Tinplate printing using the principle of lithographic offset printing, which uses the repelling physical properties of ink and water and uses the printing pressure to transfer the artwork and text to tinplate via blanket. For tinplate has special physical and chemical properties and reprocessing performance, its printing process is somewhat different from ordinary offset printing.
Tinplate printing has special requirements for ink, kindly check as below:
1. Should be with good adhesion and mechanical properties
Printed tinplate needs to be cut, bent and stretched before they can be made into the tin box. Therefore, the printing ink is required to have good adhesion and mechanical properties. In order to improve the adhesion of ink, the tinplate needs to be white priming before printing. White is the primary color of all pictures, which has high brightness. After the addition of other high-energy Hue, it could improve the value of each hue and form color levels.
2. Requirements for white ink
In order to make the surface of tinplate silver (or yellow) with a metallic luster, the varnishing is needed before printing. With the limit of ink coverage, monochromatic often needs to be varnished twice and its whiteness can reach 75%. The whiteness is an important quality factor of the tinplate printing products, so it requires white ink has good cohesion with the ground ink, which will not turn yellow or fade after several times of high-temperature baking and steam. Applying ground ink to tinplate can increase the adhesion of the tinplate which is good for white ink adhesion. The common ground ink is epoxy amine type with the characteristics of light color, non-yellow and non-aging after baking for many times and processing properties of good flexibility and impact resistance.
3. Requirements for color ink
The color ink for tinplate printing should not only have a certain degree of water resistance, but also have some special character. As the surface of tinplate does not penetrate water and solvent, it needs to be baked and dried, so its ink should be thermosetting. It has a high requirement on the tinting strength and durability. In addition to the basic properties of general offset ink, iron ink should also have heat resistance, strong adhesion of ink film, impact resistance, good hardness, cooking resistance and light resistance and so on.
4. Drying process of ink
In the production of tinplate printing, ink drying is a complex physical and chemical reaction process. To control the ink drying spread reasonably and its physical and chemical mechanism well, then we can effectively carry out printing operation fast and ensure the printing quality. If the ink dries too fast, it will reduce the normal transmission performance of the ink and affect the normal process of production, like printing empty, light color.
The dried ink on the roller and forme will block the transfer of ink, and then cause the printing plate graphic dry knot layer expand outwards;
If the ink drying fast, more desiccantis needed and will cause the ink adsorption ability higher which will cause the blank part dirty and so on.
On other hands, if the ink drying too slow can cause bad trapping, bad adhesion and firmness performance lower; it is easy to cause scratches in the transmission process. So ink drying speed should be appropriate, too fast and too slow is unfavorable.
5. Special structure of printing equipment
Because of the difference of printing products, the auxiliary machinery of tinplate printing press is different from paper offset press. Tinplate is insoluble in Water, does not absorb solvent, so printing ink needs to be baked at high temperature to make it volatilization and forming film and curing. Therefore, a drying room is required during printing process .
The total length of the tinplate printing automatic line is about 50 meters, so it has large and precision property. In addition, the tinplate printing machine is controlled by magnet splitting and double splitting, and the high hardness of tinplate also determines the the advanced continuous iron conveying system cannot be used in the printing process, which is different from the paper offset press in the structure of feeding section, alignment system and stacking section.
