After the inner coating and outer coating process, the tinplate can be printed on the machine. Because the surface of the tinplate is smooth, does not absorb ink and has a certain hardness and thickness, the tinplate printing generally adopts offset printing. The printing principle is: oil and water are incompatible. Dot conformation and color formation, selective adsorption. Tinplate lithography is basically the same as sheetfed offset printing, but due to the special nature of tinplate, tinplate printing is different from ordinary offset printing, mainly in the following aspects.
(1) The drying and heat resistance of the ink, the curing of the printing ink should be heated and dried, so the white ink after heating is required to be unchanged, not yellow, the color ink does not fade, does not change color, the glazing oil does not lose light, the printing ink is generally Curing at 150 ° C for 10-12 minutes, but usually in the post-process such as varnish baking high-frequency resistance welding, internal re-coating, internal full-bake baking, the temperature should reach 180-220 ° C, therefore, the heat resistance of the ink appears More important.
(2) solvent resistance. After printing, the varnish contains various solvents. In order to prevent bleed during glazing, the ink should have good solvent resistance.
(3) Resistant to retort: Containers filled with food need to be sterilized by high temperature. Therefore, it is required that the printing ink can not be bleed, faded or softened due to the action of wet steam.
(4) resistance to processing. Most of the printed iron products have to be stamped, punched, bent, flanged, wound and other processes, so the ink layer on the surface of the iron should have a strong structure, requiring the printing ink to have good adhesion, flexibility, surface hardness and impact resistance. strength.
(5) light and weather resistant. Various daily necessities generally have a long storage time, so it is required that the printing iron ink cannot be discolored and aged due to light irradiation or climatic factors.
(6) The ink layer does not contain heavy metals. Many printed iron products, such as toy boxes, chess boxes, battery cases, etc., the heavy metal content in the ink should also meet the corresponding standards of the United States, Japan, and the European Union.
To complete all printed iron prints, a layer of varnish should be applied to increase the gloss and scratch resistance of the printed surface, and also increase the hardness to make the printed surface film have certain flexibility and corrosion resistance. Varnish should have the following properties:
(1) The color retention performance is good, and the solvent in the varnish cannot make the ink bleed and fade.
(2) has sufficient hardness and firmness to withstand the processing deformation of the post process.
(3) Match with white paint or base oil, can not erode white paint or base oil.
Commonly used in varnishes are epoxy resins, alkyd resins, acrylic resins, and acrylic acid amino groups. The composition is different, the properties are different, and the uses are different. It is usually necessary to use different types of varnish depending on whether there is deep stamping deformation in the post process, local high temperature heating of the weld and whether high pressure cooking is required. In order to increase production efficiency and save energy, the last printing process is generally combined with the varnish process.