Causes and Prevention of Similar Printing Failures

In the past month, the author found in the visits of packaging and printing companies in Henan, Beijing, Heilongjiang, Liaoning, Shandong, Jiangsu, Shanghai, Zhejiang and Hunan that a similar packaging and printing problem has plagued people and caused difficulties for enterprises. After more than 30 days of practice, we now try to talk about the following points and communicate with our peers.

First, the shrink hole

Shrinkage is one of the failures of printing ink film due to poor leveling. After the packaging printing operation, the wet ink film retracts during the leveling process, forming a small round bare substrate or ink layer. The main cause of shrinkage is the difference in the surface tension of the upper and lower parts of the wet ink film. A wet printing ink can be considered as a double layered liquid film. In the film-forming process of package printing, shrinkage occurs when the surface tension of the upper wet ink film is lower than the surface tension of the lower wet film (due to wet substrate). It can also be explained that shrinkage occurs when the cohesive force of the ink after printing is rebounded due to too much plasticizer in the packaging printing ink system. Occasionally, when this phenomenon is about to occur, the wet ink film loses its leveling property, and it happens to suppress the occurrence of shrinkage holes. In some quick-drying gravure or thermosetting offset printing, as the solvent increases in the hot air blow or the baking temperature increases, the wet ink film is leveled again, which may cause secondary shrinkage cavity failure to occur. .

The preventive measure is to improve the wet ink film flow performance of the packaging printing ink during printing, the surface tension of the printing ink is lower, and the wettability of the substrate is better. Theoretically speaking, when the spreading factor of the printing ink is Ws>0 (the difference between the interface between the upper wet ink film and the upper and lower wet ink films and the surface tension of the wet film on the lower substrate surface), the surface of the system is printed after printing of the packaging printing ink. Can fall, packaging printing ink will be developed into a complete graphic ink film. In practical applications, it can be solved by adding an appropriate leveling aid (chemical or physical) or a low surface tension solvent. In addition, it should also pay attention to the surface treatment of printing materials to improve the wettability of substrates (ie, substrates), such as the use of solvents or rags or electric sparks to clean the packaging before printing.

Second, pits and pits

As the wet ink film of the packaging printing ink is accompanied by foreign particles, these foreign particles are spread on the wet ink film, repelling the surrounding ink to form pit-like craters, also known as “pockets”. It can be clearly seen that there are foreign particles in the center of the pit, such as fish eyes, so it is called "matte" and generally does not reveal to the substrate of the substrate.

Preventive measures with shrinkage holes should pay more attention to the environment of the packaging and printing shop, the influence of the solvent or diluent used, etc.

Third, the spot

Spots (commonly known as graining) are the generic term for irregular patches on the surface of the ink film after printing. This is due to the poor dispersion of pigments, the presence of insoluble soft polymer particles or the precipitation of insoluble metal salts in the binder used in color inks, and the indiscriminate dissolution of small pieces of ink film skin in ink, etc.; Dust or dust on the surface of the substrate being printed is also a cause of spotting; also, when the roller is coated with a non-uniform material, once it is compounded, the surface of the substrate will form irregular spots, especially pure aluminum foil. Compounding is more obvious. Therefore, to prevent the occurrence of such failures, it is necessary to pay attention to the dispersion and purification of printing ink and the cleanliness of the printing shop environment.

Fourth, shrinking

After the packaging printing ink is printed, the wet ink film is retracted, so that a larger area of ​​the printing material is exposed. This type of failure is called "shrinkage." The reason is that the spread coefficient Ws of the printing ink is a large negative value. This failure is due to poor wetting of the printing material by the printing ink. For example, oily surfaces, plastic surfaces, and electrodeposited printing ink surfaces that contain too much anti-scratch additives on the surface; plates that are not centered or offset by a few microns or even a few wires, shrink in advance (but not in the longitudinal direction), often Causes partial shrinkage (breakage).

The precautionary measure is to carry out strict degreasing treatment on the surface of the substrate before printing, and after processing, it must be verified by the water film test before printing. For the quality of printing plates, one is to change the printing plate, and the other is to add printing ink or slow thinner for printing. However, for plastics and aluminum foils, measures should be taken to improve the wettability (for example, surface modification), and special printing inks with low surface tension or non-degraded inks should be used.

Fifth, the dew

After the packaging printing ink is printed, the ink film at the corner of the substrate is retracted, resulting in a thinner ink film at the place, or even a bottom exposed. This failure is another form of shrinkage cavity failure due to the high surface force of the wet printing ink film, which tends to minimize the surface energy of the system and the wet ink film retracts from the corners.

Preventive measures are mainly to make the packaging and printing of the workpiece as possible without sharp corners, sharp edges, chamfering, inverted edge, or in the packaging and printing only in the edge, corner complement. In general, thixotropic agents are added to the printing ink to increase the fluidity of the packaging printing ink to prolong the leveling of the ink film.

Sixth, thick edge

The corners of the printed ink film are particularly thick, and the finished printed product looks like a frame, and is often called "crush." This is due to the fact that the wet ink film solvent volatilization on the corners is faster than elsewhere, and the concentration of the film-forming image is often higher than elsewhere, and the temperature is also lower than elsewhere, forming a high surface tension area, which makes the adjacent printing. The ink flows to the corners and thickens the ink film there.

The preventive measure is to appropriately increase the thixotropy of the printing ink or reduce the printing pressure, and add an appropriate amount of a rheological aid through experiments.

The above failure analysis and suggestions are just the author's extremely simple approach. In order to get rid of this "non-inflected law," we must first cultivate a high degree of enthusiasm for work, a strong sense of responsibility, and dedication, and will be able to handle the rigorous breakdowns of the packaging and printing in a complicated job. Pushing it will help reduce mistakes.

(Author/Liu Jiaju)

From - China Packaging