In packaging printing, the background color is often printed first to enhance the high quality of the pattern decoration and pursue high added value of the product. In practical operation, it has been found that this printing sequence is prone to ink crystallization. What is the reason behind this?
1、 In order to achieve a bright and bright background, the ink layer is usually printed thick or reprinted once or with increased printing pressure, and more dry oil is added during printing. Although the ink layer completely covers the printing carrier, the rapid drying results in a very smooth ink film layer on the surface of the printing ink after film formation, making it difficult to overprint well, like glass. This makes the ink printed unevenly or completely impossible to print. The oil ink printed on the cover (stack) presents bead like or weak colored printing patterns on the base color, and the ink connection is poor, some of which can even be erased. The printing industry refers to it as ink film crystallization, vitrification, or mirrorization.
In order to improve the clarity of image and text edges, most manufacturers have added silicone oil to ink systems in recent years. However, excessive silicone oil often causes vertical shrinkage of the ink film.
There are currently several different opinions on the reasons for the crystallization of ink films. According to crystallization theory, crystallization is the process of forming crystals from a liquid (liquid or melt) or gas state. A substance whose solubility significantly decreases with decreasing temperature, and whose solution can reach saturation and crystallize through cooling; A substance whose solubility decreases slightly with decreasing temperature, crystallizes when some solvents evaporate and then cool down. Some people believe that the crystallization of packaging printing images and texts (ink film layer) is called recrystallization... The printing ink film system is formed by solvent evaporation (evaporation) and then cooling, also known as recrystallization.
2、 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 pigment crystals are anisotropic, their crystalline state is needle or rod like. When forming ink film, the length direction is easily arranged along the flow direction of the resin (connecting material) in the system, resulting in significant shrinkage; However, there is no directional arrangement during spherical crystallization, resulting in small shrinkage. Inorganic pigments in packaging printing ink systems typically have spherical crystals, such as cadmium based packaging printing ink, which also has small shrinkage (crystallization).
The particle size also affects the molding shrinkage rate and molding shrinkage ratio. When the pigment particles are large or small to a certain extent, the molding shrinkage rate and shrinkage ratio are the smallest. On the other hand, resins with large crystals and spherical shapes exhibit small molding shrinkage, while resins with large crystals and non spherical shapes exhibit large molding shrinkage.
In short, 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 their chemical structure, but also largely depends on their physical properties, such as crystal particle size distribution, condensation phenomena, solid solutions, and other influencing factors; We should also make a fair evaluation of the advantages and disadvantages of both inorganic and organic pigments, so that they coexist, and the latter holds the primary position.
When selecting packaging printing ink (pigment), it is also necessary to 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) Covering power (the absorbance characteristics 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, and the molecular structure symmetry of the pigment are higher than those of the symmetrical low crystal form).
The covering power of the crystalline form is greater than that of the rod shape, and the covering power of the pigments with high crystallinity is greater than that of the pigments with low crystallinity. Therefore, the greater the covering power of the packaging printing ink ink film, the more likely it is to have glass failure. The heat resistance, migration resistance, weather resistance, solubility resistance, and the interaction with polymers (resins in oil ink systems) or additives cannot be underestimated.
3、 Some operators believe that improper selection can also cause crystallization failures. It is because the base ink dries too hard (thoroughly), resulting in a decrease in surface free energy. Currently, if 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 desiccants, if rapid and intense drying methods, such as drying, are used, crystallization phenomenon will occur.
Post time: Nov-22-2023
