Flexographic printing, offset printing, gravure printing and screen printing are the four most common printing methods in the world, each with its own characteristics. Because flexographic printing has unique flexibility, economy, and is beneficial to the protection of the environment, it meets the hygiene standards of food packaging printed products. This is also one of the reasons for the rapid development of flexographic printing processes abroad. However, judging from the current situation in China, offset printing is more popular, and gravure printing has also occupied a large market in the packaging industry, and the technology of flexographic printing is relatively late to start, although it has indeed been achieved in recent years. Great progress, but there is still a big gap with the international advanced technology level.
1. Flexible plate-making process flow:
From the perspective 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:
Original → Electronic Separation or Photography → Positive Image → Back Exposure → Main Exposure → Development and Rinse → Drying → Post-processing → Post-Exposure → Compared with offset platemaking, there are mainly the following differences
1. Reproducible color value range. Offset printing is 1% to 99% (or 2% to 98%), flexo printing is 3% to 95%.
2. Expansion of outlets (at 50%). Offset printing is 15% -20%, and flexographic printing is 30% -40%.
3. Add the number of network cables. Offset printing is generally up to 175 lines / inch, while the number of flexo and screen lines is generally not more than 150 lines / inch.
Second, the dimensional deformation of the color separation film
The most obvious feature of the flexographic plate is its elasticity. When the flexographic plate is installed on a cylindrical cylinder, the printing plate is bent and deformed along the surface of the cylinder. This deformation affects the patterns and text on the surface of the printing plate, making the printed image The text is not the correct reproduction of the design manuscript, and even severely deformed. After the flexographic plate is mounted on the cylinder, such static deformation (tensile deformation) in the circumferential direction of the cylinder is always unavoidable. In order to compensate for the distortion of the printed image, it is necessary to reduce the size of the corresponding graphic on the negative film. Before designing the manuscript or color separation, the elongation of the printing plate should be considered, and the corresponding value should be subtracted from the circumferential length of 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 material. When making a flexographic plate by planar exposure, the following formula is generally used to calculate the reduction rate of the color separation film:
Reduction ratio (percentage) = K / R × 100%
Where R is the printing length of the plate cylinder and K is the coefficient, which depends on the thickness of the plate used. For example, when the plate thickness is 1.70mm, the K value is 9.89mm; and when the plate thickness is At 2.29mm, the K value is 13.56mm.
Third, the transmission rule of the flexible version of the dot
1. Reasons for the expansion of outlets
The expansion of dots in printing is inevitable. There are two main reasons for the expansion of dots: one is physical or mechanical. At the moment of imprinting, the ink on the dots of the printing plate will be squeezed. There is a certain deformation caused by the action, which leads to the expansion of the dot; another reason is the optical reason, that is to say, the expansion of the dot is caused by the reflection of light, and the light is scattered at the edge of the ink film of the dot, thus visually Produced a considerable network expansion. Optical dot expansion depends on the transparency of the ink and the smoothness and absorption properties of the paper.
2. The transmission rules of the flexible version of the outlets:
In flexographic printing, due to the relatively high elasticity of the photosensitive resin plate used, and a certain printing pressure needs to be applied during the printing process, although light pressure printing is used in flexographic printing, it will still cause image dots on the printed matter The expansion of the enlarged image causes changes in color and gradation reproduction. In practice, by measuring and drawing the corresponding flexographic printing characteristic curve, it can be seen that the dot expansion in the flexographic printing process is very serious, and the dots below 10% are difficult to control, so special treatment should be done for the highlight area. The net should be placed as far as possible.
Compensation for dot expansion can be done on the scanner side or on the imagesetter, preferably on the imagesetter. The edges of the small dots generated after exposure are neat and of good quality.
3. Influencing factors
(1) The number of screens added, the higher the number of screens, the more serious the network expansion.
(2) The size of the printing pressure. The greater the printing pressure, the more serious the dot expansion, and otherwise the smaller the dot expansion. Therefore, flexographic printing should try to maintain "zero pressure".
(3) Dot shape. Circular dots, square dots, chain dots and elliptical dots, their dot expansion at different levels are also different. Chain-shaped dots are commonly used in flexographic printing; for small dots in high-gloss areas, FM dots (FM) are the best.
Fourth, the production of flexible version
At present, the plate materials used in flexographic printing are basically solid photosensitive resin plates, and the photosensitive mechanism is: the photosensitive resin is rapidly decomposed under a certain amount of light, and the active and extremely unstable high-energy state groups are generated ( Free radicals), high-energy state groups then lead to polycondensation reaction of resins containing unsaturated bonds. The flexographic plate making process mainly includes the following procedures:
1. Back exposure of the plate material, the purpose is to determine the height of the relief on the printing plate, that is, the depth of corrosion, and solidify the substrate.
2. Put the printing plate and negative film together, and use ultraviolet light for front exposure to form the graphic part on the printing plate and cure it.
3. The printing plate is placed in a solvent and brushed, the purpose is to brush away the unexposed part of the plate material, so that the graphic part forms a relief.
4. Place the printing plate in a dryer to dry, so that the solvent absorbed in the printing plate evaporates as soon as possible, so that the thickness of the printing plate is restored to the original standard value.
5. Post exposure and debonding treatment. Post-exposure and debonding treatment of the dried plate material can further solidify the shoulders and the base, improve the printing performance of the flexographic plate, and improve the printing durability of the flexographic plate.
Fifth, the points to be noted in the flexographic plate making process:
1. Try not to place small prints, dots and other details on a large area, even if it is the same color, try to divide it into two versions. If it cannot be separated (such as the limitation of the number of color groups of the printing press), you can consider Appropriate local padding.
2. Try to avoid large-area solid block multi-color overprinting.
3. The text specifications should not be too small, especially for the negative text. Otherwise, when the pressure of the printed matter changes, the printed graphics will show a large amount of deformation, making the Yang graphics thicker, and the Yin graphics thinner or 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 inking hole on the anilox roller is generally 45 °. Therefore, when printing with an ordinary anilox roller, the printing plate should avoid using a 45 ° screen angle. Avoid the appearance of mottled prints.
6. Avoid designing wide and long bars and ground along the horizontal direction of the printing cylinder, which will cause machine vibration. Ideally, slashes, curves, wave lines and other irregular curves.
7. Avoid designing large circular patterns on the layout. Because when the printing plate is elastically stretched or compressed, the regular geometric pattern becomes irregular and the circle becomes an ellipse.
8. Avoid the requirement of precise overprinting of tight stitching.
9. The color setting of the original should take into account that the printing machine can print a maximum of several colors.
9. When using ink to overlap colors, it is not advisable to use two color blocks of equal size to overprint to avoid inaccurate overprint and affect the printing quality. You can use local areas to overprint text or pattern patterns and overprint local color blocks on a large area of ​​solid color blocks.
6. Common faults and solutions of flexographic plate making
1. Sticky layout
the reason:
1) The plates are not cleaned after being washed and corroded;
2) The rinsing liquid is used for too long or the effective components are insufficient.
Solution:
1) The plate is washed and corroded, and then thoroughly cleaned;
2) Change the flushing solvent.
2. The straight line is not straight, curved or wavy
the reason:
1) Insufficient rear exposure;
2) Insufficient main exposure;
3) Insufficient drying time.
Solution:
1) Adjusted exposure time;
2) The main exposure time is tested again;
3) Extend the drying time.
3. Small text and thin lines are lost
the reason:
1) Insufficient rear exposure;
2) The main exposure is insufficient.
Solution:
1) Exposure time after readjustment;
2) Re-do the main exposure test.
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