The printing principle of tinplate:
Tinplate printing also utilizes the physical property of water and ink repelling each other. With the help of printing pressure, the image on the printing plate is transferred to the tinplate through the rubber blanket, which belongs to the principle of offset lithography. Due to the special physical and chemical properties of tinplate and the reprocessing performance of the printed product, its printing process is quite different from that of ordinary offset printing.

Printing on tinplate has special requirements for ink:
1. The ink is required to have good adhesion and mechanical properties.
Since tinplate printed products are ultimately made into food cans, toys, metal photo frames, and buckets and cans for chemical products, they need to undergo processes such as cutting, bending, and stretching. Therefore, the printing ink is required to have good adhesion to tinplate and corresponding mechanical properties. To enhance the adhesion of the ink, a white base coat is first applied to the tinplate before printing. White is the base color of all images and has a high brightness. When combined with other high-brightness colors, it can increase the brightness of each color and form a color scale.

2. Requirements for White Ink
The surface of tinplate is silver-white (or yellow) with a metallic luster. Before printing color graphics, the surface needs to be coated or printed white. Due to the limitation of ink coverage, single-color machines often require two white printing passes, and the whiteness can reach 75%. Whiteness is an important indicator of tinplate printing product quality. White ink is required to have good adhesion to the primer, not turn yellow after multiple high-temperature baking, and not change color after high-temperature steam treatment. Applying a primer to tinplate can increase adhesion to tinplate and have good adhesion to white ink. The commonly used primer is epoxy-amine type, which has the characteristics of light color, no yellowing or aging after multiple baking, and good flexibility and impact resistance in processing performance.

3. Requirements for Color Inks
The color inks used for tinplate printing, in addition to having a certain degree of water resistance, also have special requirements. Since the surface of tinplate does not absorb water or solvents and needs to be dried by baking, the inks should be heat-curable. The coloring power and durability of the pigments are required to be high. Besides the basic properties of general offset printing inks, tinplate printing inks should also have the characteristics of heat resistance, strong adhesion of the ink film, impact resistance, good hardness, resistance to boiling and steaming, and lightfastness, in accordance with the characteristics of tinplate printing.

4. Drying Process of Ink
In the production of tinplate printing, the drying of ink is a complex physical and chemical reaction process. To effectively carry out rapid printing operations and ensure product quality, it is necessary to reasonably control the drying speed of ink and understand the physical and chemical mechanisms involved in ink drying. If ink dries too quickly, it will reduce the normal transfer performance of ink, affect the normal production process, cause the printed marks to be blurred, the ink color to be pale, and the ink to dry on the surface of the printing plate and ink roller, which will hinder the ink transfer; it will also cause the dry layer of the printing plate's graphics to expand outward; and it will increase the ink's adsorption due to excessive use of drying agents, causing dirt on the blank areas. On the other hand, if ink dries too slowly, it can lead to difficulties in overprinting, sticking, reduced adhesion and firmness, and is prone to scratches during the transmission process. Therefore, the drying speed of ink should be appropriate; both too fast and too slow are unfavorable.

5. Special Structures of Printing Equipment
Due to the difference in substrates, the auxiliary mechanisms of tinplate printing machines are distinct from those of paper offset printing machines. Tinplate is insoluble in water and does not absorb solvents, so the printing ink needs to be baked at high temperatures for the solvents to evaporate and form a film to solidify. Therefore, drying rooms are generally required in the printing process equipment. The entire automatic tinplate printing line is usually about 50 meters long and is characterized by its precision and large size. Additionally, tinplate printing machines use magnets for sheet separation and double-sheet control. The high hardness of tinplate also determines that advanced continuous tinplate feeding structures cannot be used in the printing process. The structures of the pusher, alignment, and stacker sections are also different from those of paper offset printing machines.