Experiences in prepress output quality management
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Despite the growing maturity of CTP technology, laser imagesetters continue to maintain their strength in production. In the printed publications such as commodity packaging, book catalogues, etc., there is still a considerable part of the process of pre-press designing, outputting photos, and finally printing finished products. The quality of the photo film will be directly related to the quality of the final printed product, which indicates that there are still many areas to be studied and explored in the traditional process. The author will talk about the experience of pre-press output quality management from the perspective of process management.
Original quality
From the perspective of total quality management, the quality of the control photo film should start from the original, because the quality of the original determines the quality of the photo, so the original must be carefully examined.
1. Check the surface of the original for scratches, dirt, text and lines, and whether there are any missing or broken pictures.
2. Check if the picture is color cast. Under natural light or a standard light source close to daylight, observe whether the white, gray, black and other decolorizing parts on the reflective paper interfere with other colors.
3. Check to see if the main colors are accurate. In general, the color of a reflective manuscript (such as a photo) will be slightly more exaggerated than the color of the actual scene, such as more vivid than the original color. The reflection of the person as the main body should be based on the ruddy complexion of the face.
4. Check if the contrast is moderate. The contrast of the reflective draft includes luminance differences, color contrast, and contrast balance. The brightness difference is moderate, there will be a rich transition color level between the high-tone part and the dark-adjusted part; the color contrast is moderate, the color density of the reflective draft is large, and the stereoscopic feeling is strong; the contrast is good, and the same color is in the high-profile part. The color reproduction of the dark areas will be consistent.
Output document quality
The main check items for documents that need to be output as follows:
1. Check whether the set line, color mark and various printing and cutting lines are fully set. The set line should be black with four colors (C100, M100, Y100, K100). If there is no set line, the output of four or more colors will be equal to a pile of waste.
2. Check the image color mode. When the laser imagesetter outputs the color separation film, all color images are in CMYK color mode. Therefore, the color mode of the linked image should be checked carefully by linking the image table before output. If the color mode of the color image is RGB or Lab mode, the mode conversion should be performed in the image processing software. Otherwise, only the black plate will have an image when the color separation film is output, or the color separation may not be ideal, or even output; if it is black and white Image, image mode should be grayscale or binary mode.
3. Check that the resolution of the image used meets the printing requirements. Make sure that the scanned image, photo and other bitmap images have the right resolution, which is necessary to ensure the quality of the output photo slide.
The resolution of printed images is generally set to 1.5 to 2 times the number of dot lines. The setting criteria for resolution of some common publications are as follows: the scanning resolution of newspapers is 125-170 dpi, and the number of dot lines used for newspaper printing is 85~ 133 lines/inch; journal magazines and promotional materials have a scanning resolution of 300 dpi, because the number of dots used for printing is 133 lines/inch or 150 lines/inch; the scanning resolution of high-quality albums is 350-400 dpi. Most of the beautifully printed booklets use 175 to 200 lines per inch. It is best not to use pictures downloaded from the Internet, because the resolution of these pictures is generally only 72dpi, and some even lower, no matter how to deal with them, the output will not be good.
4. Interpret the results of the preview by RIP. The RIP interpretation preview is consistent with the final release result. Check whether the image of the file link is correct by pre-viewing the original or proof, and whether the image interpretation is intact (if the image is copied incorrectly, the RIP interpretation will be wrong), and the image color is Corresponding to the original, whether the text is garbled, whether there are any misplaced elements in the file, whether the black version is overprinted, etc. This is the last check before the output, so be careful to avoid quality problems.
Phototype machine operation
After the imagesetter is started, its working status can generally be seen in the graphical interface of the control software of the output workstation. Items that need to be checked:
1. The setting of the laser imagesetter matches the system equipment and runs normally. In order to ensure the stability of the laser energy, the laser head should be tested and corrected periodically.
2. Whether the concentration and temperature of the liquid medicine of the film processor are normal. It is necessary to set the parameter values of development and fixing according to the performance of the film and the liquid medicine, and regularly test with the test strip and the ladder rule, and at the same time, do the daily cleaning and maintenance work of the film processor to prevent the sediment from being dirty.
3. Is the performance of the film and the liquid itself stable? In order to ensure the stability of the quality of the photo film, the variable factors should be minimized in the use of raw materials, so that the relative use of raw materials is relatively fixed.
4. Photographs are easily scratched by foreign objects during the operation and transmission of the equipment. The transmission components should be cleaned and wiped regularly. Once the parts are found damaged, they should be replaced in time.
Output film quality
Checking the quality of the output film, as the last process of the film, to really play the role of tonic plugging, should pay enough attention to it.
1. Check the fit accuracy. In general, the misalignment error of a four-color or multi-color film that is continuously output is small, if one job requires a one-color or two-color version, and the other color of the film is one day, two days or more. For the output, the fitting precision of each color film should be carefully checked. If the fitting error is large, all the color plates should be continuously output. Of course, this should be based on the time of use of the imagesetter and the stability of the work. Some imagesetters have a large tolerance for the continuous output of four-color film. In this case, consider repairing or updating the machine.
2. Check the imprint relationship of each color version against the original or proof. Sometimes, due to personnel operations or machine failures, layout elements may be lost or misplaced. For book publications, the best way to check is to attach the film to the original or proof, and it will be clearly visible if there is a mistake. Of course, in this process, we must pay attention to avoid scratching the film and affect the quality of the printing plate.
3. Check if the film is dirty. Film dirty is a common problem in the output process. Generally speaking, the washing tank should be cleaned before starting every day. Before the water is injected, the water purifying equipment of the external water supply switch should be turned on to prevent the sediment deposited in the pipeline from being taken into the washing tank, scratched or attached to the film, affecting Film quality. After the work is finished, drain the washing tank and wash it carefully.
4. Check the angle of the network cable of each color output film. In order to avoid hitting the net, in theory, the difference between the four colors should be 22.5°. In the actual production, the four-color printing adopts 15°, 45°, 75° and 90°. Among them, the yellow version has weaker visual stimulation and poor visual perception, so it is generally set to 90°. Since the human vision is most sensitive to 45°, the main color version of the original is generally set to 45°, and the specific setting should be determined according to the design of the printed product. The angle of the network cable can be checked by the magnifying glass on the angle of the screen of each color film, or by the angle measuring tool. In addition to the special requirements of customers, the general network cable angle setting should not be changed at will, so as to avoid unnecessary losses.
5. Check the solid density of the output film. The density of the film in the field is checked by a transmission densitometer, which is generally required to be 3.5 or more, preferably about 4.2. If the density is too high, it will produce a paste phenomenon; the density is too low. For prints with text and solid layouts, the ink will be light and look unreal. At this time, it is necessary to consider replacing the fixing solution or cleaning the laser head to ensure the stability of the output film quality.
In summary, the pre-press output process can be improved not only by improving the quality of the photo-distribution film, but also by improving the production efficiency by carefully and accurately checking and taking corresponding measures to control it.