In flexographic color printing, how to arrange color sequence reasonably is an important issue for process technicians. An important role of printing is to copy manuscripts artistically, which requires the printing workers to have a certain degree of artistic appreciation and master the ability to express color changes. Improper arrangement of color sequence may cause color deviation, color mixing and reverse overprinting of the printed product; scientific color sequence arrangement will make the color of the printed product more close to the original, and even strengthen a certain color atmosphere to make the printed product layer clear and the dots clear , Overprint is accurate, the color is true, natural and coordinated.
1. Problems that should be paid attention to in the flexo color sequence
The printing color order refers to the order of printing of each color plate in multi-color printing. The color effect is different when the printing color sequence is different.
For color printing in flexographic printing, yellow (Y), magenta (M), cyan (C), and black (K) four-color plates are usually used, and overlay printing is performed in a certain order. In theory, four-color printing can have 24 different printing color sequences, but in fact, due to the different printing color sequences, the color rendering effect of the printed matter is also different. Reasonable printing color sequence will make the color of the picture closer to the original, and even strengthen the atmosphere of a certain color, adjust the level of the picture, the tone is soft, and it is also convenient for overprinting. Therefore, in color printing, especially packaging printing, more than four colors or spot colors and full version, we should pay more attention to the arrangement of printing color sequence.
When determining the printing color sequence, it should be noted that the flexographic printing color sequence is basically the same as the relief printing color sequence, but the lithographic color sequence must not be used. For the printed manuscript of the four-color screen version, the color of the pattern should be analyzed. The commonly used color sequence is yellow, magenta, cyan, and black. But some people choose the color sequence of yellow, black, magenta, and cyan. Because the water-based ink has high color density, strong hiding power, thick ink layer, bright colors and low light transmittance. The printing color sequence of the four-color screen version cannot be changed at will.
The color sequence printed in flexographic printing is a variable, and there are many factors that affect the color sequence of the printing. Especially for multi-color screen printing, the determination of the color sequence of the printing should fully consider various factors, according to the specific conditions and the printed product. The quality requirements are reasonably determined. In general, the following aspects should be paid attention to when determining the printing color sequence.
1. Color rendering effect and printability
Printing color sequence affects the rendering effect. There is only one possibility when printing the first color, that is, the ink layer is directly printed on the surface of the printing material, and the color rendering effect is relatively stable. When printing the second color, there are two possibilities, that is, part of the dots are directly transferred to the surface of the substrate, or part of the dots are printed on the dots of the first color (that is, overprinting). In both cases, the color rendering effect will change. There are three possibilities when printing the third color, that is, some dots are printed directly on the surface of the substrate, some dots are printed on the ink layer of the first color or second color dots, and some dots are printed on the first color and second color dots Overprinted ink layer. If the dots are printed directly on the substrate, there may be an increase in dots. If it is dot printing, the bottom ink layer has three conditions of moderate drying, insufficient drying and excessive drying, and there will be problems such as dot growth or cross-coloring and vitrification, which will cause difficulties in printing. Therefore, the more overprints, the more complicated the problems. For printing suitability, there are several possibilities for ink overprinting. One is that the underlying ink is dried properly; the second is that the underlying ink is over-dried, resulting in vitrification. The dots shrink after the new ink layer is printed, or even not printed at all. , There is a printing failure.
2. The principle of color rendering on the screen
In the high-profile parts of the printed product, the dot arrangement is mostly in a parallel state, and the color sequence has little effect on the color mixing of the high-profile parts. In the process of transition from high-key to mid-tone to dark-tone parts, the color components of the subtractive color principle gradually increase, so that on the same screen, different printing color sequences cannot get good tone reproduction in different parts. According to the color change rule, the printing color sequence has little effect on the high-key color mixing, and has a great influence on the dark tone.
3. Transparency of ink
The ink has a certain hiding power, and the post-printed ink is overprinted on the first printed ink, which will affect the color rendering. Ink with good transparency, after the two colors are overprinted, the color of the lower ink layer can pass through the upper ink layer (subtractive color mixing), showing the correct new color. In color inks, due to the difference in transparency and hiding power of each color ink, even if the same color plate is printed in different color sequences, different color rendering effects will be obtained. For example, if the yellow ink used is transparent yellow, the transparency order of the four inks is: yellow> magenta> cyan> black. At this time, the ink with poor transparency should be printed first, and the ink with good transparency should be printed later.
4. Features of the picture
Generally speaking, if the dot area is small, print first, and if the dot area is large, the main color plate on the screen can be post-printed or post-printed. The operator should choose the color sequence of printing on the basis of fully analyzing various factors.
5. Measurement of the influence of printing color sequence on color rendering effect
In color overprinting, the overprint rate is generally used as a measure of the effect of the printing color sequence on the color rendering effect, that is: Overprint rate = [(solid density of the first and second colors overprinted-monochrome solid density of the first color) ÷ 2 Solid color density of color monochromatic] × 100%. Obviously, different printing color sequences have different overprint rates, and different printing effects are obtained. For spot color printing, the order of light to dark is generally adopted, which can make the dark ink appear bright after overprinting. For spot colors printed by the superimposed method, the secondary colors are printed first, and the main colors are printed later.
2. Determine the color sequence of flexo printing
General principles
Since the printing color sequence has a great influence on the printing effect, how should the color sequence of flexo printing be determined? In general, the following principles can be followed:
1. Arrange the color sequence according to the content and characteristics of the original
Printing is a means to express the graphic design of the layout, and the emotional components reflected in each color are also different. Therefore, the arrangement of color sequence must first consider the content and characteristics of the original. When designers design graphic layouts, they often have chosen a good tone of layout colors. This tone represents the overall feeling of the entire hue and becomes the dominant color during printing. In terms of chromatics, those based on red, orange, and yellow are called warm colors; those based on green and blue are called cool colors. Due to the hiding power of ink, black and cyan are printed first mainly in warm tone, and magenta and yellow are printed later in cold tone; magenta first is printed in cold tone and then cyan is printed. For example, people want to see the familiarity
The watermelon crumbs are red, and I do n’t want to see the prints appear purple or green. Different color sequences have different color shift effects. Therefore, the colors that need to be enhanced in the color atmosphere of the picture can be printed in the last color.
For general-tone originals, the printing color sequence can be determined according to the rule of the printing plate with a small dot integral area (such as a black version) and the printing with a large dot integral area (such as a yellow version), that is, according to black → cyan → Magenta → yellow order printing.
2. Determine the color sequence according to the quality of ink transparency
Generally speaking, yellow ink is the most suitable for transparency, followed by magenta ink and cyan ink, and the worst is black ink. Therefore, the printing color sequence first prints the black version, then the cyan or magenta version, and finally the yellow version.
3. Determine the color sequence according to the characteristics of the picture
The main color version on the screen should generally be placed in the last or second final printing, and the other color versions should be printed in the first or second color to facilitate the reproduction of the characteristics of the picture. For example, a picture dominated by a large area of ​​landscape (cool tone is the keynote), with black, magenta, cyan, and yellow as the color sequence. Arrange black printing first, and use black to outline, to facilitate the printing of other colors. The transparent yellow final print can be used to adjust the brightness of the entire screen to form shiny, vivid colors. The picture (warm tone) generally dominated by characters adopts black, cyan, magenta, and yellow sequences. Ink with poor transparency is printed first and does not cover other colors. The magenta and yellow backprints make the character's face rosy, rich in color, and realistic in effect.
4. Determine the color sequence according to the characteristics of plate making and other processes
Since the effect of moire is to be minimized in screen tone printing, the screen angle of the adjacent two color groups differs by at least 30 during printing, which is helpful to prevent color defects and moire. In addition, when the screen version and the field version are printed, the screen version can be printed first and then the field version.
5. Determine the color sequence according to the ink absorption performance of the printing material
When the ink absorption of the printing material is large, the first color is printed with high-tech ink first. For example, for poor quality paper, considering its low whiteness and smoothness, loose fibers, poor ink absorption, and easy to powder and fluff, yellow ink can be printed first to make up for the above defects of the paper. When printing at night, due to the recognition of the human eye, weak color inks with low lightness should not be arranged in the first color printing. The order of lightness of the ink is: white <yellow <orange <green <green <red <blue <purple.
6. Arrange the color sequence according to the difficulty of overprinting
In the flexographic printing of paper, due to the objective defects such as deformation and expansion of the paper, the main colors of the printing products with relatively high register requirements should be arranged in two adjacent color groups for printing. For example, the two-color machine can be printed in the same unit at one time to avoid problems caused by paper expansion and contraction. The large area of ​​the field can be arranged for post-printing to avoid problems such as smudging during paper handover. Although objective defects such as plastic deformation and expansion are better than paper, this is also an objective fact. In addition, the single-color machine is to overprint the second color after the first color is dried, while the multi-color machine is a process of "wet pressure wet". Ink is printed first, and ink with low viscosity is printed later. This is crucial, otherwise it will cause the disadvantage of reverse overprinting.
7. Refer to the ink price to arrange the color sequence
At present, the price of domestic ink is the most expensive with magenta ink, followed by yellow ink and cyan ink, and the price of black ink is the lowest. Therefore, in order to reduce printing costs, under the premise of meeting the printing quality requirements, cheap black and cyan inks are printed first, and the expensive magenta and yellow ink plates are printed afterwards, which can save costs.
8. Color sequence of multi-color machine overprint
Since printing from the first color to the last color printing, the printability will gradually drop, so you can put the dark tone version in the first color printing, bright tone version in the final printing.
Due to the characteristics of flexo ink and related factors, the above principles can be used as the basis when determining the printing color sequence, but due to different printing content, equipment, substrates and the characteristics to be expressed, the arrangement of the printing color sequence Not the same. In addition to the above general principles, when arranging the printing color sequence, it should be properly considered from the perspective of art, so that the printed matter can be more artistically appealing and meet the needs of readers.
According to the experience that people have summarized from the printing practice, the following two points can be considered in determining the printing color sequence: When multi-color dot printing, the printing color sequence that can be considered is yellow → magenta → cyan → black or yellow → black → product Red → Cyan; if it is color block overprint, the large color block should be printed first, and the small color block should be printed later. [next]
3. How to determine the color sequence of plastic film flexo printing
Reasonably determining the printing color sequence is an important process in plastic film flexo printing. The inks used for plastic film printing are divided into surface printing inks and internal printing inks, and the printing processes of surface printing and internal printing are also different, so the printing color sequence is also different. To sum up, the difference in composition and operation of surface printing ink and internal printing ink mainly includes the following aspects:
1. The difference between link materials
The bonding material of surface printing ink is mainly polyamide resin, which has good adhesion and good gloss, but it is not suitable under high temperature conditions and has poor fastness when compounding. The linking material of non-cooking lining ink is mainly chlorinated polypropylene, and nitrocellulose and vinyl chloride-vinyl acetate copolymer resin are also used abroad. The high-temperature retort ink-linking material used in polyurethane printing is polyurethane vinegar. When used, a certain amount of hardener is added, the two liquids are mixed, and the cross-linking reaction occurs.
2. The difference between using solvents
The solvents used in surface printing inks are mainly xylene and isopropyl alcohol. In general, the printing ink solvents are mainly toluene and ethyl acetate. High-temperature cooking inks are mainly based on ethyl ketone and ethyl acetate. Liyin ink solvent is suitable for high-speed printing, the volatility of the solvent is relatively fast, and the residual amount of the solvent is particularly small.
3. Difference in wear resistance
Because the polyamide resin has good flexibility and greater elasticity, the added additives make its wear resistance better and improve, and it is more firm against external objects. The chlorinated polypropylene resin of Liyin ink is particularly rigid, and its wear resistance is poor. Because it is Liyin, the requirements for wear resistance are correspondingly lower.
4. The difference between using auxiliary materials
The surface printing ink and the inside printing ink are different in their auxiliary materials and additives due to the difference in connection. The surface printing ink is often added with dehydrated malate and kiwifruit ester to improve adhesion, increase gloss and increase viscosity. Liyin ink is also added with various pigment dispersants, enhancers, defoamers and other additives.
5. The difference of printing process
The process of printing on the inside is exactly the same as that of the general process, but the image on the printing plate is the reverse image when the plate is printed, and the image is the positive image after printing. The surface printing ink drying speed is slow, and the inside printing ink drying speed is faster than the surface printing speed.
According to the difference in composition and use of surface printing and internal printing inks, the general color sequence in plastic film flexographic printing is:
(1). Printing color sequence of surface printing process
The "surface printing" of the plastic film uses white ink as the laying background color to set off the printing effect of other colors. The main ones are as follows
Advantage. First, plastic white ink has good affinity with PE and PP films, which can improve the adhesion fastness of printed ink layer. Secondly, the background color of white ink is total reflection, which can make the colors of printed products more vivid. Third, the printed background color can increase the thickness of the ink layer of the printed product, make the printed layer more abundant, and have a visual effect of embossing. Therefore, the printing color sequence of the plastic film surface printing process is generally determined as: white → yellow → magenta → cyan → black.
(2). The printing color sequence of the printing process
In order to get the same visual effect as "table printing", the printing color sequence of the Indian printing process should be the opposite of "table printing", that is, the white ink background color is placed in the final printing, so that the white ink background color can be seen from the front of the printed product To set off the role of various colors. Therefore, the printing color sequence of the Liyin process should be: black → cyan → magenta → yellow → white.
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