People can often see the standard banner hanging on the roof of the shopping mall, and some of them also have new designs, which are printed with giant banners and colorful patterns. There are also giant light boxes erected on the top of the building such as "CCB" and "ICBC". They are all screen-printed works. Generally, if the printing area is greater than 850 mm × 1189 mm, it can be called a large-scale silk screen.
The usual screen-printing plate-making process does not need to be described here. Here, a brief discussion is made on the special part of large-scale screen-printing plate-making printing, which can be used as a way to throw a brick.
1. Manuscript
The originals of large advertisements are mostly smaller than the finished products, and the originals must be enlarged by several hundred times. Therefore, to carefully examine the original (color reversal film) with a magnifying glass of more than 10 times, it is required that the original must have a clear outline, uniform hair, clear color and moderate contrast. Large advertising manuscripts must be carefully photographed with professional photography equipment and professional advertising photography technicians. Printing is to copy the original, and the copy is beyond the original. Everyone always says that Chinese prints are not as exquisite as foreign ones. The reason is that most manufacturers refuse to invest more in manuscripts and advertising ideas. Large screen-printed manuscripts should be produced with larger negatives such as 6-inch, 8-inch, 12-inch, etc. Kodak single color reversal film. If a manuscript is a fine work of art in terms of creative composition, colors, shadows, distances, layers of reality, and theme, then no matter what kind of printing method is used, the copies must be fine.
According to the creative design of the advertisement, the original manuscript may not be a single shot. It is required to recreate more than two photographic works with a computer, to stitch together the flowers, and also to combine the advertising slogan, trademark, product name, factory name, address, Telephone and other implants, seamless use of the right to use, can also be adjusted tones and local color changes. Finally, enter various technical data: magnification, dot line number, dot type and grid angle, partitioned plate-making data, etc., after confirmation, store, print out color samples, submit to customer for approval, and confirm or modify the information The data is input into the hard disk, and the laser phototypesetter outputs the color separation film, yellow Y, magenta M, cyan C, black BK, four negatives, if there is a spot color, it also needs a spot color backplane. Once you get the color separation film, you can make a silk screen version.
The size of the trackable film of the laser line machine is generally less than full sheet or equal to full sheet, ie 850mm × 1189mm, some can output lengthened film, that is, the axial direction is less than or equal to 1189, and the radial direction can be extended to 850 decimal or integer For example, output 1010mm × 3010mm film, so it is no problem for making large screen printing of type A and letter A.
Second, the dot, point type and angle
The dot, pattern and angle used for large screen printing.
The printed dots are still screen meshes.
The transitional gradation on the printed image is formed by dots. The printed dots, the center distance between dots and dots are equal, and the diameter of the dots is changed according to the shade of the original layer.
The change of the center distance of the dot, that is, the number of lines of the dot, the number of lines: the number of dots within a unit length. The printing industry is accustomed to using the English system. For example, there are 150 dots in an inch length called 150 lines, converted to metric system 150 / 2.54 ≈ 60 lines per cm length, a set of printing plates M, C, B, K must use the same line number.
The dots change their diameter in the same number of lines to form gradations. The more gradations, the richer the hues. The fewer gradations are compressed, and the subtle gradations will be lost. The tool for checking the number of levels is the ladder. The ladder without dots is called a gray ladder. The dots are used to represent the level.
The selection of the number of dots and lines is related to the viewing distance of the printed matter, and has nothing to do with the color and imagination of the screen. Offset prints are generally viewed at close range, such as albums at bright line-of-sight. The calendar is an interior decoration, so offset printing uses 150 lines and 175 lines. The fine dots are almost indistinguishable from the retina and can only be seen as a full color Continuous picture.
For large screen printing, the general viewing distance should be several meters or even tens or hundreds of meters to watch. There is no need to use thin lines. The higher the number of lines, the more technical difficulties are related to screen printing. Big.
A type of printed products, such as telephone booth advertisements, interior decoration posters, etc., use 30 lines / in (12 lines / cm) is enough, the thick network cable will also have a sense of freshness and a certain decorative effect.
Letter A type printed products such as roadside giant billboards are very good for 20 lines / in (8 lines / cm). At the French Basic Science Exhibition (Beijing), a set of 6-meter-long and 2-meter-wide white cloth curtains each printed a monochrome photo with a large dot line number, and a combined harvester harvested in Mailang. One line per inch, a 50% dot area is 1.27cm and a 100% dot area is 2.54cm!
Dot type
There are many types of dots in offset printing, such as conventional cloth dots, elliptical dots and chain dots, etc. They are all developed from cloth dots, and the dots are arranged vertically and horizontally. In electronic printing, a chaotic dot pattern is also used, which is also regular. The basic unit is a network with a 90 ° angle and the top of the corner as the center point, and then radiates five layers in sequence, and three points on the second layer. Three layers and five points, four layers and seven points, and five layers and nine points. The points have the same size and the same spacing, and then develop into a full version in turn, becoming a mess.
Now the dot type has developed artistic dot type, such as straight line, ripple, wave pattern, sand mesh (thick and fine), brick pattern-etc., it will have a strong decorative effect when used in large-scale monochrome screen printing.
The cloth type dot type is most commonly used, but it must be considered that the superimposition of various colors along the net and the superimposition with the screen mesh appear abnormal interference patterns. The superimposed angle of the meshes of various color versions in the world is the dark version (CM, C, BK). The angle difference is 30 °, that is, the first quadrant of the coordinates is 15 °, 45 °, and 75 °, which is called unequal difference. Angle system, their angle difference is 15 ° -30 ° -30 ° -15 °. My country has also adopted this set of perspectives. The light (Y) version generally uses 0 degrees. This set of angles is sufficient for offset printing, but it is not enough for silk printing, because although it solves the abnormal interference between color plates, it does not solve the problem of interference between color plate dots and screen meshes.
When two mesh textures are superimposed, interference fringes will be found. The size of the interference fringes varies with the size (line, mesh) and superimposition angle of the two mesh textures. After experiments, the large screen printing uses 250 mesh screens, and when printing 20 or 30 lines of dots, you can safely use the above-mentioned conventional angle M, C, BK dark version, and use 15 °, 45 °, and 75 °, respectively. The Y-line color version is fixed at 0 °. If the number of dots and the number of screen meshes are changed, when printing a silk screen version, it is necessary to find the printing angle without abnormal interference patterns through the experiment before printing. The normal interference pattern is conducive to the transmission of the color party and reproduces the outline of the image. The abnormal interference pattern destroys the color transmission and destroys the image outline. Care must be taken when printing.
Large screen printing can also adopt chaotic dot or sand dot dot type. However, the dot generator of the laser imagesetter is required to have the function of generating such a dot pattern.
Third, the screen version
The screen version of the printed color dot image should use a flat screen (S type). The mesh number does not need to be too high, just select 250 mesh yellow (red) color screen.
For the selection of the screen frame, the cross-section should be larger. If the cross-sectional size is small and the strength is sufficient, internal tension deformation will occur when the screen is stretched. The tension of the large color screen printing is very important. The four screens must have the same tension and the tension is inconsistent. Overprinting is not accurate when scraping, and the image is falsely mixed. Under normal circumstances, the tension should be controlled between 16N / cm ~ 20N / cm, and the tolerance of the tension of each color version should be below 0.5N / cm.
Due to the large effective scraping area of ​​large screen printing, the distance of the empty net should be increased accordingly. The distance of the empty net should not be less than 20cm ~ 30cm.
For large-scale wire mesh stretching, the first choice is the mechanical lever-type large-scale stretching machine, followed by the pneumatic (oil pressure) large-scale stretching machine, and the second is the manual stretching method.
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