Relationship between packaging printing process and film quality of ink
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In the packaging and printing, the background color is often printed to highlight the high quality of the pattern decoration to pursue the high added value of the product.
First, in order to achieve the purpose of bright and bright background, generally, the ink layer is printed thickly or reprinted once or increased in printing, and the dry oil is also added more. Although the ink layer completely covers the printing carrier, the surface of the printing ink becomes a very smooth ink film layer due to excessive drying. It is difficult to overprint as well as the glass, so that the ink printed on the back is not printed uniformly or completely. Up, the ink printed on the cover (stack) presents a bead-like or block-colored print pattern on the ground color, the ink connection is poor, and some can even be wiped off. The printing industry calls it ink film crystallization, vitrification or mirroring.
In order to improve the sharpness of the edges of the graphic, in recent years, most manufacturers add silicone oil to the ink system, but too much silicone oil often causes vertical shrinkage of the ink film.
At present, there are several different opinions on the causes of crystallization of the ink film. According to crystallization theory, crystallization is the process of forming crystals from a liquid (liquid or solvate) or gaseous state. A substance whose solubility is significantly reduced as the temperature is lowered. The solution is saturated and crystallized by lowering the temperature; the substance having a small decrease in solubility with a decrease in temperature is cooled and crystallized when a part of the solvent is evaporated. Some people think that the crystallization of the package printing image (ink film layer) is recrystallization... The ink film system is evaporated (volatility) and then cooled, which is also called recrystallization.
Second, some people believe that the crystallization (crystallization) of packaging printing ink is mainly caused by the crystallization of pigments in the ink system. We know that when the pigment crystal is anisotropic, its crystal state is needle and rod. When the ink film is formed, the longitudinal direction is easily aligned along the flow direction of the resin (linking material) in the system, so that a large shrinkage occurs, and in the case of the spherical crystal, there is no direction alignment, and thus the shrinkage is small. The inorganic pigment in the packaging printing ink system usually has a spherical crystal, for example, a packaging printing ink of a cadmium pigment, and its shrinkage (crystallization) is also small. Also, the particle size also affects the molding shrinkage ratio and the molding shrinkage ratio. For example, when the pigment particles are large to a certain extent or small to a certain extent, the molding shrinkage ratio and shrinkage ratio are minimized. On the other hand, the resin having a large crystal spherulitic crystal has a small shrinkage, whereas the crystal is large, and the aspherical crystal has a large shrinkage. In short, whether it is the subtractive mixing of color pigments or the additive blending of color shades, the correct use of pigments is not only related to chemical structure, but also depends on its physical properties, such as crystal grain size distribution and agglomeration. Influencing factors such as solid solution; it should also make a fair evaluation of the advantages and disadvantages of inorganic pigments and organic pigments, so that the two coexist, while the latter has the primary position.
In the process of selecting packaging printing ink (pigment), it must also consider its tinting strength (the finer the dispersion, the higher the tinting strength, but there is a limit value, beyond which the tinting strength will decrease), covering Force (the absorption characteristic of the pigment itself, the difference in refractive index of the resin binder required for pigmentation and coloration, the pigment particle size, the crystal form of the pigment, the symmetry of the molecular structure is higher than the opacity of the symmetrical low crystal form; The shape is larger than the rod-like hiding power, and the pigment with high crystallinity has a larger hiding power than the crystallinity, so the greater the hiding power of the packaging printing ink film, the more the probability of failure of vitrification), heat resistance and migration resistance. Weather resistance, solvent resistance, and interaction with polymers (resins of ink systems) or additives added can not be underestimated.
Third, some operators believe that if the printing ink film is crystallized improperly, it will also cause crystallization failure. Because the underlying ink dries too hard (dry), the surface free energy is reduced. After the current one color printing, 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 a rapid and intense drying method, such as drying, is used, crystallization occurs.
In view of the crystallization failure of the packaging printing ink after film formation, the author proposes the following solutions and preventive measures:
One is to use a quick-drying ink cover with more solvent (not to wait for it to dry to print the second color, although the effect is better, but this method is laborious, time-consuming and costly), and the solvent used can be immersed. And soften the underlying layer of ink film (but sometimes it doesn't help).
The second is to master the overprinting time, and the second color should be printed as soon as possible after the previous color printing.
The third is to add some slow-drying polymerization aids (such as hydroquinone) or slow-drying materials (Vaseline, lanolin, wax additives) to the ink.
Fourth, different basic colors that are easy to overprint can be used in the design of the packaging and printing decoration.
Fifth, cobalt desiccant should be used less in the ink formulation.
Sixth, a fast drying synthetic resin type binder can be used 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. If the zero varnish or alkyd resin, epoxy resin and polyamide resin are the materials with higher adhesion, the adhesion can be enhanced by adding the ink, but the miscibility of these resins with the offset ink is not good. Because too much addition will often dilute the color of the printing ink, and too little effect will not be good.
Eight is to add an organic solvent to the printing ink to dissolve the crystallized ink film layer. This is one of the commonly used methods of the printing operator, but the smoothed ink film layer which has become the main mesh structure of the crystallized ink film layer is not easily 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, making it easier to print, but at the time it seems to be printed, but after drying it is not firmly attached and it is easy to wipe off.
Looking at the above treatment methods and preventive measures, it is possible to ensure that the quality of the printed and printed images is the dry column of the ink to be mastered in the printing, so that it is best to start printing the second color without allowing the ink to form a film over the head.