In flexible packaging printing, the phenomenon of reverse adhesion of printed matter often occurs, that is, when the printed matter enters the next process (printing product is usually slicing process; lithographic product is usually composite process), the graphic part of the printed matter is glued to the back of the printing substrate, the pattern text are destroyed. This phenomenon is called adhesion.
Reverse adhesion of printed matter is the most fatal quality problem in soft package printing, which will lead to the scrap of products bring about great losses. In the high temperature season, the negative adhesion is more likely to occur, but the reasons for the negative adhesion are various, high temperature is not the only reason. Therefore, in the actual production, we should consider analyze from many aspects, find out the root causes combined with the specific situation, so as to avoid such failures, ensure smooth production reduce unnecessary economic losses.
The author will analyze the causes preventive measures of the negative adhesion of soft package printing from the following aspects.
Improper selection of printing ink
Some printing factories often only pay attention to the price of ink, blindly choose low-cost ink in order to reduce costs, neglect the printing suitability of ink. If the printing suitability of the ink used is poor, it will often cause poor ink adhesion or poor drying, which will lead to the ink falling off naturally after winding or the reverse adhesion of the printed matter. Therefore, when choosing ink, attention should be paid to the use instructions of the ink to ensure that the performance of the selected ink meets the printing requirements. In addition, when printing, the adhesion of ink should be tested to ensure that the requirements can be met before batch printing.
Poor ink drying
Poor drying of ink is one of the causes of negative adhesion of printed matter, the phenomenon is more obvious. The poor drying of ink mainly includes two aspects: one is that the drying speed of ink is too slow; the other is that the ink is pseudo-drying, that is, the ink surface has dried, but the ink interior has not been completely dried.
Volatile drying inks are commonly used in plastic flexible packaging printing. Solvent evaporation is used to achieve ink drying. Therefore, the speed of solvent evaporation is an important factor to determine the drying condition of ink layer the quality of printing. The slower the solvent evaporates, the better the reproducibility of the printing layers, the brighter more beautiful the color is, but it is prone to adhesion; the faster the solvent evaporates, the easier the ink dries, but the faster the volatilization, the whiter the printing will be. Therefore, the selection of solvent with appropriate volatilization rate is the key to plastic flexible packaging printing.
In the process of drying analysis identification of printed matter, the ink can be touched by hand. If the imprint is not dry, the drying speed of the ink is too slow. The dilution solvent with faster volatilization can be replaced to adjust the ink. The drying speed of the ink can be accelerated by increasing the temperature of hot air increasing the volume of hot air. If the ink surface is dry, but the drying is not complete, it indicates that it belongs to the phenomenon of ink pseudo-drying. It is necessary to adjust the solvent ratio, reduce the content of fast-drying solvent, properly increase the content of solvent with slower drying speed, at the same time, increase the hot air temperature the amount of hot air to ensure that the ink layer on the printed matter is completely dry.
Solvent residue is another cause of negative adhesion of printed matter, which is difficult to prevent by printing factories. When identifying, a large area of ink can be pressed with the palms of both hands. If a sticky his found in the printed matter a few minutes later, it indicates that there is residual solvent.
Solvent residue is mainly due to improper proportion of true false solvents in ink solvents insufficient purity of solvents. Printers should high purity solvents to prepare ink diluents contact ink manufacturers to obtain the ratio of ink diluent solvents to ensure that the solvent evaporates completely avoid the negative adhesion caused by solvent residues.
Thin film with static electricity
The printing surface of the film can be touched by hat the winding point to check whether the film has static electricity. If the film has static electricity, it is necessary to check the ground wire of the printing press to ensure that the ground wire is connected, so as to avoid the transfer of ink due to the influence of static electricity cause the reverse adhesion of the printed matter.