not exceed ±0.5 mm, and Mark the positioning edge, which is the rule edge; the paper smoothness refers to the level of the paper surface is smooth, smoothness determines the degree of closeness between the paper and the rubber touch, obviously under the same printing pressure, printing with low smoothness paper The resolution of printed artwork is poor. For example, under the same printing conditions, the use of coated paper is better than the use of coated paper; the water content of the paper is between 6% and 8%, and the water content is too high. When the height is high, the surface tension and surface strength of the paper are reduced, the 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, flounces, curls, or unequal deformation will occur. After printing, defects such as pleats 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 viscosity of the ink increases the viscosity of the ink and decreases the flowability of the ink. Conversely, the ink increases the fluidity. An ink with a large consistency does not equal to a large viscosity. Only the ink with the same viscosity has a higher consistency and a more fluidity. difference. If the fluidity is too small, the ink transfer 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 supply failures. The fluidity is too large, the imprinting cannot be accurately transferred, 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. During the printing process, the viscosity and fluidity of the ink can be adjusted by using a varnish, but it is not advisable to add corn flour to the ink in order to allow the print to dry. It can also be achieved by using fast-drying inks. Therefore, in the printing process, the ink viscosity and fluidity should be adjusted. The preservation of ink has always been a headache for printing companies. Because the ink rotates 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, oxygen contained in water also oxidizes the ink, and the contact time between water and ink is easy to emulsify the ink. When printing on the machine, there will be no inking phenomenon. Although the amount of ink is large, but uniform Ink rollers (mainly hard rollers) are still not inked. It is advisable to use wax paper to isolate the ink from the 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. 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. By using the principle of ink repulsion, a barrier is established between the blank part and the ink so that the ink is not stained and the printing plate is blank. section. General requirements Fountain solution PH value between 5.0-6.0 is appropriate. 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, small outlets are easily washed away by water, and the printed 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 based on the area and distribution of the layout text, 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 rule line is not on the paper (the paper is slightly farther away from the rule line), you can manually draw a ruled line on the plate corresponding to the edge of the paper, and apply a little ink on the rule line and then print to check the overprint; It is also possible to print the print once more and check whether it can be accurately overlaid after printing twice.
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, to ensure 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, to ensure that the paper pull force is proper and overprinting is accurate. The use of small offset printing rules should pay attention to the distance between the paper and the regulatory panel baffle, 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 is accurately related to the position of the paper. Since small offset presses have their own particularities, different from large offset presses, the continuous feed used by large-scale presses with four or more web formats is usually direct or indirect. In contrast, small offset presses generally use transcendent paper feed. In the paper, the renewal of the paper is given to the impression cylinder by the friction 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, during the process of pulling the paper by the pull rule, the front ends of the two front-side hooks of the paper just touch each other. In this way, the paper is an unstable movement in the process of being pulled, or the paper is not pulled into place due to the collision with the front gauge side end surface, resulting in inaccurate overprinting. Another situation is that the paper does not touch the front face of the front gauge, but in the second positioning, the edge of the paper just touches the small end surface of 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 printer prints, the printing interval between color and color is short, and the ink does not have drying time, which is a wet and superimposed overprinting mode, and the printing interval between color and color is longer in the monochrome machine 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 the different overprinting methods, the color sequence 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 and India. 2. According to the three primary colors ink transparency and hiding power arrangement 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. For overprinting, the overprinting of the most difficult colors is first done. 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 must pay attention when changing the ink. To maintain the purity of the ink, it is best to add a little of the next color ink to be printed, and then wash it again when the cleaning ink is about to be completed or completed. 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 use the powder in the last color printing. In the process of four-color overprinting, it is better to print at the same speed and not to 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 the overprinting due to paper deformation can be reduced.
In summary, the monochrome offset press overprints color prints, both for the machines and for the operators. Only by mastering the overprinting technology, can we print fine and high-quality printed products under the conditions of sophisticated equipment and printing consumables
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) does not exceed ±1 mm. For printed covers and finer pictures, the error of the web size (width × length) does not exceed ±0.5 mm, and Mark the positioning edge, which is the rule edge; the paper smoothness refers to the level of the paper surface is smooth, smoothness determines the degree of closeness between the paper and the rubber touch, obviously under the same printing pressure, printing with low smoothness paper The resolution of printed artwork is poor. For example, under the same printing conditions, the use of coated paper is better than the use of coated paper; the water content of the paper is between 6% and 8%, and the water content is too high. When the height is high, the surface tension and surface strength of the paper are reduced, the 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, flounces, curls, or unequal deformation will occur. After printing, defects such as pleats 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 viscosity of the ink increases the viscosity of the ink and decreases the flowability of the ink. Conversely, the ink increases the fluidity. An ink with a large consistency does not equal to a large viscosity. Only the ink with the same viscosity has a higher consistency and a more fluidity. difference. If the fluidity is too small, the ink transfer 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 supply failures. The fluidity is too large, the imprinting cannot be accurately transferred, 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. During the printing process, the viscosity and fluidity of the ink can be adjusted by using a varnish, but it is not advisable to add corn flour to the ink in order to allow the print to dry. It can also be achieved by using fast-drying inks. Therefore, in the printing process, the ink viscosity and fluidity should be adjusted. The preservation of ink has always been a headache for printing companies. Because the ink rotates 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, oxygen contained in water also oxidizes the ink, and the contact time between water and ink is easy to emulsify the ink. When printing on the machine, there will be no inking phenomenon. Although the amount of ink is large, but uniform Ink rollers (mainly hard rollers) are still not inked. It is advisable to use wax paper to isolate the ink from the 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. 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. By using the principle of ink repulsion, a barrier is established between the blank part and the ink so that the ink is not stained and the printing plate is blank. section. General requirements Fountain solution PH value between 5.0-6.0 is appropriate. 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, small outlets are easily washed away by water, and the printed 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 based on the area and distribution of the layout text, 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 rule line is not on the paper (the paper is slightly farther away from the rule line), you can manually draw a ruled line on the plate corresponding to the edge of the paper, and apply a little ink on the rule line and then print to check the overprint; It is also possible to print the print once more and check whether it can be accurately overlaid after printing twice.
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, to ensure 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, to ensure that the paper pull force is proper and overprinting is accurate. The use of small offset printing rules should pay attention to the distance between the paper and the regulatory panel baffle, 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 is accurately related to the position of the paper. Since small offset presses have their own particularities, different from large offset presses, the continuous feed used by large-scale presses with four or more web formats is usually direct or indirect. In contrast, small offset presses generally use transcendent paper feed. In the paper, the renewal of the paper is given to the impression cylinder by the friction 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, during the process of pulling the paper by the pull rule, the front ends of the two front-side hooks of the paper just touch each other. In this way, the paper is an unstable movement in the process of being pulled, or the paper is not pulled into place due to the collision with the front gauge side end surface, resulting in inaccurate overprinting. Another situation is that the paper does not touch the front face of the front gauge, but in the second positioning, the edge of the paper just touches the small end surface of 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 printer prints, the printing interval between color and color is short, and the ink does not have drying time, which is a wet and superimposed overprinting mode, and the printing interval between color and color is longer in the monochrome machine 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 the different overprinting methods, the color sequence 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 and India. 2. According to the three primary colors ink transparency and hiding power arrangement 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. For overprinting, the overprinting of the most difficult colors is first done. 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 must pay attention when changing the ink. To maintain the purity of the ink, it is best to add a little of the next color ink to be printed, and then wash it again when the cleaning ink is about to be completed or completed. 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 use the powder in the last color printing. In the process of four-color overprinting, it is better to print at the same speed and not to 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 the overprinting due to paper deformation can be reduced.
In summary, the monochrome offset press overprints color prints, both for the machines and for the operators. Only by mastering the overprinting technology, can we print fine and high-quality printed products under the conditions of sophisticated equipment and printing consumables
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