Liquid packaging, especially for high temperature cooking and sterilization, has developed rapidly in recent years. Matching printing inks are also required to be more environmentally friendly-black and white milk PE film for milk special packaging, etc. Most of them use soluble surface printing inks to meet environmental standards.
How to judge the quality of liquid packaging film
Liquid packaging film refers to the packaging film of soy sauce and vinegar, which can withstand acid, alkali, and endurance. It is commonly used in the mixed production of LLDPE, LDPE, EVA and color masterbatch. To determine whether a liquid packaging film is good or not, there are the following requirements:
1. The static and dynamic friction coefficient of the liquid packaging film should be low, generally controlled between 0.2-0.4, in order to fill quickly in a short time on the high-speed filling and packaging machine. If the friction coefficient is high and the filling speed is low, the filling will be insufficient in a certain period of time.
2. The liquid packaging film can only be printed on the surface, the fastness should be high, it can withstand the hot filling of pasteurization, and the ink does not fall off at a temperature of -20 ℃ in cold storage.
3. Must have sufficient strength to ensure that it will not be pulled under the tension of the high-speed automatic filling machine. Generally speaking, this kind of film is a single layer with a thickness above 10S. It is often mixed with two grades of LDPE with a high melt index of 4-6g / 10min and a low MI of less than 1g10min. Or, it is made by mixing HDPE and color masterbatch and blow molding. The high temperature resistant film must be subjected to 121 ° C sterilization treatment, with or without shrinkage, and good heat sealability. Therefore, it is necessary to select ink and color masterbatch correctly. If the resin in the formula is too sticky, the melt resin will stick to the edge of the seal during fusion sealing; if too much slip agent is used in the formula, the strength of the fusion sealing is not high and it is easy to open.
Ink fastness is not good, easy to discolor
The factors that cause the low ink adhesion of the liquid packaging film are:
1. The corona treatment of the substrate is less than 40 dynes / cm, and the surface is dense and smooth and has poor affinity for ink.
2. The addition of oily materials in the masterbatch is not enough. Although the gloss is improved and the friction coefficient is reduced, the printing fastness is reduced and the heat seal fastness is reduced. Therefore, the choice of the masterbatch is crucial. Improve the adhesion fastness of the ink to the printing substrate, and enhance the compatibility of the binder in the ink with the printing substrate film. Choose a solvent that has good swelling or solubility to the printing substrate film, increase the content of binder in the ink, and add the amount of tackifying resin in the ink to improve the adhesion of the ink.
3. The misalignment of the added solvent does not meet the actual standard.
4. The ink is too old and contains impurities, which must be filtered.
5. The added solvent contains moisture.
Liquid packaging film has a noise when printing, and it sticks after winding
After the plastic film is wound up by gravure, divided and cut into packaging bags and stacked, the ink or ink between the printing surfaces and the blank side of the printing are stuck together and cannot be separated (strong pulling will cause the ink to fall off and the product will be scrapped) . It should be analyzed from the causes that may lead to adhesion, that is, the ink is sticky, the humidity is too high, and the pressure is too high.
1. The internal factors that cause blocking: the ink itself has poor heat resistance and has a certain viscosity.
When a large amount of solvent remains in the printed ink, it is as if the paint is not dry-although it may seem dry to the naked eye, it is actually dry and sticky. After the printed matter is rolled up, the residual solvent is difficult to evaporate, and the resin in the ink cannot be dried and solidified, which causes serious adhesion of the finished product.
If the product with adhesion is measured by gas chromatography, the residual solvent content often reaches tens of thousands of PPM, and the residual solvent will make the product contain odor. This not only affects the compound strength, but also the food flavor and hygiene standards. Start with the performance, drying conditions, and load of the dryer; secondly, use fast-drying flux as much as possible; finally, the printed material should be stored in a moisture-proof place, in order to prevent the moisture in the raw material from expanding the resin in the ink and causing stickiness.
2. Improper ink ratio causes stickiness. For inks with special properties (heat resistance, oil resistance), a hardener is added to this ink, and the hardener is a low-molecular substance, which is inherently sticky. The resin has poor heat resistance. In the preparation of gravure inks with synthetic resins, resins with a high ring and ball softening point should be used. Generally, the ring and ball softening points selected from 105 ° C to 0 ° C are too low for heat resistance.
3. Rewinding is too tight. During the printing process, the film should not be too stretched or too long or too tight. Because the printing machine keeps running, the winding volume is increasing, especially when a certain part of the network is too deep, the area is too large, and the ribs tend to stick, and blocking will occur.
4. Ink and substrate are not bonded well. Solve this problem can be tried from two aspects, on the one hand is the correct selection of ink, control the quality of the ink to adapt to the substrate. On the other hand, the surface of the substrate should be treated to improve its affinity for ink. It is best to use single-sided treatment to prevent the phenomenon that both sides of the tension are good. This is because the double-sided electrical treatment has too good affinity for the ink surface that is stuck on, resulting in anti-sticking.
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