In packaging offset printing, many products need post-press processing. Among the many processes of post-press surface treatment, there is the processing of coating. However, many quality problems will be encountered in the process of processing, many of which are related to packaging offset printing. We said that the effect of lamination is not only related to the material of the film and the process of the film, there are many reasons related to printing. In order to ensure the quality of film processing, we must analyze those factors related to quality, find out the reasons, and solve the quality problems. This article discusses some of my views on the impact of packaging offset printing on lamination.
The factors related to packaging printing are roughly: the thickness of the ink layer of the printed matter, the area of ​​the graphic, the nature of the printed ink, the nature of the printing paper, the printing process and so on. These factors affect the mechanical binding force, physical and chemical adhesion. As a result, the adhesive properties of the film on the surface of the packaging printed matter have changed, resulting in quality problems.
1. The thickness and area of ​​the ink layer of packaging and printing
Some packaging and printing products have a large graphic area, and some are printed on the ground, so that the ink layer is relatively thick. The thick ink layer is often difficult to adhere to the plastic thin film, and it will delaminate and foam soon after the adhesion. This is because the thick ink layer changes the porous surface characteristics of the paper, closing the paper fiber capillary pores, hindering the penetration and expansion of the adhesive on the paper. In this way, the fastness of the coating is greatly affected. In addition, changes in the thickness and area of ​​the ink layer cause changes in surface tension, which in turn causes changes in adhesion fastness. Therefore, when packaging and printing, we must strictly control the amount of ink printed. Control the field density value. Compared with other printing, the ink layer of packaging offset printing has the lowest thickness of the ink layer, which is beneficial to the lamination, and is generally controlled at 2-3µm.
2. Ink for packaging and printing
The film products for packaging and printing should use the fast-setting bright offset printing ink. Because the fast-set bright offset printing ink, its connecting material is composed of synthetic resin, dry vegetable oil, high boiling point kerosene and so on. The molecular groups in these synthetic resins and the molecular groups in the cross-tackifier will expand and penetrate each other, produce cross-linking, and form a physical and chemical binding force, which is conducive to coating. In the application of packaging offset printing ink, it should be noted that the kerosene should not be excessive, otherwise it will cause the ink to dry too fast, crystallizing the surface of the ink layer, but affecting the coating effect. When printing light colors, some diluent needs to be added. High-quality diluent should be used in application. White oil is an emulsifier and should not be used. Inferior white ink has obvious powdery particles that are not firmly bound to the binder. After the printing, the binder will penetrate the paper, and the pigment will float on the paper surface and hinder the adhesion. Weili oil is a transparent paste made of aluminum hydroxide and dry vegetable oil. Although it has better printing suitability, aluminum hydroxide is light in weight and will float on the surface of the ink layer after printing, resulting in poor adhesion or blistering. Velly oil has a slow drying property, and inhibiting the drying of the ink is not good for the coating. Therefore, we should use caution when using Villi oil. Quick-drying varnish can make the blots shiny and dry, and the printing performance is good. We say that it is best to use quick-drying gloss oil as a diluent.
3. Addition of printing powder
In order to adapt to the high-speed printing of multi-color machines and prevent the back of the paper from sticking during printing, packaging offset printing is often solved by powder spraying technology. The powder sprayed is mostly composed of cereal starch and natural suspended substances. These powder particles are coarse. If too much powder is sprayed during the printing process, these powder particles will float on the surface of the printed product. The adhesive will not be combined with the ink layer everywhere when coating, which seriously affects the quality of the coating. Therefore, the amount of powder sprayed should be strictly controlled during packaging offset printing. On the premise of non-sticky, use the minimum amount of powder.
4. The surface and dryness of the inner surface of the printed matter
The surface ink layer or inner layer of the printed matter is poorly dried, which is very harmful to the quality of the coating, and will cause quality problems such as blistering and delamination. There are many factors that affect the drying of the ink layer on the surface of the printed matter, such as the type of ink, the amount of desiccant, the structure of the paper, the temperature and humidity of the workshop environment, and so on. Therefore, we must pay special attention to the drying of the ink layer when printing. Strictly control the balance of water and ink during printing, pay attention to the emulsification of ink, and control the acid value of fountain solution. At the same time control the temperature and humidity of the workshop. Make the surface and inside of the packaging print really dry, so that the quality of the film is guaranteed.
In short, in order to ensure the quality of the film on the packaging print, we must not only consider the situation of the film process, but also from the beginning of printing, otherwise the quality of the previous process will affect the quality of the subsequent process, this concept must be remembered. It is necessary to establish the idea that the former process serves the latter process.
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