Metal is an important packaging material and plays a very important role in packaging. Metal printing is different from general paper and plastic printing, and has its own suitable printing characteristics. It has special requirements on the ink materials used.
1. Metal printing coating, varnish
The reasonable selection of paint and varnish is the key to affecting the quality of metal printing products. Reasonable selection of paints and varnishes is the premise to ensure that metal printing products have good firmness, color, whiteness, glossiness and processability. According to different uses, metal printing coatings can be divided into primer coatings, white coatings, varnishes, inner wall coatings, side seam coatings and so on. Side seam coatings can be divided into inner side seam coatings and outer side seam coatings. The inner side seam coatings must meet food hygiene requirements after drying.
2. Metal printing ink
Metal printing ink is a special ink printed on tinplate and other metal thin plates, mainly composed of pigments, binders, fillers and auxiliary agents.
In addition to the printability of general offset inks, metal printing inks should also have the following characteristics:
(1) Impact resistance and strong adhesion. In the post-printing process, tinplate and other metal materials are subjected to cutting, bending impact, seam and other can making processes, so the ink layer should have sufficient adhesion so that the printed tinplate can withstand the stamping of the mold. And has excellent mechanical processing properties (including flexibility, surface hardness and impact strength, etc.). At the same time, the ink layer is required to have good lipophilicity, so that it can be well combined with the varnish to ensure a certain adhesion between the tinplate and the ink layer and the varnish.
(2) Firm and hard. In the process of making cans, the printed tinplate prints inevitably collide with each other, and the cut tinplate is very sharp, which is easy to damage the pattern on the metal surface and make the ink layer fall off, so the ink layer is required. Firm and hard.
(3) Heat resistance. In the drying process, the printed part must be heated and dried at a certain temperature and time. Therefore, pigments and binders with strong heat resistance should be selected so that the ink is not easy to yellow after heating.
(4) Solvent resistance. In order to make the appearance of metal prints beautiful, increase the hardness, and prevent the damage caused by friction, glazing treatment is required. However, the varnish contains a large amount of strong solvents, which require that the dried ink layer does not change color, swell, or shrink.
(5) Light resistance. The metal container is required to keep the pattern color unchanged under daylight and light.
(6) Cooking resistance. Some metal cans need to be boiled for sterilization or heated before drinking. Therefore, the ink layer should not change color, not stain, generate bubbles and lose gloss after being steamed.
Since the surface of the metal printing material cannot penetrate, it is easy to cause the dot increase phenomenon. Compared with paper printing inks, metal printing lithographic offset printing should use high viscosity inks. The metal surface is a non-absorbent surface. Too much wetting water on the plate surface is likely to cause emulsification of the ink. Therefore, the water and ink balance on the plate surface should be achieved by controlling the water film thickness and the amount of ink on the plate surface.
In metal printing, in order to improve the adaptability of post-press processing and make the surface of the printed matter have a certain gloss, it should be glazed (post-press coating) before the printing ink is completely dried to form a uniform and smooth coating. Film to avoid bleeding. At the same time, the metal printing ink should have a certain hardness and toughness, and its properties cannot be changed when repeatedly heated, and the base coat and varnish should have good adhesion.
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