How to Determine the Color Sequence of Flexographic Printing? Issues to Consider when Determining the Color Sequence in Flexographic Printing
Printing color sequence refers to the order in which various color plates are printed in multi-color printing. Different printing color sequences lead to different color effects.
In color printing with flexible plates, the commonly used four-color plates are yellow (Y), magenta (M), cyan (C), and black (K). These are overlaid in a certain order. In theory, there are 24 different color sequences for four-color printing, but in practice, the color sequence affects the color reproduction of the print. A reasonable color sequence can make the colors on the print closer to the original, even enhancing the atmosphere of certain colors, adjusting the tonal levels, and facilitating trapping. Therefore, in color printing, especially in packaging and situations involving more than four colors or spot colors, attention should be paid to arranging the color sequence.
When determining the color sequence, it is important to note that the color sequence in flexographic printing is generally similar to that in letterpress printing, but it should not adopt the color sequence used in offset printing. For the printing original of the four-color wire frame version, the color sequence is typically analyzed based on the colors of the pattern, commonly using the sequence yellow, magenta, cyan, black. Some may opt for the sequence yellow, black, magenta, cyan. This is because water-based inks have high density, strong coverage, thick ink layers, bright colors, but low transparency. The color sequence for the four-color wire frame version should not be changed arbitrarily.
The color sequence in flexographic printing is a variable, influenced by various factors, especially for multi-color wire frame versions. The determination of the color sequence should comprehensively consider various factors based on specific conditions and the quality requirements of the prints. Generally, when determining the color sequence, attention should be focused on the following aspects:
Color Reproduction and Printing Suitability: The color sequence affects color reproduction. When printing the first color, there is only one possibility: the ink layer is directly printed on the substrate's surface, resulting in a relatively stable color reproduction. When printing the second color, there are two possibilities: either some dots are directly transferred to the substrate's surface, or some dots are printed on the dots of the first color (i.e., trapping). These two cases result in variations in color reproduction. When printing the third color, there are three possibilities, and so on. The transparency of the ink also plays a role in the printing suitability.
Color Rendering Principles on the Image: In areas with high tones in the print, dot arrangements are mostly in parallel, and the color sequence has little effect on the color mixture in high-tone areas. However, as you transition from high tones to mid-tones and then to dark tones, the color sequence's impact on the color representation increases.
Ink Transparency: Inks have a certain degree of coverage, and when overprinting one color over another, it affects color rendering. Good transparency in inks allows the color light of the lower ink layer to pass through the upper ink layer (subtractive mixing), showing the correct new color. Different inks have different levels of transparency, even when printing with different color sequences on the same color plate, resulting in different color effects.
Characteristics of the Image: Generally, it is advisable to print the plate with fewer dots first, followed by the plate with more dots. The main color tone plate on the image can be printed last or second to last. Operators should choose the color sequence based on a thorough analysis of various factors.
Degree of Impact of the Color Sequence on Color Reproduction: In color overprinting, the trapping rate is commonly used as a measure of the degree of impact of the color sequence on color reproduction. The trapping rate is calculated as [(solid density of overprinting between the first and second colors - solid density of the first color alone) ÷ solid density of the second color alone] × 100%. Different color sequences result in different trapping rates and, consequently, different printing effects. For spot color printing, a general rule is to use a sequence from light to dark, which can enhance the brightness of dark colors.
General Principles for Determining the Color Sequence in Flexographic Printing
Since the color sequence has a significant impact on printing effects, how should the color sequence for flexographic printing be determined? In general, the following principles can be followed:
Arrange the Color Sequence Based on the Content and Characteristics of the Original: The color sequence should be arranged considering the content and characteristics of the original. Designers often choose the color tone when designing the layout, representing the overall feel of the colors. The color tone can be warm (with red, orange, yellow as the base) or cool (with green, blue as the base). The arrangement of the color sequence should consider this base tone.
Determine the Color Sequence Based on the Transparency of Inks: Considering the transparency of different inks, the color sequence should be arranged based on the transparency of the inks. Yellow ink is usually the most transparent, followed by magenta and cyan, with black being the least transparent. Therefore, a sequence that prints less transparent inks first and more transparent inks later should be considered.
Determine the Color Sequence Based on the Characteristics of the Image: Consider the characteristics of the image. For images dominated by large areas of scenery (with a cool tone as the base), a color sequence of black, magenta, cyan, yellow may be suitable. For images where characters are the main focus (with a warm tone), a color sequence of black, cyan, magenta, yellow may be appropriate.
Consider the Transparency of Inks: Different inks have different transparency and coverage. When determining the color sequence, consider the transparency of the inks used and arrange the sequence accordingly. Inks with lower transparency should be printed first, followed by those with higher transparency.
Consider the Difficulty of Trapping in Overprinting: In flexographic printing on paper, it is important to minimize the impact of trapping issues. For adjacent two-color groups, the angles of the screen lines should differ by at least 30 degrees, which helps prevent color shifts and trapping issues.
Consider the Absorbency of the Substrate: When the substrate has high ink absorbency, the first color should use an ink with high viscosity. For example, if the paper quality is poor, with low whiteness and smoothness, using yellow ink as the base can compensate for these defects. During nighttime printing, due to the human eye's recognition capabilities, low-brightness light-colored inks should not be arranged as the first color.
Consider the Color Sequence in Multi-Color Presses: In multi-color presses, the printing suitability gradually decreases from the first color to the last color. Therefore, dark tone plates can be placed in the first color printing, and light tone plates in the last printing.
Consider the Price of Inks: Currently, the price of domestically produced inks is highest for magenta, followed by yellow and cyan, with black being the least expensive. To reduce printing costs while meeting quality requirements, arrange the sequence with cheaper black and cyan inks printed first, and more expensive magenta and yellow inks printed later.
Determining the Color Sequence for Plastic Film Flexographic Printing:
Determining the color sequence in flexographic printing on plastic film is an important process. Due to differences in the inks used for surface printing and reverse printing on plastic film, as well as variations in the printing processes, the color sequence determination also differs. The main differences in composition and operation between surface printing (surface ink) and reverse printing (inside ink) are as follows:
Binder Differences: The binder for surface ink is mainly polyamide resin, which has good adhesion and gloss but is not suitable for high-temperature conditions and has poor bonding when laminated. The binder for non-cooking reverse printing ink is mainly chlorinated polypropylene, and in foreign countries, nitrocellulose and vinyl chloride-vinyl acetate copolymer resin are also used. For high-temperature-resistant reverse printing ink, the binder is polyurethane, and a certain amount of hardener is added during use for cross-linking reactions.
Solvent Selection Differences: Surface ink mainly uses solvents such as xylene and isopropanol. Generally, the solvents for reverse printing ink are mainly toluene and ethyl acetate. For high-temperature-resistant reverse printing ink, solvents such as acetone and ethyl acetate are used. Solvents for reverse printing ink are suitable for high-speed printing, with faster volatility and minimal residual solvents.
Abrasion Resistance Differences: Polyamide resin in surface ink is flexible and has good elasticity, and additives are added to improve abrasion resistance. Chlorinated polypropylene resin in reverse printing ink is rigid and has lower abrasion resistance. However, as reverse printing does not require high abrasion resistance, the demands are correspondingly lower.
Auxiliary Material Differences: Due to differences in binders, auxiliary materials, and additives for surface ink and reverse printing ink also differ. Surface ink often includes desiccant acrylate ester, kiwi fruit acid ester to improve adhesion, gloss, and viscosity. Reverse printing ink includes various dispersants, enhancers, defoamers, and other additives.
Printing Process Differences: While the plate-making process for reverse printing is identical to general plate-making, the image on the plate is reversed compared to surface printing. Surface ink has a slower drying speed than reverse ink.
Considering the differences in composition and use between surface ink and reverse ink in flexographic printing on plastic film, the general color sequence for the printing process is as follows:
Color Sequence for Surface Printing Process: In the surface printing process for plastic film, white ink is used as the base color to enhance the printing effects of other colors. The color sequence is generally determined as white → yellow → magenta → cyan → black.
Color Sequence for Reverse Printing Process: To achieve a visual effect similar to surface printing, the color sequence for the reverse printing process should be the opposite. White ink, serving as the base color, should be printed last so that, when viewed from the front of the print, the white ink can enhance the effects of other colors. Therefore, the color sequence for the reverse printing process is generally black → cyan → magenta → yellow → white.
These considerations for color sequence determination in plastic film flexographic printing take into account the differences in inks, solvents, abrasion resistance, auxiliary materials, and the plate-making process between surface and reverse printing.