In modern China's printing industry, commercial rotary printing is emerging. The launch of commercial rotary machines actually raises commercial printing to a higher level. It adapts to the requirements of the modern printing market with shorter production cycles, large print batches, and high quality, speed, and efficiency. With the standardization of working environment and machine operation, in rotary printing, the quality control of pre-press production plays an important role. If the preliminary work is not in place and the pre-press production is not suitable for commercial rotary printing, it will cause a delay in the genuine product rate and a large amount of paper waste, which will seriously affect the printing quality and timeliness of its products. To give full play to the advantages of commercial rotary printing, pre-press production that meets actual needs is indispensable.
One. Prepress production process
1. Design
High-speed commercial rotary presses generally use web printing, the paper path is relatively long, the physical and optical properties of the paper surface are not very uniform, mainly rely on drying to accelerate ink drying, and the requirements for pre-press design are different from offset offset and newsprint offset .
(1) The use of gray component substitution: GCR (abbreviation of GreyComponent Replacement) refers to the achromatic value of the multi-color area that is overprinted or mixed with three primary inks in color image reproduction, which can be equal to the weakest of the three primary colors Replace the ink color ink and get the same visual effect.
In the actual production process, sometimes the commodity color of some hypermarkets (referred to as the DM list) is difficult to adjust the ink color, especially the large area of ​​dark tone or field parts. The reason is that the Y, M and C versions are too heavy to reach. Neutral gray balance, after many times of practice, it is found that when the yellow, magenta, and cyan plates are all dark, not only is the ink color difficult to print uniformly, it is easy to paste the plate, but it is not conducive to registration. In the front-end production, you must use GCR technology to increase the depth of the black version, use black to replace the neutral gray, and reduce the amount of ink used in other colors. In this way, as long as the black version is normal, the other color version uses a small amount of ink. Fluctuations and the color of the printed product are also easy to maintain stability, which is mainly due to the skeleton effect of the black version. By reducing the amount of color ink, not only is it easy to meet the requirements of proofs, but also reduces production costs.
(2) Application of background color removal
Undercolor removal (UnderColourRemoval, abbreviated as UCR) is different from the above-mentioned GCR. GCR is an achromatic structure process, which is full-level undertone removal, while undercolor removal is partial removal, and its black version is a short version.
In commercial rotary printing, there are often sticky, ink pile, title or field false, these phenomena can not rule out the printability of the paper and ink used, and equipment factors, but part of the reason is that the pre-press design is not very good Good combination with the actual requirements of printing production.
The reason for the pre-press design to cause stickiness is the sum of the tone levels of the picture, that is, the total amount of ink is set to be unrealistic. The sum of the tone levels of the cold-set type without drying requires that the single sheet of paper does not exceed 260%, and the heat setting Type web printing does not exceed 300%. Many people do n’t understand this, they only pursue full color, and the sum of the tone values ​​is set very high, which causes the title or the large area of ​​dark areas that are pressed out of the field to accumulate ink and make it false. The thickening of the ink layer also requires drying temperature. The rise of the surface caused wrinkles on the printed surface. And when the sum of tone values ​​is high, phenomena such as poor overprinting, through-printing on the back, and smudging on the back due to insufficient drying of the ink will occur.
In the process of continuous printing practice, effective solutions have been found:
1. Reasonably set the sum of the tone values ​​of the picture and keep it as low as possible to the national standard value, so that the printed image can have both layers and sharpness, and at the same time, it can reduce the ink drying time and make the printed product not sticky.
2. Reasonable use of background color removal.
The background color is partially removed, the neutral gray balance is reasonably controlled, the image contrast is increased, and the saturation is increased. This can effectively ensure that the pictures are colorful, and it is not easy to pile up ink and dull. Considering the expansion of dots in commercial rotary printing, replacing the field with about 90% of the deep web is very effective to solve the problem of the falseness and color cast of the title or the field.
3. Printing plate printing
The commercial rotary press uses a soft-soft design in which the rubber cylinder is the impression cylinder. The paper completes the ink transfer under the pressure of the rubber cylinder. The dot expansion (mainly the mechanical dot expansion) is better than that of a sheet-fed offset printing machine. It is much larger. It is tested in multiple printing processes, and for different papers, considering the mechanical and optical dot expansion. When using X-Rite528 spectrodensitometer to measure the start-up printing of Heidelberg M-600, the printed dots expand. Generally between 13% and 19%. When printing plates used in commercial rotary machines, it is necessary to increase the exposure and expose small dots of 3% or less.
The increase of mechanical dots is affected by many factors, such as: ink, fountain solution, blanket, paper surface performance, etc. The main reason for the increase of optical dots is that the refracted light increases the apparent area of ​​the dots. Because the expansion of the dot area will have a certain effect on the grayscale of the printed matter and the balance of each color, the 13th step ladder section will burn out the third level, and the 8μm yin and yang lines of the concentric circle sections of the yin and yang micrometers will be aligned. If it is not 175 lines / inch, it should be adjusted reasonably according to the actual situation of each printing plant.
4. Plate scanning
Commercial rotary presses have fast printing speeds, and a large amount of waste and defective products will appear before reaching the normal ink color and registration. In order to improve work efficiency, qualified prints appear as early as possible, and reduce unnecessary waste of ink and paper. Plate printing should be used for pre-press production. It can control the color difference between printed matter and design in a relatively small range.
The general printing plate scanning can reach about 80% of the normal ink consumption of the printed product during actual printing. Then, through the adjustment of the overall ink characteristic curve, the genuine product can be quickly produced, which can greatly improve the work efficiency. For those who do not have proofing The printing work is particularly important. The printing plate scanning work is more effective for short-run printing with proofing. It can greatly shorten the time for commercial rotary machines to open normal ink colors, saving a lot of paper, ink and other raw and auxiliary materials. Enterprises reduce costs and increase benefits. If you encounter a discrepancy between the scanned curve and the normal ink curve of the printed product during actual printing, you should generally check the following aspects before printing:
First of all, see if the width of the long red film on the edge of the film is correct. The width of the red film of different models is different. For example, the roller of Heidelberg M-600 with a circumference of 598mm requires a red film width of 23mm, and the M-600 with a circumference of 578mm The requirement of the roller is 23.1mm, the greater the difference from the actual required width, the greater the impact of the scanning results. Therefore, the actual operation should be combined with the model used, through the red film to dry the solid block of appropriate width .
Secondly, check whether the smoothness of the plate used is ideal. Sometimes the plate is dented and the surface is not flat, and the scanning results will also be affected. The flatness of the general domestic printing plate is slightly inferior to that of the original imported foreign version. Of course, the influence of plate unevenness can be reduced by adjusting the inspiratory volume of the plate scanner. In general, when scanning imported plates, choose the level of inhalation as the first level, and when scanning domestic plates, you can choose the second or third level of inhalation, depending on the smooth surface of the printing plate, because after increasing the inhalation, The printing plate will be adsorbed on the scanning panel more evenly, which is conducive to correctly reflecting the ink demand of the graphic part on the printing plate. However, blindly increasing the suction volume may suck in a large amount of dust, and the suction hole will be blocked over time, affecting the scanning quality.
In short, in pre-press production, we should gradually realize that we can only use pre-press production to adapt to commercial rotary printing, not commercial rotary printing to adapt to pre-press production. The pre-press production of commercial rotary printing is complicated and esoteric, and it needs continuous exploration and summary in actual work to continuously improve the production level.
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