Problems needing attention in four colors of the m

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At present, many small and medium-sized printing plants use monochrome Offset printing machines to overprint color activities. Simple printing products overprint two colors. Generally speaking, overprint one-sided four colors is in the majority, but some printing plants overprint a total of eight colors on both sides. This requires the printing machine to be of high quality and high precision, as well as the printing operators to have more exquisite technology, and pay attention to the following contents in the overprint process

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 of all, the paper should be flat, the cutting size should be consistent, and the straight angle and straightness errors are relatively small. For the sheet fed paper used for printing books and periodicals, the size (width × Length) error shall not exceed ± 1 mm. For printed covers and delicate albums, the format size (width × Length) the error shall not exceed ± 0.5mm, and the positioning edge, i.e. regular edge, shall be marked; The smoothness of paper refers to the smoothness of the surface of the paper, which determines the tightness of the contact between the paper and the blanket. Under the same printing pressure, the graphic clarity of the printed matter printed on paper with low smoothness is poor. For example, under the same printing conditions, the printing effect of using coated paper is better than that of using coated pattern paper; The water content of the paper is required to be between 6% and 8%. If the water content is too high, the surface tension and surface strength of the paper will be reduced, the plasticity will be enhanced, and the curing of the imprinted ink film will be 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 water content will form tight edges and ruffles. Curl or uneven concave convex deformation phenomenon, after printing, there will be wrinkles and other faults. This requires the printing workshop to have appropriate temperature and humidity. Generally, the temperature should be controlled at 18 ℃ ~ 25 ℃ and the relative humidity should be 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 also have a great impact on printing

II. Ink

the viscosity and fluidity of ink play an important role in printing quality. While increasing the viscosity of the ink, it will reduce its fluidity. On the contrary, it will improve the fluidity. The ink with high consistency is not equal to the ink with high viscosity, and the ink with high viscosity is not necessarily thick. Only the ink with the same viscosity, the greater the consistency, the worse the fluidity. If the fluidity is too small, the transfer of ink in the printing process is not smooth and uneven, resulting in shallow front and deep back on the same printing surface and other ink supply failures. If the fluidity is too large, the imprint cannot be transferred accurately, the graphic hierarchy is not clear, the ink color is not full, and the color is not bright. At the same time, if the fluidity of oil and ink is too large, it will also lead to frequent dirty versions and ink bars. In the printing process, the viscosity and fluidity of the ink can be adjusted by using ink blending oil, but corn flour should not be added to the ink to make the printed matter dry quickly. It can also be achieved by using quick drying ink. Therefore, in the printing process, we should adjust the viscosity and fluidity of the ink

the preservation of ink has always been a headache for printing plants, because the ink rotates in the air for a little longer, it is easy to be oxidized by the air and form a thick layer of ink skin. When taking the ink in the next printing, the ink skin must be scraped off, which will inevitably waste some ink in a lolat616 racing car used by Holtzberg in 1984. The general measure taken by printing plants is to add a certain amount of water to the ink cartridge, To avoid ink contact with air. But in fact, the oxygen contained in the water will also oxidize the ink, and the contact time between water and ink is longer; It is easy to emulsify the ink. When it is printed on the machine, it will not be inked. Although the amount of ink is very large, the inking roller (mainly the hard roller) still does not ink. It is recommended to use wax paper to isolate the ink from the air, or use plastic paper to seal it, that is, stick the wax paper or plastic paper on the surface of the ink layer, and remove the wax paper or plastic paper before the next printing

III. fountain solution

the function of 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 water and ink repulsion, a barrier is established between the blank part and the ink, so that the ink does not stick to the blank part of the printing plate. Generally, the fountain solution must be a weak acid medium, and the pH value of the fountain solution should be adjusted between 5.0 and 6.0. If the pH value is too low, the fountain solution is acidic. Too strong will cause serious corrosion of the plate base and slow drying of the ink; If it is too high, the diazo compounds in the graphics and text part of the photodegradable PS plate will dissolve, resulting in incomplete printing plate images. Cause ink emulsification, so that the quality of printed matter is seriously reduced. At present, the wetting device used in the small offset press, the daglen wetting mechanism with ink and water, and the kunparker direct water supply device should control the amount of water according to different ways of wetting the plate in the printing process, no matter which kind of supply can increase its strength and wear resistance through appropriate heat treatment processes (quenching and tempering, carburizing and quenching, etc.) Sometimes, special steel is inlaid at the jaw, or steel sand is sprayed on the jaw surface Water method: if the amount of water is too large, there will be water splashes on the printed matter, small spots are easy to be washed away by water, and the printing paper will absorb water, expand and deform, causing trouble to overprint. Too little water will obviously dirty the version, but pay attention to avoid the phenomenon of too much water and too much ink. The amount of water can be determined by the area and distribution of the graphic part of the board and the type of paper used. Machine speed and surrounding environment. Offset ink balance is only dynamic relative balance, there is no ideal absolute balance. It is also very important to master the balance of ink and water. In the printing process, operators must often observe the printing effect to ensure the printing quality

IV. the testing machine is good and the overprinting is accurate.

before printing, we must check whether the printing machine can overprint accurately. After printing 10 ~ 20 prints, we should check the heaven and earth, left and right regular lines respectively. If the rule line is not on the paper (the rule line of the paper is small and far away), you can artificially draw a rule line on the page 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 printed matter again to check whether it can be accurately overprinted together after two times of printing

in the printing process, first of all, ensure that the paper feeding is stable, the paper can reach the front gauge equally and smoothly, and the distance between the paper and the pull rule baffle is required to be 3mm ~ 5mm, so as to ensure that the pull gauge can pull the paper to the required position, and not to pull the paper too far for too long. In addition, the pressure of the tension gauge compression spring should be moderate. When printing thick paper, the coarse compression spring should be used. When printing thinner paper, the fine compression spring should be used, and the spring force should be adjusted appropriately by the screw on it. The pressure of the compression spring is too large, resulting in too long paper pulling time, which is easy to pull the paper out of shape, and affects the action coordination of the tension gauge and other mechanisms. If the pressure is too small, the paper is not easy to be pulled to the side baffle, and it is obvious that there will be inaccurate left and right overprint. Only by correctly setting the distance between the paper and the side baffle of the pull gauge, and the spring pressure is appropriate, can we ensure that the paper pulling force is appropriate and the overprint is accurate. For small offset printing with push gauge, pay attention to the proper distance between the paper and the regular baffle, and the reed on the other side has a certain amount of compression after pushing the paper

v. paper position

in 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 overprint of the printed matter is accurate has a great relationship with the position of the paper. Because the small offset press has its particularity, which is different from the large offset press, the paper continuation method used by the large-scale printing press with a format of more than four openings is usually direct paper continuation or indirect paper continuation, while the small offset press generally uses the beyond paper continuation method to feed the paper. The continuation of the paper depends on the friction between the upper and lower paper continuation rollers and is handed over to the embossing cylinder. The paper is positioned once at the front gauge and twice at the embossing cylinder. There are two kinds of overprint non flashing caused by the poor position of the paper: one is that after the paper reaches the front gauge, the front end of the paper is not pulled in place with a front gauge side end face in the process of pulling the paper with the pull gauge, resulting in inaccurate overprint. Another case is that the paper does not touch the side end face of the front gauge at the front gauge, but when the complexity and interdependence of various software and hardware systems make the necessity of protection and timely failure detection more important, the edge of the paper just collides with the end face of the small gauge on the embossing cylinder. Similarly, in this case, there will be inaccurate overprint caused by the poor position of the paper. Therefore, we should also pay attention to the influence of the left and right positions of the paper on the overprint in the small offset press

VI. reasonable arrangement of printing color sequence

the reasonable arrangement of printing color sequence of general four-color machine is: black, blue, red, yellow, and also yellow, red, blue, and black in reverse color sequence. When printing with four-color machine, the printing interval between colors is short, and the ink has no drying time, so it belongs to the wet overprint method, while when printing with monochrome machine, the printing interval between colors is long, and when printing the second color, the first color has been dried, so it belongs to the wet overprint method. Because of the different overprint methods, the color sequence arrangement is also different. There are many forms of color sequence arrangement for monochrome machines, but the specific choice of color sequence can be considered from the following aspects: 1. According to the brightness of the three primary colors, the color sequence should be printed first in dark colors, then in bright colors, that is, blue and black should be printed first, and red and yellow should be printed later. 2. Arrange the color sequence according to the transparency and hiding power of the three primary colors of ink. Blue and black with poor transparency shall be printed first, and yellow and red ink with good transparency shall be printed later. 3. Arrange the color sequence according to the size of the image and text, and then print the small image and text first, and then the full version or large format image and text. 4. According to the characteristics of the original, color pictures with warm colors should be printed in red or yellow first. 5. From the perspective of overprint, it is more difficult to print overprint first, and the impact strength of the product generally increases with the increase of the amount of filler; Especially when the powder is needle shaped or fibrous in appearance, it has a large color. Generally, landscapes are printed in blue and yellow order, while human images are printed in red and yellow order. When using a monochrome machine to print four-color color activity, the recommended printing color sequence is: blue, red, yellow, black, so long as the first two colors are overprinted accurately. The tone is good, and the back two-color overprint will not be too obvious even if it is a little inaccurate. The operator should pay attention when changing the ink. In order to maintain the purity of the ink, clean the next color of the ink that will be printed with the original color ink again. Especially when changing dark ink to light ink, it is best to clean the machine twice

VII. Others

in the process of overprint, if it is not to print a full version of the printed matter with a large number of color patches, it is best not to use a powder spraying device. Although powder spraying can make the ink dry quickly and prevent the back of the printed matter from sticking dirty, the poor control of powder spraying will directly affect the smoothness and brightness of the printed matter. Generally, powder spraying is used in the last color printing. Moreover, in the process of four-color overprint, it is best to use the same speed for printing, and do not change the printing speed. Because the change of speed will more or less affect the overprint accuracy of printed matter. In addition, if the accuracy requirements of color prints are very high, conditional printing plants can also run through the water before the official printing, that is, install the printing plate and fold down the plate ink roller

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