Variations in dot gain values ​​between different colors can cause color balance problems. If the dot gain values ​​are too far from the values ​​in the above table, the relationship between the various colors will be seriously affected and the printed sheets will also have an extra color.
The balance of color is usually evaluated at the 50% tone. Although the dot gain tolerance for each color in the 50% dot area is +/- 4%, the dot gain difference between the two colors cannot exceed 4%. For example, if the cyan dot gain value is 2% larger than its corresponding recommendation value, then the yellow dot gain value cannot be 2% larger than its corresponding recommendation value. Otherwise, the extension between the two colors will exceed 4%, and the color balance will be seriously affected.
For direct offset printing, the direct offset process is used. When the printing plate is exposed, the emulsion of the negative is face up, which means that the film emulsion layer does not directly contact with the surface of the printing plate. This exposure method from the film to the printing plate will result in a dot gain/TVI value of 8% to 10% on the printing plate. There are many potential changes in dot gain/TVI between direct offset and normal offset. SNAP recommends actual results for verification.
Print Contrast Print contrast is an objective method of describing the openness of dark portions of printed images. Print contrast is calculated using the density of solid and dark dot areas. If the print density meets the SNAP rules, a higher print contrast indicates a higher ability to maintain the dark details of the print. The following is the formula for calculating the contrast:
[(Field Density)-(density at 75% tone) × 100]/Field Density
If the density meets the SNAP rules, the result of the formula will be as shown in the following table. If the density does not comply with the SNAP rules, it is necessary to make appropriate adjustments according to the SNAP parameters to obtain the correct printing effect.
Print contrast at 75% tone
Print Contrast (75% tone) Newsprint Offset Print 13% 20%
Magenta 12% 20%
Yellow 15% 20%
Black 16% 20%
Tolerance +/- 5 +/- 5
Note: The above does not include the embossed data. Currently there is no provision for embossing the relevant data.
An important factor in image registration, registration, and four-color printing on the printer is the accurate registration of the four colors. These colors must be carefully arranged to allow them to be printed in precise locations. Arranging bad color reproductions will appear blurry, unfocused, and accurate positioning will give clear results.
No color or black registration can exceed 0.012 inches in each direction, including lateral, peripheral, or oblique. In addition, the registration between the two colors cannot exceed 0.012 inches in any direction, and also includes lateral, peripheral or oblique directions.
The 0.015 inch is equivalent to 100 dpi, and the length of 1.5 rows of dots when the screen angle is 90 degrees.
Printing color sequence
SNAP's recommended print sequence is shown in the following table:
Offset Flexo Printing Embossing Cyan - Magenta - Yellow - Black - Cyan - Magenta - Yellow - Magenta - Cyan - Black Cyan - Magenta - Yellow - Black
Note: Many tests in the industry show that when using CMYK to print color sequences, offset and embossing will get the maximum color rendering range. In a newsprint flexo process, opaque yellow is usually printed first so that it does not obscure the back color.
Paper newsprint paper and non-coated wood pulp paper vary in brightness. Their difference will affect the hue and overprint of the printing ink. The following table lists the characteristic parameters of the materials used by SNAP. SNAP has obtained the parameters based on these materials. If the characteristics of the printers used by the printer differ greatly from the parameters listed in the table below, appropriate adjustments must be made.
Substrate reference
L* A* B* transparency brightness
SNAP material 83.0 -0.3 4.2 94.5 58
Ink The following table lists the target values ​​for ink and overprint in the printing process. Although the primary colors (primary colors) may reach the target values ​​listed in the table below, this does not necessarily mean that the secondary color can also achieve the target value. The target values ​​for red, green, and blue also vary depending on the printer, paper, and ink.
CIE* L* a* b* target value
L* a* b*
Qing 57 -23 -27
Magenta 53 48 0
Huang 79 -5 60
Black 40 1 4
Green + Yellow 53 -34 18
Cyan + Magenta 41 7 -22
Magenta + Yellow 52 41 25
Note: The above values ​​are from ISO 12647-3. They only represent offset and letterpress inks. There is no data on flexo printing yet. The test method is based on ISO 13655. (20 observers, light D50, 450/00, black back)
The CIE referred to in the table represents the International Commission on Illumination. CIE L* a* b* is a system that describes colors, where L* stands for brightness, a* refers to the red-green range in the color, and b* refers to the blue-yellow range in the color.
Many cold-fix printers see CIE L*a* b* as an objective method to describe or communicate ink, dye hue, and print quality information. In addition, the use of reflection densitometers to measure the color error, grayscale and overprint and other values ​​are all effective tools for objective measurement, description and exchange of information.
Hue error, grayscale, and overprint measurements can be used as an effective tool to repeatedly check SNAP parameters. If the print results are good, and the hue error, grayscale, and trapping measurements are consistent with the SNAP parameters, then it can be said that the parts in the process are operating normally. If the measured values ​​differ greatly from the SNAP parameters and the printed image looks good, consult the ink, printer, and paper supplier for reasons. If the measurement results are close to the SNAP parameters but the effect of the printed image is not good, the supplier must also be consulted.
The hue error, grayscale, and overprint values ​​listed in the following two tables belong to the cold-fix printing process. Because different printing plants may use two measurement methods, the table includes absolute hue, gray values ​​(including paper density), and relative hue errors and gray values ​​(excluding paper density). SNAP recommends using an absolute method to measure hue errors and grayscale values ​​so that the measurement results include the paper density value, because the device “sees†the ink and the paper as the human eye sees it during the measurement.
The data in the table is just a recommended reference for the printer to SNAP. It is not an absolute operating parameter. Printers and newspapers should conduct a print quality test on their own paper, ink, printing press, printing sequence, and other process components. When satisfactory print quality is achieved, the test target value should be measured to determine hue error, grayscale, and overprint values.
Hue Error/Hue Value Hue Error/Grayscale Offset (Excluded, Relative) Offset (Including Paper, Absolute) Flexo (Excluded, Relative) Embossing (Excluded, Relative)
Green Magenta 28/10 White Magenta 28/42 33/15 32/16
Magenta Yellowish Yellow 56/11 Yellowish Yellow 58/34 61/15 57/16
Yellow Magenta 8/1.4 Magenta 10/25 5/1 8/7
Note: The absolute, relative hue error/grayscale is listed in the table because both methods are common methods. However, SNAP still recommends that printers and newspapers use absolute values ​​to evaluate the process.
Ink Overprint Ink Overprint Offset Flexo Printing Ink Ink Order CMY YMC CMY
Blue 69% 78% 63%
Green 80% 98% 80%
Red 50% 89% 50%
When printing on substrates of similar densities, the inks that meet the NAA parameters are similar to the inks that meet the SWOP parameters. However, there are still substantial differences between the two. The following table lists the recommended inks for newspaper and commercial printing.
Recommended Ink Market Offset Flexo Embossing Newspaper Printing NAA AD-LITHO® NAA AD/FLEXTM NAA AdPro
Commercial Printing AD-LITHO
Commercial inks differ from NAA AD-LITHO inks only in intensity, and their hue characteristics are the same. NAA has validation software for coldset inks. For membership fees, NAA evaluates ink samples submitted by printers or ink manufacturers to identify inks that meet the NAA AD-LITHO, AD/FLEX, and AdPro parameters. GATF Association also identifies ink for SNAP and charges a fee