Discussion on Color Balance and Color Sequence in Color Printing

Color printing is now used by many products, such as newspapers, books, magazines, advertising, and various types of packaging. In spite of this, rare color prints that really qualify as fine prints are rare. Of course, this is related to many factors, such as printing equipment, printing materials, printing technology, etc. However, among many factors, the balance of colors and the arrangement of color sequences play a relatively important role in the quality of printed products. In the following, the author will talk about the color balance and color sequence issues in the color printing of general books and newspapers for peer reference.

A color balance problem

1. Dot increase

Dot gains between different colors can cause problems with the overall color balance. If the difference between the increase value of each outlet and the normal (recommended) value is too large, the relationship between various colors will be seriously affected and surplus colors will be produced on the printed sheets. In actual printing, the balance of colors is usually evaluated in a 50% tone. Although at 50% dots, the allowable error for dot gain for each color is ±4%, the dot gain difference between the two colors cannot exceed 4%. Otherwise, the balance of printing colors will be seriously affected.

2. Print contrast

Print contrast is an objective method of describing the openness of the dark portion of a printed image to indicate the ability to maintain the dark details of the print. If the print density conforms to the SNAP rule, a higher print contrast indicates a higher ability to maintain the dark detail of the print. The contrast can be calculated by the following formula.
Contrast = (field density - 75% density at the tone) × 100% / field density If the density is in line with the SNAP (printing data advertising printing parameter specification) rule, the results of the formula will be as shown in Table 1. If the density does not comply with the SNAP rules, you need to make appropriate adjustments according to the SNAP parameters to get the correct printing effect.

Table 1 Print contrast ratio Print contrast (75% tone) Newsprint offset flexo Green 13% 20%
Magenta 12% 20%
Yellow 15% 20%
Black 16% 20%
Tolerance ± 5% ± 5%

3. Image registration and registration issues on printing presses

Accurate positioning and registration of four colors is an important task in four-color printing. To perform positioning and registration, first of all, we must carefully arrange the color sequence so that each color can be printed at a precise position. If the image registration is overprinted, the copied colors appear blurred, the image is distorted, and the image is not clear. In actual printing, care should be taken to ensure that any color and black registration error in each direction cannot exceed 0.012 inches, including axial, circumferential and diagonal directions. In addition, the registration error between any two colors in any direction should not exceed 0.012 inches. It also includes axial, circumferential and diagonal directions.

Second printing color sequence problem

1. Printing color sequence of general color offset printing products

The so-called general color offset printing product refers to a color print printed with a single-sheet single-color or multi-color offset printing machine.

(1) Printing color sequence of single-sheet single-color offset printing press When printing by single color offset printing machine, the ink is superimposed by wet-dry dry printing. The interval between printing of two colors of ink is relatively long and short. Hours, long hours, dozens of hours. In overprinting, if the adhesiveness of the previous color ink is greater than the adhesiveness of the latter color ink, the ink of the latter color can adhere well to the ink film of the previous color ink, and the overprinting effect is good; otherwise, the ink of the latter color not only cannot be very good. It adheres well to the ink film of the previous color ink, and it also sticks part or all of the ink film of the previous color with it. The overprint effect is naturally not good, and color reproduction cannot be achieved. It can be seen that the adhesion of the ink is crucial to the color arrangement of the ink overprint.

Drying methods of printing inks for sheetfed offset printing machines are mainly based on oxidative polymerization drying and supplemented by osmotic drying. In the drying process of the ink, the adhesiveness of the ink also changes, first increases, then decreases, and the ink film formed when the ink is completely dried loses its adhesiveness. According to the law of ink adhesion with drying time, as long as the time interval between printing of two inks is mastered, printing can be achieved when printing with the ink of the previous color is large and ink adhesion of the latter ink is small. However, due to the long time required for the offset ink to completely dry, the “overprinting” or “ink crystallization” failures are generally not allowed to occur during color sorting. Therefore, the use of a single color offset press for printing color prints is more flexible in the selection of the print color sequence, and is generally considered in terms of overlay accuracy, ease of operation, and printing cost. For example, taking into account that the deformation of paper during the printing process will gradually become stable, you can print yellow ink with a low requirement for overprinting accuracy, or print ink with a smaller area, so that the printing sequence can be reduced. The degree of inaccurate overprint of the ink after printing is small; if the printing color sequence is arranged from light to dark, it is easier to replace the ink and the operation is more convenient; if the inexpensive, opaque chrome yellow ink is printed on the first color, It can reduce printing costs.

(2) Printing color sequence arrangement of multi-color offset printing press Using multi-color offset printing, if a four-color offset printing machine is used, the color printing will be printed out once. When printing with a multi-color offset printing press, the printing interval between colors is very short. The first printing ink has just been printed for a fraction of a second, and the ink of the one-color ink is to be overprinted. The ink transfer printing is a wet-wet wet printing process. Since the wet printing is always carried out in the wet and wet manner at the moment of overprinting, the adhesiveness of the previous color ink has not yet increased with the drying time, so in order to ensure the normal operation of the overprinting, the adhesiveness of the various inks is ensured. It must be gradually reduced from front to back in the printing unit.

Printing with a multi-color offset press has a lower trap rate than that with a single-color offset press, because in dry printing, the post-printing ink is transferred to pre-printed ink that has been fixed and gradually dried. The post-printing ink splits in the middle of its own ink film, one part is transferred to the previous color ink film, and the other part remains on the surface of the printing plate. However, wet printing is not the case. The latter color ink is generally split between the total ink films where the two color inks are laminated. Only a small portion of the ink is transferred to the previous color wet ink film, and most of the ink remains on the plate. From the position where the wet printing ink film is split, the thickness of the ink film transfer sequentially increases from the front to the back of the printing unit, which facilitates the overprint of the wet printing ink.

The density of the ink has a certain relationship with the thickness of the ink film. Generally, as the ink density value increases, the thickness of the ink film also gradually increases. According to China's printing industry standard CY/T
The field density range for fine prints specified in 5-99 is: yellow 0.85 to 1.10, magenta 1.25 to 1.50, cyan 1.30 to 1.55, and black 1.40 to 1.75. If all are calculated according to the upper limit of the standard range, the experimental results show that the ink film thickness at the yellow ink density of 1.10 is approximately 1.43
The thickness of ink film with μm, magenta ink density of 1.50 is approximately 0.96 μm, and the ink film thickness with cyan ink density of 1.55 is approximately 0.82.
Îœm, the ink film thickness at a black ink density of 1.75 is approximately 0.75
Îœm. If the above density ink printing is used, printing is performed in the order of black, cyan, magenta, and yellow, and the overprinting effect is good; if the printing sequence is changed to black, cyan, yellow, magenta, the ink film thickness becomes 0.73 in order. Îœm, 0.82
Μm, 1.43 μm, and 0.96

Îœm, obviously, the superimposition of magenta ink is not good. Even if the amount of magenta ink is increased in order to increase the overprinting effect of the magenta ink, color misalignment, damage to the gray balance of the printed matter, exacerbation of the dot gain, and the like will occur.

Brightness