Talking about flexographic printing plate making technology
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Flexographic, offset, gravure and screen printing are among the world's four most popular printing methods, each with its own characteristics. Because flexographic printing has unique flexibility and economy, and is beneficial to the protection of the environment, it meets the hygiene standards of food packaging and printing, which is one of the reasons why the flexographic printing process has developed rapidly in foreign countries. However, from the current situation in China, offset printing is more popular, gravure has also occupied a large market in the packaging industry, and the technology of flexographic printing is relatively late, although it has indeed achieved in recent years. Great progress, but there is still a big gap with the international advanced technology.
First, the flexographic plate making process:
From the point of view of manuscript design and plate making process, the flexographic printing process is self-contained and has its own unique features. The plate making process is basically as follows:
Originals, electronic color separation or photography, positive image, back exposure, main exposure, development and rinsing, drying, post-processing, post-exposure, and paste are mainly compared with offset printing:
1. A range of color values that can be reproduced. Offset printing is 1%~99% (or 2%~98%), and flexo printing is 3%~95%.
2. The outlets are expanded (50%). Offset printing is 15%~20%, and flexo printing is 30%~40%.
3. The number of screen lines. Offset printing is typically up to 175 lines per inch, while flexo screens typically do not exceed 150 lines per inch.
Second, the size of the color separation film deformation
The most obvious feature of the flexographic plate is its elasticity. When the flexographic plate is mounted on the cylindrical drum, the printing plate produces a bending deformation along the surface of the drum. This deformation affects the pattern and text on the surface of the printing plate, so that the printed image is printed. The text is not a correct reproduction of the design manuscript, or even a serious deformation. This static deformation (tensile deformation) generated in the circumferential direction of the drum after the flexographic plate is attached to the drum is always unavoidable. In order to compensate for the deformation of the printed image, it is necessary to reduce the size of the corresponding image on the negative film. The design of the original or color separation before the plate making should take into account the elongation of the plate. The corresponding value should be subtracted from the circumferential length dimension in the original to compensate, so that the printed product will meet the size requirements.
In addition to the radius of the plate cylinder and the thickness of the double-sided tape, the reduction rate is also related to the thickness of the plate. When the flat plate is used to make the flexographic plate, the following formula is generally used to calculate the reduction rate of the color separation plate:
Reduced rate (percentage) = K / R * 100%
Where R is the printing length of the plate cylinder, K is the coefficient, which depends on the thickness of the plate used, for example, when the thickness of the plate is 1.70 mm, the K value is 9.89 mm; and when the thickness of the plate is At 2.29 mm, the K value is 13.56 mm.
Third, the flexible version of the dot transmission law
1. Reasons for the expansion of outlets
The expansion of dot in printing is inevitable, and there are two main reasons for the expansion of the dot: one is physical reason, or mechanical, and the ink on the plate dot will be squeezed at the moment of imprinting. The effect is a certain deformation, which causes the dot to expand; the other reason is the optical reason, that is, the dot enlargement is caused by the reflection of light, and the light is scattered at the edge portion of the dot ink film, thereby being visually A considerable expansion of outlets has occurred. The expansion of the optical dot depends on the transparency of the ink, the smoothness of the paper, the absorption properties, and the like.
2, flexible version of the dot transfer rules:
In flexographic printing, since the photosensitive resin plate used is relatively elastic, and a certain printing pressure needs to be applied during the printing process, although light pressure printing is used in flexographic printing, it still causes image dot on the printed matter. The expansion of the image is elongated and causes changes in color and layer reproduction. In practice, by measuring and drawing the corresponding flexographic printing characteristic curve, it can be seen that the dot enlargement in the flexographic printing process is very serious, and the dot below 10% is difficult to control. Therefore, special treatment should be performed for the highlight zone. Should try to put the net down.
For the expansion of the dot gain, it can be done on the scanner side or on the imagesetter. It is best done on the imagesetter. The edge of the small dots generated after exposure is neat and the quality is better.
3. Influencing factors
(1) The number of screen lines is increased. The higher the number of screen lines, the more serious the network points are expanded.
(2) The size of the printing pressure, the greater the printing pressure, the more serious the dot enlargement, and the smaller the dot enlargement. Therefore, "zero pressure" should be kept as much as possible in flexographic printing.
(3) Dot shape. Circular dots, square dots, chain dots, and elliptical dots have different dot spreads at different tonal levels. In the flexographic printing, chain-shaped dots are commonly used; for small dots in the high-light zone, the frequency modulation dots (FM) are the best.
Fourth, the production of flexible version
At present, the plates used in flexographic printing are basically solid photosensitive resin plates. The photosensitive mechanism is that the photosensitive resin is rapidly decomposed by a certain amount of light, and a highly active and highly unstable group is produced. The free radical group, the high energy state group re-initiates the polycondensation reaction of the resin containing the unsaturated bond. The flexographic plate making process mainly includes the following processes:
1. Back exposure of the plate to determine the height of the relief on the plate, ie the depth of corrosion, and to cure the substrate.
2. Put the printing plate and the negative film together, expose the front side with ultraviolet light, form the graphic part on the printing plate, and make it solidify.
3. The plate is washed in a solvent, the purpose is to brush off the unexposed portion of the plate, so that the graphic portion forms a relief.
4. The plate is dried in a dryer to promote the evaporation of the solvent absorbed in the plate as soon as possible, so that the thickness of the plate is restored to the original standard value.
5. Post exposure and debonding treatment. Post-exposure and de-bonding of the dried plate can further cure the shoulder and the substrate, improve the printing performance of the flexographic plate, and improve the printing durability of the flexographic plate.
Fifth, Points to note during the flexographic plate making process:
1. Try not to put small parts, dots and other details on a large version of the field. Even if it is the same color, it should be divided into two pieces. If it is impossible to separate (such as the number of printing machine color groups, etc.), consider The stencil is suitably applied locally.
2. try to avoid large-area solid block multi-color overprint.
3. the text specification can not be too small, the Yin text is even more so, otherwise, when the pressure of the printed matter changes, the printed graphic presents a large amount of deformation, making the positive image thicker, the negative image is thinner or more dead.
4. The width of the individual thin lines should be greater than 0.2 mm.
5. In flexographic printing, the engraving angle of the ink-receiving hole on the anilox roller is generally 45°. Therefore, when printing with a common-type anilox roller, the plate should avoid the 45° angle of the screen. Avoid print moiré.
6. Avoid designing wide and long bars and solid areas along the horizontal direction of the printing cylinder, which will cause the machine to vibrate. Ideal for slashes, curves, wavelines, and other irregular curves.
7. Avoid designing large circular patterns on the layout. Because when the plate is elastically stretched or elastically compressed, the regular geometric pattern becomes irregular and the circle becomes elliptical.
8. Avoid precise overprinting requirements for tight seams.
9, the original color should be considered that the printing machine can print a few colors.
10. When using ink overlay, it is not advisable to use two equal-sized blocks to overprint to avoid inaccurate overprinting and affect printing quality. It is possible to use overlaid text or pattern patterns and overprinted partial color patches on a large area of the solid color block.
Sixth, the flexo plate making common faults and solutions
1. the board is sticky
The reason:
1) The plate is not washed after being washed and corroded;
2) The rinse solution is used for too long or the active ingredient is insufficient.
Solution:
1) The plate is washed and corroded, and then thoroughly cleaned;
2) Replace the rinse solvent.
2. the line is not straight, curved or wavy
The reason:
1) Post-exposure is insufficient;
2) The main exposure is insufficient;
3) The drying time is not sufficient.
Solution:
1) Adjusted exposure time;
2) The main exposure time is tested again;
3) Extend the drying time.
3. Missing small text and thin lines
the reason:
1) Post-exposure is insufficient;
2) The main exposure is not sufficient.
Solution:
1) Re-adjust the exposure time;
2) Perform the main exposure test again.