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Method for reducing failure rate of poor ink adhesion fastness
Time: 2015-07-30 Source: Cutting Edge of Packaging Read: 12696 times
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Plastic printing and packaging is a highly professional technology. Plastic films often produce a series of problems when gravure printing is performed. One of the common problems is poor ink fastness. How to make poor ink fastness is a quality problem. The probability of minimization is a common concern for packaging and printing companies.
In the actual production process, the test of ink adhesion fastness generally uses transparent tape (the adhesive force of the tape is generally more than 2N / 15mm) to stick and pull the printing surface. After sticking and drawing, the ink does not transfer or transfers to the tape in a small area. Then it is judged that the ink adhesion fastness is up to standard. If the ink is transferred in a large area or completely after sticking, it means that the ink has poor adhesion on the printing film. The poor adhesion of the ink will cause the composite film to have a large area or all of the ink transferred during peeling, resulting in low composite strength, and no effective combined force can be formed between the layers of the composite film. Performance, and because the composite strength is low, the bag may risk delamination during the heat resistance process, which ultimately affects the use of the composite packaging bag. Therefore, there is no composite film with sufficient ink adhesion fastness, even if the printed pattern is even more beautiful, it is as if there is no The foundation of the building can not stand the test of time.
The reasons that affect the ink adhesion fastness are as follows:
First, the impact of the ink resin system Resin is an important part of the ink, its important role is first as a carrier of the pigment, powdery pigments, fillers and other solid materials are mixed and connected, and secondly, the ink can be dried on the substrate Consolidation and film formation. The resin system is one of the most critical factors affecting the fastness of the ink to the substrate. For plastic packaging printing, there are several substrates such as BOPP, PET, NY, etc. Different substrates have different molecular structures. In order to obtain good ink adhesion, the ink resin used in printing must have a good Affinity. Chlorinated polypropylene resin-based inks are generally used for printing polypropylene (PP) films, and polyurethane- based inks are used for printing polyester (PET) films. Although with the development of ink technology, it is suitable for substrates such as BOPP, PET, and NY. General-purpose inks have begun to be used, but in actual use, it is still necessary to effectively identify the type of ink. Misuse of ink or mixing of inks from different manufacturers will cause quality accidents with poor ink adhesion.
Second, the effect of pigment dispersion Pigment as a carrier of the colorful colors of the ink, because most of it is small molecules of organic or inorganic matter, can not directly form effective adhesion with the printing substrate, so the pigment can only be crushed into small, uniform The particles are evenly distributed into the resin to achieve wetting and coating of the pigment surface by the resin, so as to obtain a stable suspension system. The degree of pigment dispersion, in addition to directly affecting the fineness, gloss, viscosity, stability, tinting power and hiding power of the ink, has a great impact on the adhesion of the ink; a good dispersion system makes the pigment At the same time that the fineness is optimal, it also makes the resin's coating of fine pigment particles more intact. After the film is consolidated on the surface of the plastic substrate, a uniform and dense resin interface wrapped with pigment particles is formed, increasing the resin and plastic film. The contact area of the surface improves the adhesion fastness.
Third, the impact of solvents Most plastic gravure printing is still based on solvent-based inks, as an essential organic solvent in the ink system has a great impact on the adhesion of the ink on the surface of the plastic substrate.
1. Whether the solvent in the ink after printing completely affects the adhesion fastness. To make the ink suitable for printing production, it is necessary to ensure that the ink can be fully dried after printing. Generally, two or more mixed solvents are used as the organic solvent in the ink. The release ability of ink resin to organic solvents and the volatilization rate of solvents directly affect whether the solvents in the ink can be completely dried. Solvents with good volatility have better solvent release properties than solvents with slower volatility, but an excessively high proportion of high volatility Soluble solvents will cause the surface of the ink to dry too quickly, but the release of the inner layer of the ink will be hindered, and the drying will not be thorough. In the actual production process, you need to add an appropriate amount of slow-drying or middle-slow-drying solvents to moderate the overall solvent evaporation gradient and improve the solvent volatilization and release ability. .
2. The ratio of the true solvent to the fake solvent in the ink solvent will also affect the adhesion of the ink. Although the holiday solvent can't actually dissolve the resin, its combination with the real solvent can make the polarity of the mixed solvent system in a wide range. Adjustable so as to adapt to the polarity of the ink resin itself and achieve the best dissolving power. Resin molecules are microscopically stretched in a solvent system of suitable polarity, which is conducive to dispersion and wetting during the grinding process with pigments, and forms a uniform encapsulation state. However, if the false solvent is added in an improper proportion, the stability of the resin system will be deteriorated, the micro resin molecules will become warped, and even a plurality of resin molecules will be wrapped together by the false solvent, which will not be able to wet the pigment particles well during the dispersion process. Wetness affects the uniformity of the film formation of the ink on the surface of the substrate, and the adhesion decreases.
3. Swelling effect The organic solvent in the solvent-based ink will cause the ink to interact with the plastic substrate surface when it contacts the plastic substrate, that is, the swelling effect. The swelling effect makes the surface of the plastic substrate rough and forms tiny "pits". After the film is formed, the ink produces a physical anchoring effect, which plays an important role in improving the adhesion fastness.
Fourth, the impact of film surface treatment effect Plastic packaging printing commonly used substrates are BOPP, PET, NY, etc. These substrates are flat and smooth from the surface, after processing and molding without surface treatment can not be directly used for printing, even It is because polar materials such as PET and NY have a large surface tension and cannot fully meet the requirements. Therefore, the surface of these substrates needs to be corona treated. After corona treatment, more active groups are formed on the film surface. It is more easily wetted by most inks and solvents, and improves the adhesion fastness of the ink. Therefore, the surface treatment state of the plastic film needs to be confirmed before printing. In general, the surface tension of BOPP needs to be above 38mN / m, the surface tension of PET needs to be above 52mN / m, and the surface tension of NY needs to be 56mN / m or more.
Fifth, the impact of film moisture absorption performance
Hygroscopic films such as NY have a great impact on printing performance after moisture absorption. Once absorbed, the fastness of ink adhesion decreases. Therefore, when storing and printing such films, keep them as dry as possible, and do not let it get wet. The film must be wrapped with high-barrier materials such as aluminum foil to prevent the film from getting wet. For films that have already absorbed moisture, they should be dried in a curing room before being evaluated for use.
Sixth, the impact of light screen printing In the design of packaging patterns, in order to perfectly reflect the effect of the pattern, the design generally uses a variety of colors of light screen printing to superimpose complex colors. Due to the small amount of ink on shallow screen printing and the problem of dry plate, it is easy to cause the problem that the original amount of ink is small and only transferred to the film. At the same time, due to the small amount of ink on shallow screen printing and insufficient solvent to wet the film, the tiny air interface between the ink and the film cannot be completely ruled out, resulting in poor ink adhesion. For shallow screen printing, in order to effectively avoid the problem of poor ink fastness, slow-drying solvents should be added to the shallow screen ink, the printing speed should be increased as much as possible, and the drying temperature should be increased, and the corrosion plate should be used as little as possible.
7. Conclusion In summary, in plastic gravure printing, there are many factors that affect the fastness of the ink to the film surface. Only comprehensive analysis and evaluation of the ink composition, solvent performance, substrate performance and platemaking process can effectively improve printing. Ink fastness.

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