Section 6 Printing of Plastic Products - 6

(3) Features of flexo printing of plastic film flexographic printing:
1 printing plate is soft, elastic, excellent ink transmission performance;
2 light pressure, printing speed, no noise;
3 The ink thickness of the flexographic printing is similar to the embossing;
4 The flexographic printing dot enlargement is large, the color reproduction level tone reproduction is not ideal, and the reproduction tone is shorter;
5 The use of anilox roller for ink simplifies the ink transfer device. Easy to operate, easy to maintain;
6 Because of the certain elasticity and thickness of the flexographic plate, elongation will occur under pressure, affecting the overlay accuracy;
7 The rubber and resin plates used in flexographic printing are swelled and corroded by solvents, and therefore are subject to certain restrictions on the use of raw materials;
8 The printing material has a wide adaptability and is suitable for printing all kinds of plastic flexible packaging;
9 Flexographic printing can also be produced online with postpress processing, such as compounding, indentation die cutting, forming, and sealing.
1. Requirements for the design of manuscripts for flexographic printing of plastics 1 The blackness of patterns and texts should be deep and even, and black and white are distinct. If the graphic outline is blurred and dark, it will affect the effect of separation negative film.
2 When the large area is in the same color as the finer texts, lines, and dots, the amount of ink and pressure needed are different. When the two cannot be taken into account, they should be printed separately.
3 Try to avoid large-area multi-color overprinting. It is not appropriate to superimpose two equal-sized color patches to avoid overprinting, and the words, patterns, or small patches should be partially overprinted on a large-area solid color patch, which will make the screen appear rich in color. In the use of ink overprinting, the ink overlay effect cannot be inferred from the effect of the deployment of the paint, because the characteristics of the ink and the paint are different.
4 Avoid designing wide and long bars and solids in the horizontal direction of the print cylinder. This can cause machine vibrations, ideally slashes, curves, wavy lines, and other irregular curves.
Avoid designing larger circular patterns on the 5th page because the flexible media will stretch in the circumferential direction.
6 The white characters of the color manuscript, especially the small characters, should be boldly stroked when the ink is drafted. The strokes should be clear so that when the printing is done, the ink stretches inwards along the edges of the hollow words, making the strokes thinner or crushed. .
7 to avoid the strict registration requirements of tight seams. Two-color overprinting parts generally need to leave a cross section of 0.1-0.2mm to avoid overprinting.
8 The original coloring should take into account that the printer can print a few colors at most.
9 When the layout design manuscript contains both continuous-motion video, the manuscript should be selected from the perspective of the plate-making process. For the black-and-white transmissive manuscript, the manuscript should be selected that has sufficient tonality and does not require fine reproduction. For natural color positive films, the original is required to have good color reproduction and moderate density distribution. The color of the main pattern is composed of primary and secondary colors. Both sides are clear. The main color is neither bright nor dark. Tone.
10 Designers should understand the “elongation phenomenon of the printing plate”. When the flexographic plate is mounted on the imprinted platen roller, under the effect of pressure, the size of the picture and text along the printing method (circumferential direction) is elongated, and the roller axis direction remains unchanged. According to the "print elongation" law of flexographic printing, when designing a manuscript or photographing color separations, the designer should subtract the corresponding value in the circumferential length dimension of the manuscript for compensation according to the elongation of the plate.
If the radius of the plate cylinder is R, and the total thickness of the printing plate and the double-sided adhesive tape is t=5 mm, the length of the printed graphic must now be exactly 100 mm. When layout, the length of the original document should be:
x=(2πR)/[2π(R+t)]×100=R/(R+t)×100=100/105×100(mm)=95.24(mm) Printing plate elongation=100-95.24= 4.76 (mm)
It can be seen from the above that if a graphic image with a length of 100 mm is to be printed, the circumferential length of the flexographic plate should be 95.24 mm, and the length is 4.76 mm, and the axial dimension is unchanged. This formula is only an approximate calculation. In fact, the elongation of the plate pattern is also related to the height of the raised portion of the pattern. The height of the printing plate graphic part protrudes high, and the deformation after pressing is obvious.
According to foreign materials and years of experience in the use of dolrel cyrel photosensitive resin version of the United States, to obtain the total thickness of the cyrel plate and the entire circumferential elongation of the table when the 3-48 full circumferential printing (printing plate full cylinder circumference). Relationship, for reference only.
Table 3-48 Thickness and total circumferential elongation of cyrel plate

It should be noted that when the same plate is attached to a plate cylinder with a different circumference, its circumferential extension is not equal, and the length of the printed material is not equal. For example, when a 200mm long and 2.84mm thick printing plate is attached to a 300mm and 500mm circumference plate roller, the elongation is 11.39 and 6.83mm respectively, and the length of the printing product is 4.56mm, so use this table. When correcting the amount of elongation, the plate cylinder to be used should be selected first.
2. Process requirements for flexo printing (1) Requirements for color screen negatives 1 Generally, thicker screens (30-45 lines/cm) are used to reproduce the short-tone adjustments. This is due to the fact that the flexographic printing dots have been enlarged and the picture contrast has been reduced. The colors are not clear and they are boring. The larger the number of network cables, the more serious the network points are expanded. It is better to use lower screen number than high screen number screens in order to obtain printed products with a sense of contrast. Flexographic printing can not be screened at high light, and must have a small network (2% -3% of screen area) dark tone due to the impact of the amount of network expansion, should reduce the number of outlets;
The angle of the 2 color screens is: Y82.5°, M67.5°, C7.5°, Bk37.5°. This is because the flexographic printing uses an anilox roller to supply the ink. Due to the influence of the angle of the anilox roller, the same color-screening line angle as the plain printing cannot be used.
(2) Selection of Flexible Plates Currently, the most commonly used flexographic plates in China are the following: Cyrel from DuPont; Flex-light from Uniroyal; Elaslon from Tokyo Chemical Industry; and Asahi Kasei Industrial APRF lex edition and Shanghai Printing Technology Institute GS flex edition. Among them, DuPont's Cyrel plates are the most widely used. The types, specifications, and models of Cyrel plates suitable for flexible packaging plastic printing are shown in Table 3-49.
Table 3-49 List of Cyrel Plates Applicable to Flexible Packaging Plastics Printing
1 soft, suitable for smaller version of the drum
2. Decrease beat
3. Sharp part of anti-white
4. Applicable color dot and line printing
5. Economic 1. Porous surface helps ink transfer
2. The printed area is a blue easy area
3. In addition to the PQS-type features, the exposure also has:
1. Decrease beat
2. Soft, suitable for small-diameter version of the drum 1. Edition thin
2. With anti-halo base
3. Network expansion rate
4. Porous surface ink transfer well
5. The print is easy to distinguish between the web size and thickness of the plate and the selection of the printing product and the conditions of the printing equipment and materials.
1 The difference of printing materials. According to the surface conditions of the material, different types of plates are used. For the surface-smooth and soft substrates, thin plate materials can be used; for rough-surfaced, hard substrates, thick plates are used.
2 prints