Offset printing inks are sticky and thick pastes, which are physically called plastic fluids. They do not deform or flow like an ideal fluid (water) with external force. They must be applied to a certain degree before they start to deform or flow.
Table 1 Comparison of ink particle area and dot area:
Print outlets
Dot area
15mm ink particle area (micron)
Ratio of dot area to the largest particle area of ​​ink
1133 line 4 points
14589
177
82: 1
1133 line 1 point
3647
177
20: 1
2175 lines 4 points
8426
177
47: 1
2175 line 1 point
2106
177
12: 1
250 lines 4 points
4219
177
23: 1
250 lines 1 point
1032
177
6: 1
As another example, background colors are often used in packaging and printing to highlight and highlight themes, usually magenta, cyan, yellow and other colors, which need to be formulated. This requires an understanding of the color properties of the ink to be handy To obtain the required color, the following points should be noted:
1. Ink color matching
The color mixing of the ink is a subtractive color effect, and the colors of the mix are getting darker and darker, so first of all, you should try to use fewer colors. Generally, you should use two colors to adjust it, and adjust it to a very close one, then add a third color to make micro ink Adjust to achieve the desired color. (As in Table 2)
Table 2 Introduction to the color properties of the Shanghai primary colors:
Variety
density
Color difference%
Grayscale%
Color efficiency%
transparency%
05 ～ 24 Medium yellow
1.032
4.84
7.75
89.83
89.0
05 ～ 28 transparent yellow
1.00
6.25
4.00
93.00
94.5
05 ～ 15 pink
1.465
36.08
12.97
71.33
98.0
05 ～ 14 magenta
1.46
47.07
12.30
67.04
93.2
05 ～ 32 sky blue
1.53
22.70
11.70
78.23
98.0
2. Dilution of ink
When diluting the color, the ink color of the transparent dilute ink looks dark, but it is not dark when printed on white paper, and the ink color looks brighter with pink ink. The light color is required to be brighter without pink, and some people often add a small amount of white ink when using a large amount of transparent dilute ink. Although the color tone meets the brighter requirements, the white ink makes the ink with poor light resistance easier to fade. The shortcomings must be noted. Therefore, it is better not to add white ink when printing paper with light white background. As for paper with poor whiteness, it is another matter. [next]
3. Tone flesh color
Some people often use gold and red ink to dilute the flesh color. Although the color tone is good, gold and red dilute easily fade, so it is not appropriate. Pink ink also fades easily after being diluted. Use 05 ~ 06 scarlet or 05 ~ 09 dark red to dilute the color, although the tone is slightly darker, the fading is much less. If the yellow tone is not enough, orange ink can be added a little. This ink has good light resistance and is not easy to fade. Light yellow and lake blue should be diluted with sky blue ink, not medium blue or dark blue, because it is easy to fade. There are two kinds of green ink and green lotus ink which are not good in light resistance. When diluting the color, good light resistance should also be used, otherwise it will easily fade. The current yellow ink is less lightfast than sky blue and green ink. Therefore, when adjusting the green, you should consider that the yellow color will fade in the future. You need to pay attention to the packaging and printed matter that requires lamination and display windows. In recent years, people have added an appropriate amount of antioxidants to easily faded inks, and excessive amounts will directly affect the drying of the oxidized conjunctiva during printing.
4. The issue of transparency
When using aluminum foil, aluminum foil paper, or gold or silver cardboard, this should be considered as an important point when printing background colors or patterns. For example, the effects of opaque yellow ink and transparent ink printed on the above materials are completely different. Many factories often print a layer of transparent yellow ink on this, which can make the metallic luster reflect through the yellow ink layer and become a golden color. Now usually add the appropriate amount of gloss paste to the ink to achieve the above requirements. Note: â‘ The effect of ink with low transparency will be extremely poor; â‘¡ The opaque whiteness of the ink or thinner adjustment, the effect of adjusting the ink will be the same as above.
So: use transparent pink or magenta, sky blue, green ink to print the background color, so that the aluminum foil paper or gold and silver cardboard can show very beautiful and bright colors, and sometimes use white ink as the primer, which completely covers the aluminum foil The metallic luster of paper or gold and silver cardboard is then printed as if printing a pattern on white paper.
5. The control problem of fineness
The fineness of offset printing ink is generally controlled below 15 microns, and the thickness of the PS graphic film is generally 7 microns to 10 microns, but under the printing pressure and the pulling force of the substrate, the ink power of the ink is controlled at 0.05. The ink film is about 7.5 microns. The fineness is shown in Table 1. As shown in Table 1, the area of ​​250 lines 1 point is 6 times larger than the area of ​​15 micron ink particles, and 40 points is 23 times larger. Moreover, most of the ink particles are much smaller than 15 microns, so the qualified ink will not affect the printing quality due to fineness problems. On the other hand, when adjusting the ink, especially when adding auxiliary agents, such as corn starch, the particles that are large due to the dispersion and clumping will affect the printing quality.
However, except that the ink becomes thick due to the aggregation of particles over a long period of time, or the storage is improper, and the surface of the ink is mixed with the skin, most of the paper is powdered and fluffed and mixed into the ink will also cause the ink to thicken. [next]
6. Control of viscosity
Often used Brookfield Viscometer or domestic Emila viscometer. In fact, this is because the pigment particles in the ink are attracted to each other and flocculate at rest, so they are sticky and agglomerated. When subjected to mechanical agitation, the flocculation force is destroyed, and it becomes thin and easy to flow. But when it is still for a while, it will flocculate again. This is thixotropy. Therefore, the offset ink must determine its viscosity and yield value.
7. Measurement of viscosity and fluidity
The measurement of viscosity and the measurement of fluidity are all related to the adaptability of printing: the printing speed is slow, inks with higher viscosity can be used; the printing speed is faster, inks with lower viscosity can be used. For example, the offset ink used in high-speed four-color offset presses has a lower viscosity than the inks used in ordinary single-speed or two-color offset presses. However, the viscosity of the ink becomes smaller as the temperature increases. After the high-speed offset printing machine starts printing, due to the fast speed of the ink roller and the high temperature, the viscosity of the ink will gradually become smaller, so it does not matter if it is slightly higher. Of course, the substrate also has a relationship with the ink viscosity requirements. Smooth and solid paper can use higher viscosity ink. Loosely structured paper should use ink with lower viscosity, otherwise it will easily cause powdering, rubber hair and other phenomena.
8. Viscosity and viscosity
Viscosity and viscosity are related: that is, under normal circumstances, the viscosity of the ink with a large ink viscosity is also large, but there are times when it is not related, so some inks have similar viscosities, but different viscosities, and some inks have similar viscosities and different viscosity. In this case, the ink should be selected based on viscosity. For example, the color offset ink is more suitable in the range of 5-9, and its range is 5-11. Because of the fast printing speed, the offset printing ink is suitable for a viscosity range of 3 to 6. The viscosity of the ink has a great relationship with the printing speed, the printing speed is fast, the separation speed of the ink layer is also fast, and the viscosity of the ink is also large. These are the three reasons why the offset ink of the web is particularly low. Viscosity has a greater relationship with ink spreading. In the applicable range, ink with greater viscosity is better for ink transfer. The ink layer finally printed on the paper is also thicker, and the dots look more upright. But if the viscosity is too large, it will hit unevenly on the ink roller, and even pile up the ink rollers or plates or blankets. At this time, the dots of the printed matter are also hairy and unreal. There are also solid imported papers that can be printed very well. If you use poorly made paper, you will lose powder, fluff, or even peel. This is caused by the paper's inability to withstand the stickiness of the ink layer when the blanket is separated from the paper. There is also viscosity related to temperature. When the temperature is high, the viscosity decreases. When the offset press is printing, the paper will fluff and peel when the machine is stopped halfway. It is because it has stopped for a period of time and turned cold. . If you print some good paper waste paper first, the car will be normal after driving for a while. On four-color or more-color offset presses, the viscosity relationship between the four-color inks is also very important. Generally speaking, the viscosity of the latter color ink cannot be greater than the previous color, otherwise the ink layer of the previous color will be pulled down .
In addition, there is also a yield value that is related to the fluidity and the "body" of the ink, and is also greatly related to thixotropy. [next]
9. The relationship between ink fixation and drying
Offset printing inks are divided into three types: oil type, resin type, and light fast drying type. But now the latter two types are generally adopted. Therefore, the summary is as follows:
â‘ After the resin ink is printed on the paper, the low-viscosity petroleum solvent and dry vegetable oil can quickly penetrate into the pores on the surface of the paper, so that the pigments wrapped in synthetic resin and dry vegetable oil can solidify on the surface of the paper. This state is called For fixation. At this time, a small amount of prints will not stain on the back when the stacking pressure is not large, but it will stain when the pressure is too large. The fixed ink layer is further oxidized and polymerized to form a strong ink film, which is the real drying. Generally, resin ink has two stages of fixation and complete drying. The fast fixation ink is called fast fixation ink. The fixation is actually measured in seconds, but due to the relationship between the test method and the instrument, it is now calculated in minutes. The drying time is equivalent to the time for the printing to dry completely, in hours.
â‘¡The effect of fast-fixing ink on coated paper, coated paper, or gold and silver cardboard is remarkable. Because the pores on the surface of the coating are very small and many, it can be inhaled in an appropriate amount of petroleum solvent in a very short period of time. Because a certain amount of petroleum solvent cannot be sucked in a short time to fix the ink, the effect is not good. The fixed state of the ink is not really dry, so only a small number of prints can be stacked and cannot be moved by hand, because the stacking is too much, the pressure is too large, or the local pressure is too large when it is moved by hand, only the solidified state The ink layer will still be destroyed and stain the back.
Regardless of fixation or not, the oxygen in the air makes the dry vegetable oil become unsaturated double bonds, first oxygen, then polymerization, gradually increasing molecular weight and finally becoming a strong film, which is the real drying.
10. Ink drying conditions
â‘ Oxygen that needs air;
â‘¡The heat or chemical additives that cause and promote the polymerization-that is, dry oil, etc. In order not to cause scrapping, the instrument can be used to perform an ink drying test of 5 grams to 100 grams. For example, the amount of white dry oil is about 3% to 15% of the ink (too much drying does not increase, it will also cause the emulsification of the ink and easy to paste, etc.), the amount of red dry oil is about 1% to 3% of the ink ( Although the drying effect of red dry oil is several times faster than that of white dry oil, but the surface dries fast, while the internal drying is slow), in practical applications, the two should be mixed as well. [next]
â‘¢ Humidity
The water vapor in the air will be adsorbed on the surface of the paper and the ink layer to prevent the oxygen from contacting the ink, and the oxidative polymerization of the ink will be blocked, resulting in slower drying. When the relative humidity is 85% and 65%, the drying speed of the ink layer of the printed matter will be doubled. When the humidity is as high as 95% or more in rainy weather, the difference will be 2 to 5 times.
â‘£Temperature
Heat is one of the conditions for initiating and promoting oxidative polymerization, and the oxidative polymerization of dry oil itself is also an exothermic reaction, so the temperature level has a great influence on the drying of ink. For example, when the temperature is 25 ℃, it is dry compared with 15 ℃. The speed can be twice as fast.
⑤Wetness
Moisture can delay the drying of the ink, and the acidic substances in the wetting fluid can interact with the metal salts in the desiccant to reduce its drying effect, so the PH value of the wetting fluid above 5.5 has little effect. Below 4.5, the drying time will be doubled. When the PH value is below 3.5, and the amount of water is large, the ink will be emulsified seriously, which will cause the ink to not dry for a long time.
â‘¥Dryness and paper PH value
First, the surface structure of the paper is related to the fixation of the ink. The conjunctiva is dry, and the paper with too much water absorption will absorb the dry oil and synthetic resin in the ink in large quantities, so that the ink surface cannot be formed without dry oil and resin. The strong film layer, so heavier touch or friction will wipe off, which is called powdering on printing. Secondly, due to different manufacturing processes, paper also has different PH values. Alkali is good for ink drying, and acid is not good for ink drying. In addition, in China ’s practice of offset printing production less than 20 years ago, spot colors are often printed to highlight the theme graphics. In order to achieve the purpose of bright and bright background, the ink layer is usually printed very thick or printed twice , Dry oil is also added more. Because the ink layer completely covers the paper and dries too fast, the surface of the ink layer often becomes a very smooth film layer, like glass, so that the second color cannot be printed, and the phenomenon of poor adhesion even if printed is called an ink film. Crystallization or vitrification. The ink film has been crystallized. To print the second color, there are two methods in principle: one is to enhance the adhesion of the second color ink; the other is to destroy the crystallized ink film, alkyd, epoxy, Polyamide resin is a material with high adhesion, or adding tackifying resin to the ink can enhance its adhesion, but these resins are not compatible with the ink. Therefore, it cannot be added more, and the color hue of the ink is diluted by adding more. The third is to add a strong organic solvent to the ink to dissolve the vitrified ink film, which has been used by printing operators for many years, but the crystallized ink layer becomes a smooth film layer with a three-dimensional network structure and is not easily dissolved, so the effect is also Not very good, some add alkali and soap to the ink to enhance its polarity, making it easier to print, but although it was printed at the time, it did not adhere well after drying and was easy to wipe off. In recent years, the coupling agent and the tackifying resin have been used together to solve the problem, and the effect is very good. There are also large areas that are completely covered with clear ink or No. 0 ink or resin gloss oil. The second color ink can be printed without waiting for it to dry out, and the effect is better. However, this method is labor-intensive, time-consuming, costly, and costly.
11. High speed drying problem
One is to add high-efficiency dispersant in the ink to make the printing ink film layer dense and thin ink layer, which is not only conducive to fixing and drying, but also can improve the staining phenomenon on the back of the printed part; Class (approximately 75:25) methods, as well as methods of mixing starch and talc, are all effective ways to increase the drying speed of offset inks. Another example is the application of mixed acid dimethyl (DBE) in the past year, which has improved the molecular weight of the ink, and improved the gloss after too much kerosene, and the defects of penetrating the back.
Note: In order to achieve instant drying, a UV printer can be used to print aluminum foil and gold and silver card substrates (using UV light curing ink).
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