In packaging printing, the background color is often printed first to enhance the high quality of pattern decoration to pursue high value-added products.
First, in order to achieve the purpose of bright and bright background, the ink layer is generally printed at a very thick or reprinted once or to increase the pressure, dry oil also added more. Although the ink layer completely covers the printing substrate, the surface of the printing ink becomes a very smooth ink film layer due to too fast drying, and it is difficult to overprint the printing ink so that the ink printed afterwards is printed unevenly or completely. Up, the ink printed on the cover (overlay) shows a printed pattern with a very weak bead-like or block-like color on the background color. The ink is poorly bonded 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 graphic edge, most manufacturers have added silicone oil to the ink system in recent years, but excessive silicone oil often causes vertical shrinkage of the ink film.
At present, there are several different opinions on the reasons for the crystallization of the ink film. According to the crystallization theory, crystallization is the process of forming a crystal from a liquid (liquid or molten) or gaseous state. Substances whose solubility decreases with a decrease in temperature can be saturated and crystallized by lowering the temperature; substances with a small decrease in solubility as the temperature decreases, and then cooled and crystallized when part of the solvent evaporates. Some people think that the crystallization of the packaging and printing graphic (ink film layer) is recrystallization. The ink film system formed after printing is evaporated (evaporated) and then cooled. It is also called recrystallization.
Second, some people think that the crystallization (crystallization) of packaging printing inks 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 or 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; whereas the spherical crystals do not have a directional alignment, so the shrinkage is small. The inorganic pigments in the packaging printing ink system usually have spherical crystals. For example, a cadmium pigment-based packaging printing ink has small shrinkage (crystallization). In addition, the particle size also affects the molding shrinkage and the molding shrinkage ratio. For example, the same isoindoline minimizes the molding shrinkage and shrinkage ratio when the pigment particles are large or small to a certain extent. On the other hand, the resin with a large spherulite crystallize has a small shrinkage of molding, conversely, the crystallize is large, and the aspherical crystal has a large mold shrinkage. In short, whether it is the subtractive mixing of color materials or additive mixing of color shades, the correct use of pigments is not only related to the chemical structure, but also depends largely on its physical properties, such as the grain size distribution and condensation phenomena of crystal phases. Solid solution and other influencing factors.
The tinting power of the packaging printing ink (pigment) must also be taken into consideration in the process of selection (the more finely dispersed, the higher the tinctorial strength, but there is a limit beyond which the tinctorial strength drops). Force (absorption characteristics of the pigment itself, difference in refractive index of the resin binder required for the pigment and the coloring, pigment particle size, crystal form of the pigment, high molecular structure symmetry is lower than the hiding power of the symmetric low crystal form; Shaped than the bar-like hiding power, pigments with high crystallinity than the low hiding power of crystallinity, so the greater the hiding power of the packaging printing ink ink film, the greater the chance of failure of vitrification), heat resistance, migration resistance , Weather resistance, solvent resistance, and the interaction with the polymer (resin of the ink system) or additives added, etc. are not to be underestimated.
Third, some operators believe that if the ink film is crystallized improperly, it will cause crystallization failure. It is because the underlying ink is too hard (dry) and the surface free energy is reduced. The current one-color printing is too long to store, the workshop temperature is too high or too much desiccant printing ink, especially cobalt desiccant, if it uses a rapid and intense drying methods, such as drying will have a crystallization phenomenon.
For the crystallization failure after film formation of packaging printing ink, the author proposes the following solutions and preventive measures:
The first is to use a quick-drying ink containing more solvent to cover the ink (not to wait for it to dry and print the second color, the effect is good, but this method is labor- and time-consuming and costly), and the used solvent can be immersed. And soften the underlying ink film layer (but sometimes it doesn't help).
The second is to master the printing time and print the second color as soon as possible after printing in the previous color.
The third is to add some slow drying aids (such as hydroquinone) or slow drying materials (petrolatum, lanolin, wax additives) to the ink.
The fourth is to use different basic colors that can be easily overprinted in the pattern design of packaging and decoration.
Fifth, cobalt-based desiccants should be used less in ink formulations.
Sixth, it is possible to use quick-drying synthetic resin binders 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. Such as the use of zero-grade varnish or alkyd resin, epoxy resin and polyamide resin is a material with large adhesion. Adding ink can enhance its adhesion, but the miscibility of these resins and offset printing ink is not good and cannot be increased. Due to too much addition, the color of the printing ink is often diluted, and too little of the effect is not good.
Eighth, an 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 main network structure of the smooth ink film layer is not easily dissolved, too much effect is not good.
Nine is to add alkali or soap in the printing ink to enhance the polarity, so that it is easier to print, but at the time it seems to be printed on it, but it is not easy to be wiped off after drying.
Looking at the above processing methods and preventive measures, we can ensure that the quality of packaging and printing graphics is the drying column to master the ink in printing, so that it is best to start printing the second lane of color without letting the ink film dry.
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