The spot color inks required for book printing production are usually divided into Pantone spot color inks and ordinary spot color inks. Among them, Pantone's spot color ink is based on the reference color ratio specified on the Pantone color card. It is prepared by Pantone ink. The publisher only needs to provide Pantone color number, which is simple and easy, and the color accuracy is high. But the cost of ink is high. Ordinary spot color inks are prepared using ordinary inks as the base color. The publisher must provide the actual color to the printer (not available with the spot color chromatogram number), the color accuracy is low, but the ink cost is low. In this paper, the author will take the book's spot color printing as an example to explain in detail the method of blending ordinary spot color inks.

Figure 1 is the title page of a book, the entire page printing color can be divided into two hue: green and black. Among them, green is a large-area field, and it can be made into a spot color from the perspective of ensuring a large area of uniform color on the ground and saving printing costs. In the pre-press document production, the hue is C50, Y100 (ie C with a dot area ratio of 50% and Y with a dot area ratio of 100% can overprint the hue), and the same dot area ratio corresponds to the printing chromatogram. The color block shown in Figure 2.

Figure 1 Title page of a book

Figure 2 Printing chromatography

According to the printing process of the book, our company needs to provide the standard color sample to the printing factory, and the printing factory can arrange the spot color ink according to the standard color sample. In the current printing environment, the color palette of the spot color ink is usually a general density meter and a spectrodensitometer (X-rite densitometer). Among them, the color theory foundation has two color spaces: CIE L*a*b* and CIE L*C*H°. This paper focuses on the use of X-rite density meter, based on CIE L*a*b* color space for spot color ink blending.

CIE L*a*b* color space

The CIE L*a*b* color space is a uniform color space recommended by the International Commission on Illumination (CIE) in 1976. The lightness L* and the chromaticity a*, b* indicate the position of the color in the color space. L* indicates the brightness of the color; a* positive value indicates red, negative value indicates green; b* positive value indicates yellow, and negative value indicates blue. The analysis parameters involved are: ΔE* is the composite color difference; ΔL*=L sample *-L standard* (lightness difference); Δa*=a sample *-a standard* (red/green difference); Δb*= b sample *-b standard * (yellow / blue difference).

Get the base color ratio of the standard color sample

There are two ways to obtain the base color ratio of a standard color sample:

First, the standard color sample is compared with the Pantone color card to find the approximate color block, and the ink color matching is performed according to the basic color ratio of the approximate color block. This is only a reference data, and on this basis, it is necessary to use the density meter to perform color grading based on the CIE L*a*b* color space. The hue is calibrated on the Pantone uncoated color card (PANTONE 374U) with a base color ratio of 18.8% for Pantone Yellow, 6.2% for Pantone Green, and 75.0% for the thinner (Figure 3).

Second, the standard color sample can be scanned by the InkFormulation Manufacturer of the X-rite densitometer (Figure 4). After scanning, the color ratio of the color sample can be seen, and the spot color can be used as the reference data. Ink mixing. The basic color ratios obtained by the two methods are relatively close, and can be used as reference data for color grading.

Figure 4 Using the X-rite densitometer color matching system to obtain the primary color ratio

Spot color ink preparation process

01, prepare the ink adjustment equipment

Electronic scale (mg), squeegee, glass, ink meter, ink (yellow and green ink), thinner, drying equipment, printing suitability (or ink color meter), X-rite densitometer, D50 light source, coated paper.

02, measuring and analyzing standard color samples

Use the X-rite densitometer to measure the L1* a1*b1* value of the standard color sample, where L1* = 65, a1*=-32, b1*=38, it can be judged that the spot color hue is located at CIE L*a*b* The second quadrant of the color space is between yellow and green. According to the ratio of the a* value and the b* value and the base color ratio data, it can be judged that the spot color tends to be yellow.

03, mix samples, record data

According to the base color ratio data, the ratio of the yellow ink to the green ink is determined, and the ink amount of the primary ink involved in the blending is controlled at 30 g, quantified by an ink meter, and the specific gravity of the yellow, green ink and the thinner is weighed by the electronic scale in the glass. The ink is evenly blended by a squeegee, and the ink is evenly distributed. Then, the prepared ink is loaded into the printability meter, and ink samples of different thicknesses are printed on the printability meter, and the test is performed. The sample is placed in a dryer for drying.

After drying for two minutes, the X-rite densitometer is used to determine whether the thickness of the ink layer of the sample is consistent with the thickness of the ink layer of the standard color sample. Then measure the L2*a2*b2* value of the sample, compare the brightness value of the sample with the standard color sample, ΔL* = L1*-L2*, and when ΔL*<0, it indicates the color ratio of the sample to the standard color. The sample is light, and the sample ink is thin; when ΔL*>0, it means that the color of the sample is deeper than the standard color sample, and the sample ink is thick. In the spot color matching, when the absolute value of ΔL* is <1, it means that the ink thickness of the sample is very close to the ink thickness at the time of actual printing. The next color shift correction can be performed only when the sample ink thickness is adjusted to ΔL* absolute value <1.

According to the color correction principle of “correcting the large color cast and then correcting the small color cast” and the principle of “adding a small amount of plus”, the manual measurement is performed several times or the proof is measured by the printability meter, and the record L2* a2*b2* is recorded. The values are ΔL*, Δa*, Δb*, ΔE* until a sample with ΔE* less than 2 is formulated. ΔE is defined as the total color difference, but does not indicate the offset direction of the sample color difference, and the larger the ΔE, the larger the color difference. It should be noted that the color difference here is the relative color difference, not the absolute color difference.

04, preparation of large quantities of ink

The exact ratio of the yellow ink, the green ink, and the thinner is calculated based on the recorded data in each ink blending process, and finally the bulk ink is prepared.

Spot color ink mixing precautions

(1) The tool must be kept clean and free of other dust and mottled colors.

(2) The ink must be thoroughly mixed to ensure the hue is accurate.

(3) In the offset printing process, ink emulsification will reduce its saturation, so the saturation needs to be higher when adjusting the ink.

(4) When observing and measuring color samples, they must be uniformly performed under the D50 light source, maintaining a viewing angle of 2°.

Digital proofing of printed spot colors

In the current digital proofing, the digital proofing machine can't restore the printing spot color according to the CMYK four-color original ink ratio, but realize the spot color proofing through color simulation. Therefore, the spot colors presented in the digital proofs of the book are not accurate enough to be used for printing and color matching.

In the publication of book publishing, if you encounter spot color printing, you will be used to replace the spot color with any color plate in the document proofing and design links. For example, it will be made into a black version (or any original color version of CMYK), only to print. The link will provide the spot color to the printing factory. This practice will become one of the hidden dangers in actual production. It is recommended to use analog spot color proofing in the previous proofing.

For a long time, the effect of spot color ink blending is inseparable from the experience of the operator. The color technology requirements of the operator are particularly high. The masters who apply ink in the printing factory are mostly skilled and experienced masters. Today, with the increasing digitization and standardization of printing production management, the software and hardware facilities of color matching technology are increasing. It is believed that the dependence of this technology on the experience of operators will be gradually reduced in the near future, and operators only need to master the basic principles of color matching technology. With the basic functions of the instrument, it is possible to prepare spot color inks for digital and scientific color matching.

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