How to deal with ink crystallization in the hottes

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Is there always ink crystallization in printing? How to deal with it

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core tip: on packaging and printing, the background color is often printed first to highlight the high quality of pattern decoration in order to pursue high added value of goods. In practice, it is found that such a printing sequence is prone to ink crystallization. What is the reason? How to solve this problem

I. in order to achieve the purpose of bright and bright background color, generally, when printing, the ink layer is printed very thick, reprinted once, or the printing pressure is increased, and more dry oil is added. Although the ink layer completely covers the printing carrier, due to the rapid drying, the surface of the printing ink becomes a very smooth ink film under the concerted efforts of the member units of the XPS special committee, which is as difficult to overprint as glass, so that the oil and ink printed later are uneven or completely unable to print, and the ink printed on the cover (stack) presents bead like or weak printing patterns on the background color, The ink connection is poor, and some can even be erased. The printing industry calls it crystallization, vitrification or specularization of ink film

in recent years, most manufacturers have added silicone oil to the ink system in order to improve the clarity of the edges of graphics and texts, but too much silicone oil often causes vertical shrinkage of the ink film

at present, there are several different opinions on the reasons for the crystallization of ink film. According to the crystallization theory, crystallization is the process of forming crystals from liquid (liquid or melt) or gaseous state. For substances whose solubility decreases significantly with the decrease of temperature, the solution can be saturated and then crystallized by cooling; Substances whose solubility decreases little with the decrease of temperature crystallize when part of the solvent evaporates and then cools. Some people think that the crystallization of packaging and printing graphics and texts (ink film layer) is recrystallization... The printing ink film system is formed by solvent evaporation (researchers at the German University of technology camness developed a series of bio based fiber plastic composites that can be produced in a large scale, volatilization) and then cooling, also known as 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 length direction is easy to be arranged along the flow direction of the resin (binder) in its system, resulting in large shrinkage; However, there is no main operating factor affecting the friction coefficient during spherical crystallization: directional arrangement, so the shrinkage is small. Inorganic pigments in the packaging and printing ink system usually have spherical crystals. For example, the packaging and printing ink of cadmium pigments has small shrinkage (crystallization)

and its particle size will also affect the molding shrinkage and molding shrinkage ratio. When the pigment particles are large to a certain extent or small to a certain extent, the molding shrinkage and shrinkage ratio are the smallest. On the other hand, the resin with large crystallization and spherulite has small molding shrinkage, while the resin with large crystallization and non spherulite has large molding shrinkage

in a word, whether it is the subtractive mixing of color pigments or the additive mixing of color light, the correct use of pigments is not only related to the chemical structure, but also largely depends on its physical properties, such as crystal particle size distribution, condensation, solid solution and other influencing factors; We should also make a fair evaluation of the advantages and disadvantages of inorganic pigments and organic pigments, so that they coexist, and the latter occupies the primary position

in the process of selecting packaging printing ink (pigment), we must also consider its coloring power (the finer the dispersion, the higher the coloring power, but there is a limit value, beyond which the coloring power will decrease) Hiding power (the light absorption characteristics of the pigment itself, the difference between the refractive index of the pigment and the resin binder required for coloring, the size of the pigment particles, the crystal form of the pigment, the higher the molecular structure symmetry, the lower the hiding power of the symmetrical low crystal form.

the hiding power of the crystal form in flake shape is greater than that of the rod shape, and the hiding power of the pigment with high crystallinity is greater than that of the low crystallinity, so the greater the hiding power of the ink film of packaging and printing ink, the more the failure probability of vitrification) The heat resistance, migration resistance, weather resistance, solubility resistance and the interaction with polymers (resins of ink systems) or added additives should not be underestimated

III. some operators believe that crystallization failure will also be caused if improper selection is made

because the background ink is too hard (dry through), the surface free energy is reduced. At present, the storage time after one color printing is too long, the workshop temperature is too high, or there are too many printing ink desiccants, especially cobalt desiccant. If it adopts fast and intense drying methods, such as drying, crystallization will occur

1. Cover printing with fast drying ink containing more solvents (printing the second color before it dries completely, although the effect is good, but this method is labor-consuming, time-consuming, material consuming, and costly). The solvent used can immerse and soften the lower layer of ink film (but sometimes it will be useless)

2. Master the overprint time, and print the second color as soon as possible after the first color printing

3. Add some polymerization AIDS (such as hydroquinone) or slow drying materials (Vaseline, lanolin, wax additives) to the ink

On the one hand, it is convenient to clamp the sample

4. Different primary colors that are easy to overprint can be used in the pattern design of packaging, printing and decoration

5. Cobalt desiccant should be used less in ink formula

6. Synthetic resin binder with fast drying can be used to reduce the amount of desiccant in the ink

7. The purpose of enhancing the adhesion of the second color ink is to destroy the crystallized ink film. For example, Zero Ink blending oil or alkyd resin, epoxy resin and polyamide resin are materials with high adhesion, and their adhesion can be enhanced by adding them to the ink, but these resins are not miscible with offset printing ink, so they cannot be added more, because too much addition often dilutes the color of printing ink, and too little will not have a good effect

8. Add organic solvent to the printing ink to dissolve the crystallized ink film. This is one of the methods commonly used by printing operators, but the crystallized ink film has become a smooth ink film with a main structure, which is not easy to be dissolved, and the effect is poor when it is too much

9. Add alkali or soap to the printing ink to enhance the polarity, making it easier to print, but although it seemed to be printed at that time, it was not firmly attached after drying, and it was very easy to erase

looking at the above treatment methods and preventive measures, what can ensure the quality of packaging and printing pictures and texts is to master the dryness of the ink during printing, and it is best not to let the ink film dry too much and start printing the second color

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