On the causes and prevention of similar printing f

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Try to talk about the causes and prevention of similar printing faults

first, shrinkage

shrinkage is one of the faults of printing ink film due to poor leveling. After the packaging and printing operation, the wet ink film retracts during the leveling process, exposing the substrate or bottom ink layer in a small circle. The main reason for shrinkage is the different surface tension between the upper and lower parts of the wet ink film. A wet printing ink can be regarded as a double-layer liquid film. In the film-forming process of packaging and printing, shrinkage occurs when the surface tension of the upper wet ink film is lower than that of the lower wet substrate film (due to the wet substrate). It can also be explained that there are too many plasticizers in the packaging printing ink system, which causes the cohesion rebound of the ink after printing, and then shrinkage occurs. Sometimes, when this phenomenon is about to happen, the wet ink film loses its leveling property, which happens to inhibit the occurrence of shrinkage. In some fast drying gravure printing or thermosetting offset printing, with the increase of solvent in hot blowing or baking temperature, the wet ink film has leveling again, which may lead to secondary shrinkage failure

the preventive measure is to improve the wet ink film flow development performance of the packaging printing ink during printing. The surface tension of the printing ink should be low, and the wettability of the substrate should be good. Theoretically, when the distribution coefficient of printing ink w product use: s> 0 (the difference between the interface between the upper wet ink film and the upper and lower wet ink film and the surface tension of the wet film on the surface of the lower substrate), it means that after the printing of packaging printing ink, the surface energy of the system decreases, and the packaging printing ink will self develop into a complete graphic ink film. In practical application, it can be solved by adding appropriate leveling additives (chemical or physical) or low surface tension solvents. In addition, attention should also be paid to the surface treatment of the substrate to improve the wettability of the substrate (i.e. substrate), such as cleaning with solvent or rag or electric spark before packaging and printing

II. Pockmarks and pits

because the wet ink film of packaging and printing ink is stained with foreign particles, these foreign particles are distributed on the wet ink film, which repels the surrounding ink and forms pits like craters, also known as "pits". It can be clearly seen that there are foreign particles in the center of the pit, like fish eyes, so it is called "pitting", which generally does not reveal the substrate

the preventive measures are the same as shrinkage. More attention should be paid to the environment of the packaging and printing workshop and the influence of the solvent or diluent used

III. spots

spots (commonly known as graining) are the general term for irregular blocks on the surface of the ink film after printing. This is due to poor dispersion of pigments, insoluble polymer soft particles or insoluble metal salts in the binder used for color ink, and small pieces of ink film are dispersed and dissolved in ink; The dirty air in the packaging and printing workshop or the accumulation of dust on the surface of the printed substrate are also the reasons for the spots; In addition, the roller coating material is uneven. Once compounded, irregular spots will form on the surface of the substrate, especially the compounding of pure aluminum foil. Therefore, to prevent such failures, we must pay attention to the dispersion and purification of printing ink and the cleanliness of the printing workshop environment

IV. shrinkage

after printing, the wet ink film retracts, exposing a large area of substrate material. This fault is called "shrinkage". The reason is that the distribution coefficient ws of printing ink is a large negative value. This failure is due to poor wetting of the ink on the printed image and text to the substrate. For example, surfaces containing oil stains, plastic surfaces and electrodeposited printing ink surfaces containing too many surface scratch prevention additives; The printing plate is not centered or eccentric for a few microns, or even a few wires, and the shrinkage in the forward direction (but not in the longitudinal part) often causes partial shrinkage (fracture)

the preventive measure is to carry out strict degreasing treatment on the surface of the substrate before printing. After treatment, it must be verified by water film test before printing. For the quality problems of printing plates, one is to replace the printing plates, and the other is to add inking oil or slow and fast diluent for printing. However, measures should be taken to improve the wettability of plastic and aluminum foil surfaces (such as surface modification), and special printing inks with low surface tension or non deteriorated inks should be used as much as possible

v. exposed corner

after printing, the ink film at the corner of the substrate retracts, causing the ink film to be thin and even exposed. This failure is another form of shrinkage failure. Because the surface force of the wet printing ink film is too high, there is a close relationship between the surface energy of the system and the tension value and gauge distance in the tensile sample of rubber material, and the wet ink film retracts from the corner

the preventive measures are mainly to make the packaging and printing workpiece design as far as possible without sharp corners and edges, chamfer and chamfer, or fill only the edges and corners during packaging and printing. In general, thixotropic agent is added to the printing ink to improve the fluidity of the packaging printing ink and prolong the leveling of the ink film

VI. thick edge

is particularly thick at the corners of the ink film after printing. The printed product looks like a mirror frame, often referred to as "crushing". This is because the solvent volatilization of the wet ink film on the corner is faster than that of other places, which often makes the concentration of the pictures and texts formed here higher than that of other places. In addition to the above situation, the temperature is also moderately lower than that of other places in the transportation process, forming a high surface tension area, which makes the adjacent printing ink flow to the corner and thickens the ink film there

the preventive measures are to appropriately increase the thixotropy of printing ink or reduce the printing pressure, and add an appropriate amount of rheological additives through experiments

the above fault analysis and suggestions only fully comply with Hooke's law is the author's very simple treatment method. In order to get rid of this "Murphy's law", we must first cultivate a high degree of work enthusiasm, a strong heart, and devote ourselves to it. Only then can we deal with the troublesome fault affairs in packaging and printing in place in complicated work. Without procrastination and prevarication, it is conducive to reducing mistakes

author/Liu Jiaju

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