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The background color is often printed on the packaging and printing to highlight the high quality of the pattern decoration in order to pursue the high added value of the goods.
1. In order to achieve the purpose of bright and bright background, the ink layer is usually printed very thick or reprinted once or the printing pressure is increased, and more dry oil is added. Although the ink layer completely covers the printing carrier, the surface of the printing ink becomes a very smooth ink film layer after being dried too fast, which is difficult to overprint like glass, which makes the printed ink printed unevenly or completely Up, the ink printed on the cover (stack) shows a bead-like or patchy printed pattern on the background color, the ink connection is very poor, and some can even be wiped off. The printing world calls it crystallization, vitrification or mirroring of ink film.
In order to improve the sharpness of the edges of graphics and text, in recent years, most manufacturers have added silicone oil to the ink system, but too much silicone oil will often cause vertical shrinkage of the ink film.
At present, there are several different views on the causes of ink film crystallization. According to crystallization theory, crystallization is the process of forming crystals from a liquid state (liquid or melt) or a gas state. Substances whose solubility decreases significantly as the temperature decreases can saturate the solution and crystallize when the temperature is lowered. Substances whose solubility does not decrease significantly as the temperature decreases will crystallize after part of the solvent is evaporated and then cooled. Some people think that the crystallization of packaging and printing graphics (ink film layer) is recrystallization ... The printing ink film system is evaporated (volatile) and then cooled after cooling, also known as recrystallization.
2. Some people think that the crystallization (crystallization) of packaging printing ink is mainly caused by the crystallization of pigment in the ink system. We know that when the pigment crystal is anisotropic, its crystal state is needle-like or rod-like. When the ink film is formed, the length direction is easy to be aligned along the flow direction of the resin (linking material) in the system, which causes large shrinkage; while the spherical crystal does not have a direction alignment, so the shrinkage is small. Inorganic pigments in packaging and printing ink systems usually have spherical crystals. For example, packaging and printing inks based on cadmium pigments have less shrinkage (crystallization). In addition, its particle size also affects the molding shrinkage ratio and molding shrinkage ratio. For example, when the pigment particles of the same isoindolin are large to a certain degree or small to a certain degree, their molding shrinkage ratio and shrinkage ratio are the smallest. On the other hand, resins with large spherulites exhibit small molding shrinkage, whereas crystals with large spherulites exhibit large molding shrinkage. In short, whether it is the subtractive mixing of color pigments or the additive mixing of color shades, the correct use of pigments is not only related to the chemical structure, but also largely depends on its physical properties, such as crystal phase particle size distribution, aggregation phenomenon, Influencing factors such as solid solution; the advantages and disadvantages of inorganic pigments and organic pigments should also be evaluated fairly, so that the two coexist, and the latter occupies the first place.
In the process of selecting packaging printing inks (pigments), the coloring power must also be taken into account (the finer the dispersion, the greater the coloring power, but there is a limit value, and the coloring power will drop if it exceeds this value), cover Force (the light absorption properties of the pigment itself, the difference in refractive index between the pigment and the resin binder required for coloring, the size of the pigment particles, the crystal form of the pigment, the molecular structure is symmetrical, and the hiding power is lower than the symmetrical low crystal form; the crystal form is a sheet The shape has a greater hiding power than a rod shape, and the pigment with a high crystallinity has a greater hiding power than a low crystallinity, so the greater the hiding power of the packaging printing ink ink film, the greater the chance of glass transition failure), heat resistance, and migration resistance , Weather resistance, solvent resistance and interaction with polymers (resin of ink system) or added additives, etc. can not be underestimated.
3. Some operators believe that if the printing ink film is crystallized improperly, it will also cause crystallization failure. The reason is that the background ink is too hard (dry), the surface free energy is reduced. After the current one-color printing is finished, the storage time is too long, the workshop temperature is too high, or the printing ink desiccant is too much, especially the cobalt desiccant. If it uses a rapid and intense drying method, such as drying, it will produce crystallization.
In view of the crystallization failure after packaging printing ink film formation, the author proposes the following solutions and preventive measures:
One is to overprint with a quick-drying ink containing more solvent (the second color is printed without waiting for it to dry out, although the effect is better, but this method is labor-intensive, time-consuming, and costly), the solvent used can be immersed And soften the underlying ink film layer (but sometimes it will not help).
The second is to master the overprinting time. The second color should be printed as soon as possible after the previous color printing.
The third is to add some polymerization aids (such as hydroquinone) or slow-drying materials (Vaseline, lanolin, wax additives) to the ink to delay drying.
Fourth, different primary colors that are easy to overprint can be used in the pattern design of packaging, printing and decorating.
Fifth, cobalt desiccant should be used less in ink formulations.
Sixth, it is possible to use a fast-drying synthetic resin type binder to reduce the amount of desiccant in the ink.
The seventh is to enhance the adhesion of the second color ink, the purpose of which is to destroy the crystallized ink film. For example, the use of No. 0 ink or alkyd resin, epoxy resin and polyamide resin is a material with high adhesion. The addition of ink can enhance its adhesion, but the miscibility of these resins with offset printing inks is not good and cannot be added. Because too much addition will tend to dilute the color of the printing ink, but too little will not be good.
Eighth, organic solvent is added to the printing ink to dissolve the crystallized ink film layer. This is one of the methods commonly used by printing operators, but the crystallized ink film layer has become the smooth ink film layer of the main network structure, which is not easy to be dissolved, and the effect is not good when too much.
Nine is to add alkali or soap to the printing ink to enhance the polarity, so that it is easier to print, but although it seemed to be printed at the time, it did not adhere well and was easily wiped off after drying.
Looking at the above processing methods and precautions, what can guarantee the quality of packaging and printing graphics is to master the drying column of ink in printing. It is best to start printing the second color without letting the ink film dry.
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