The printing of spot color inks means that instead of CMYK four-color synthesis, the color is printed with a specific ink. Spot color inks are generally prepared by the printing plant itself, and can also be provided by the ink plant if they are used in large quantities or for a long time. In packaging offset printing, especially printing on paper packaging products, in addition to four colors, spot color printing is widely used. Spot color printing has its great characteristics in packaging printing: large-area solid color blocks are colorful and eye-catching, and the display effect is good; reducing the color difference caused by dot overprinting and dot deformation, stable color, easy to control and identify. The spot color ink has high color accuracy, solid on the ground, and wide color gamut. The characteristics of the spot color ink can overcome some of the defects of four-color printing.
However, most of our printing plants currently lag behind in the detection and control of spot colors. Most of them rely on the experience of teachers to prepare spot color inks. Judging by the eyes of the master when printing, depending on the experience of the master, subjective factors have a great influence. Therefore, quality problems such as color difference often occur. Bring great losses to the enterprise. Of course, some powerful large-scale packaging and printing enterprises have adopted spot color ink matching system to manage it. The effect is relatively better.
1. The deployment of spot color inks
We said that to print spot colors for packaging and offset printing, we must first prepare good spot inks. Before the deployment, first analyze the characteristics of the color samples and the copy requirements of the printed products, compare the spot color samples with the relevant color blocks of the chromatogram, and find the similar colors. Check the ratio of the three primary colors, and try to mix with the three primary colors ink matching the color spectrum. When blending spot color inks, the main color should be determined first, and then the auxiliary color ink should be gradually added to the main color ink. When the transferred spot color ink is slightly darker than the spot color sample, an appropriate amount of diluent can be added according to the error size; if it is slightly lighter than the spot color sample, some black ink or dark dark spot color ink can be added. The transferred spot color ink can not be printed immediately, but on the printed paper, the transferred spot color ink is applied on the spot and compared with the spot color sample. It is generally slightly darker than the spot color sample. The ink is dried relatively close or consistent. When mixing spot color inks, pay attention to the minimum original ink. For example, if two-color original ink can be transferred, do not use three-color or four-color original ink to adjust, otherwise the grayscale of the color increases. For the deployment of some special spot inks, we need to select some special inks according to the spot color requirements. For example, some antique copper colors can add some gold ink, and when printing peacock feathers to match the spot colors, in addition to the three primary colors, the addition of blue and gold, and so on. After analyzing the proofs, the color of the ink should be analyzed. Know the components of the hue characteristics of commonly used inks. For example, yellow ink should distinguish whether it is yellowish or blueish (or greenish). The red ink should distinguish whether it has a red phase or a blue phase (or purple phase). If blue ink with red phase and yellow ink with red phase are used to mix green ink, it is equivalent to adding red ink to blue ink and yellow ink to increase the black component, because according to the principle of color subtractive method. Then the transferred green ink will become dark.
Hue and partial color of commonly used inks
Name
Hue and color bias
Medium Yellow
Yellowish reddish
Deep yellow
Reddish yellow
Light yellow
Yellowish greenish, tender color
Pink
Magenta
Big red
Bright red, slightly yellowish
Deep red
Dark red, slightly purple
Jinhong
Red to yellow
Tian Lan
Cyan slightly bluish
Zhong Lan
Slightly dark blue
dark blue
Dark blue gray
Peacock orchid
Cyan slightly greenish
Light blue
Dark blue to purple, reflecting red light
After the allocation of spot color ink is determined, the proportion of each primary color ink must be remembered. In case you need to adjust the same spot color ink later. Enterprises with spectrophotometers should measure and record the L * a * b * value of the spot color oil for comparison when adjusting it next time. The characteristics of post-processing should also be considered when formulating inks, such as post-processing glazing, and the formulated inks must be resistant to solvents. [next]
Second, the problem of attention to spot color printing
In the printing process, there are many factors that lead to the quality problems of spot color inks. Below we analyze these factors.
1. Paper
a. Whiteness: Papers with different whiteness have different effects on the color of the printing ink layer. The difference of the printing ink layer with different whiteness is mainly reflected in the content of black ink in the spot color ink. Especially for color with brightness above 70, the effect is particularly obvious. Therefore, in the actual production, paper with the same whiteness should be selected for printing, and the same manufacturer, the same brand, the same gram weight, and the same production date and batch number should be used for printing. In order to reduce the influence of paper whiteness on printing color.
b. Absorbency: The same ink printed on paper with different absorbency will have different color effects. The difference in absorptive capacity inevitably makes the color of the printing ink layer different. For example, uncoated paper will appear dull and dull compared to coated paper.
c. Glossiness and smoothness: the color of the printed matter can only be seen by light reflection. If the gloss and smoothness of the paper are high, the color observed at this time is basically the color reflected through the ink layer. If the surface of the paper is rough and the gloss is low, the surface will form diffuse reflection. This will reduce the saturation of the main color light. Make the color we see lighter.
2. Surface treatment
The surface treatment of packaging offset printing products includes laminating (light film, sub ink), glazing, (varnish UV light) and so on. After these surface treatments, the printed matter will have different degrees of hue changes. There are physical and chemical reasons. The physical change is mainly reflected in the increase of the effect of specular reflection on the surface of the product. The chemical change is mainly the coating glue, the solvent in the UV oil reacts with the color of the ink layer of the printed matter. Cause changes in color.
3. Dilution agent: Dilution agent is a colorless transparent substance in the form of a paste. Dilute the color in the spot color. The amount of it has a great influence on the color. Especially blue is the most obvious.
4. Use of spot color ink: Before printing, spot color ink should be sealed and stored, the best temperature is 15 ℃ -25 ℃, relative humidity 65% ​​-70%. Since the spot color ink is prepared from several original inks, the ink should be fully stirred when it is on the machine to control the viscosity and stabilize the ink performance.
5. Drying density value: the printed product just printed, the ink is in a wet state, the density value is high and the color is deep, the density value decreases after the ink is dried, and the color becomes lighter. Therefore, in normal production, the ink is slightly deeper in a wet state.
If a density meter with a polarizer device can eliminate the light generated by reflection on the surface of the ink layer, the measured wet density is very close to the dry density. The thickness of the ink layer should also be controlled. If the control is not good, the color of the spot color has a great influence.
6. Ink and water balance: Ink and ink control of offset printing is very important. If the ink and ink control is not good, especially the emulsification phenomenon is serious, which greatly affects the spot color. Therefore, the control of the amount of water, the PH value of the fountain solution, and the conductivity must be strictly controlled. In short, in packaging offset printing, there are many reasons for the quality problems caused by spot colors. We have to specifically analyze and solve the problems for different reasons in actual production.
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