Offset production standardization (3)

2 ink properties. The chromaticity point (position in the chromaticity diagram) can be determined for the three primary color inks according to the ink standard, and the chromaticity point must have a deviated spatial domain (spherical domain).

Color measurement of ink. The chromaticity values ​​of the ink can be represented by x, y, and the brightness reference value Y, and these values ​​can be converted by the tristimulus values ​​x, y, z.

The advantage of using a chromaticity diagram is that the color of different saturation can be obtained by actually adding zinc white; a straight line can be obtained from the paper white to the spectrum color by the screening method. Of course, when the pigment thickness increases, the color changes and it is impossible to give a straight line.

The disadvantage of a chromaticity diagram is that the distance between two colors does not reflect the true visual color difference at all. For example, the distance between two reds is 1/10 of the length of the coordinate unit; for green, when people feel the same color difference, the distance between the two may be 10 times that of the former; for blue, both The distance may be half that of the former.

Permissible definition of color difference. The allowable color difference is the limit of the allowable color deviation of the printing ink layer, which is a function of the thickness of the ink layer (Figure 5-26). In the printing standard, the standard ink layer thickness should be specified. A higher ink layer thickness will cause a larger increment of the adjustment, while a lower ink layer thickness will cause problems in the fluidity of the ink, the smoothness of the ink layer surface, the gloss, and the like.

It is not possible to determine the thickness of a sheet of ink on an offset press. Because the weight of the paper fluctuates greatly, the weight of the printed and non-printed surfaces cannot be compared, and the thickness of the ink on a sheet cannot be determined. The amount of water on paper is about 7 times larger than the amount of ink, and the moisture balance between paper and the environment is changing at any time. Therefore, the job of determining the thickness of the ink layer can be performed on a sample printer. When determining the thickness of the reference ink layer for mechanical proofing, density measurements can be compared with proofs printed on the sample press.

3 solid color density. In order to adjust the ink delivery of the printing press, it is necessary to specify the solid density of the three primary inks, which must be derived according to the specified standards. Methods as below:

Print the standard paper and the actual printing paper with the same ink supply, and change the density of the solid within a certain range to print. Finally, compare the color printed on the standard paper with the standard, and find out the prints with the same color, then have the same color. The density of printed matter can be used as a printing standard (Figure 5-27). Do the same for all three primary colors. Therefore, the current party-matching therapy is adopted instead of the density measurement method because the standard ink and the printing standard ink are different.

The resulting printing standard can be used as a regulating standard for color adjustment devices (eg, CPC, CCI, ...). However, this is only suitable for testing with an on-line densitometer. Both the plate reader and the ink conditioning equipment work on the premise that the standardization and printing density are known.

Use the same paper as the print when proofing. If the paper is too thin (<809/m2), use two sticks. It can also be replaced with thicker paper of the same brightness.

2. Process standardization.

1 The dot value increases (indicated by symbol TZ). Survey sites with increased site adjustments should be examined at midtones (eg, 40%) and shadows (eg, 80%), and no useful dot gain information is available for bright tone (eg, 7%) sites. Because the densitometer cannot accurately measure bright spots, the measured data is meaningless.

When specifying the amount of increase in printing adjustments, it is also required that standard inks be printed with the maximum relative contrast.

It is necessary to set up the test block outside the printed image because the lack of a single-color part for measurement within the image and measurement on the three-color mixed dot block is error-free, so any precise measurement instrument cannot Measurements are made inside the image. Otherwise, the measurement error and the color difference of each color (from this print to that of the print) are superimposed on the measurement signal with a relatively high density value.

Table 5-12 shows the recommended values ​​for increasing the value of site adjustments.

Table 5-12 Increased recommended value (%)

Paper type dark tone 80% Medium 40%
Coated paper 9～13 15～21
Matte coated paper 9 ~ 13 15 ~ 21
Non-coated paper 11 to 15 18 to 24

2 density deviation in the field. Permissible deviations: Ds ± 14% for proofing and Ds ± 8% for printing. Plate readers also work based on this allowable deviation range.

3 color sequence. If it is an ideal transparent ink, the ink layer A is printed on the ink layer B or the ink layer B is printed on the ink layer A is the same, but actually the ink used is not an ideal transparent ink, and the thickness of the ink layer is the same. Under the circumstances, the post-printing ink layer predominates.

In wet wet conditions, ink overprinting cannot be complete, and the overprinting rate depends on the overprinting state:

In wet press printing, the ink that is printed first prevails.

When wet pressed in printing, the post-printing ink predominates.

The standard printing sequence is recommended as follows:

Four-color printing: black + blue + magenta + yellow

Two-color machine: blue + magenta; black + yellow

Monochromator: blue; magenta; yellow; black, or black;

When using the black skeleton, black is printed on the front.

In order to facilitate ink overprinting, it is beneficial to arrange the thickness of the ink layers in a small to large order. Such as:

Black = 0.8μm, blue = O. 9 μm, magenta = 10 μm, yellow = 11 μm.

4 ink overprint. The three-color solid ink overprint should look neutral gray, but when the overprint is insufficient, it will obviously produce a color sensation. Since the relationship between the three-color density values ​​changes with the glossiness of the upper surface and the type of paper, it is impossible to measure the neutrality of a three-color overprinting with a densitometer, and it is also unreliable to measure a two-color overprinting with a densitometer.

The observation condition of the printed image: When the illumination of the printed image changes, each color on the image may also change, which is the reason why the standard observation condition is required. In the printing industry, the observation of color requires standard illumination with a color temperature of 5000K.

The standard observation conditions also include the following recommended conditions: Illuminance Illuminance 2153 lx, no stray color illumination source in the observation field, such as colored walls, the light bulb of the light source is updated every 2500h.

There are also some important requirements that can not rely on the memory when making color judgments. It is necessary to make a color judgment by directly comparing the qualified proofs with the printed proofs. The assessed images are to be discharged into the airport for simultaneous observation. In order to achieve side-by-side observations, folded sheets may also need to be folded. Because qualified proofs are approved by the customer, they can be used as a comparison. Transmissive films cannot be used for comparison, which requires evaluation using different optical systems.






Author: Shichang

Source: "Quality Inspection and Control of Printed Materials" · Chapter 5 Printing Image Quality Measurement and Control Technology