Metal printing refers to the process of printing and polishing the designed layout on the surface of metal plate or metal mold after determining its specifications. Metal cans can be printed first and then processed and formed, or processed and formed first and then printed. Therefore, the printing method of metal cans is different because of the different printing materials, can types and substrate shapes. Metal is an inelastic material. Although there is a coating on its surface, it still cannot fundamentally improve its full contact with the printing plate, and the image transmission effect is not good. In order to improve the transfer performance of ink, no matter which printing method is chosen, there is a * * * principle, that is, the printing plate that directly contacts the metal surface or the indirect printing plate should be flexible and elastic to make up for the lack of printability of rigid metal materials, thus ensuring the transfer of ink. For this reason, the commonly used metal printing methods mainly include offset lithography, gravure offset printing, relief offset printing and screen printing. Offset roller for lithography and thick ink layer for screen printing are beneficial to ink transfer, while gravure and relief are hard plates, which are not conducive to ink color transfer. Therefore, the problem of ink transfer can be solved by installing rubber roller between the printing plate roller and the substrate. Therefore, it is called gravure offset printing and relief offset printing. Printing of metal cans. Printing after metal forming does not have absolute adaptability, and can only be carried out on metal cans with regular shape and large production capacity. Regular shape mainly refers to cylindrical metal cans, such as beer and beverage cans packaging is the most typical representative. Different from metal plate printing, metal can printing is completed by automatic production line, in which coating and printing are part of this production line. Generally, the internal coating is completed before molding, and the external coating and printing are carried out after molding. General assembly line production requires high speed, so embossing (metal embossing or flexography) offset printing is often used, and the printing color number is subject to the 4-color machine. The working principle of four-color relief offset press is shown in Figure 3. Through the rotation of the star wheel, the cylindrical metal cans are smoothly transported to the assembly line; The printing plate is a four-roll relief, which can realize four-color printing; The ink on the relief is first transferred to the rubber drum, and then transferred to the surface of the metal can by the rubber drum; Polishing the surface of the tank after printing can improve the fastness of ink and the gloss of the tank surface. Through the device in Figure 3, the surface printing and polishing of the metal can are completed, and the finished metal can is output. 3) polishing after printing. Whether it is metal plate printing or metal forming printing, the process of printing a layer of varnish on its surface after printing is called glazing. Glazing can increase the brightness of color, the luster of metal surface and the strength of ink film, thus improving the value of the goods themselves. The glazing equipment is similar to a coater (see Figure 3). After metal printing, the surface of the metal can is varnished by the glazing unit. Therefore, post-press glazing is also technically called post-coating. There are two kinds of varnishes commonly used for glazing, one is organic solvent varnish and the other is water-based varnish. The former can meet the requirements of coating in terms of brightness, beauty and firmness. However, this varnish takes aromatic hydrocarbon as the main solvent J, which is volatile, flammable, explosive, toxic and poor in environmental protection. It is not suitable for food packaging, drug packaging and beverage packaging, and its application is greatly limited. Water-based varnish replaces or partially replaces organic solvent with water, and certain additives and adhesives are added, so that it has the basic properties of varnish, but also has the characteristics of cleanliness, non-toxicity, environmental protection and baking resistance. The use of water-based varnish broadens the application field of metal printing, increases the development potential of metal can packaging, creates more business opportunities and promotes the development of metal printing. The substrate of metal printing is different from paper, and the weight and thickness of metal itself have certain restrictions on printing efficiency and quality. Therefore, in the renewal and design of equipment, efforts should be made in material supply, transmission and automatic control. (1) Magnetic intelligent gripper transfer device, double-diameter impression cylinder and transfer roller can prolong gripper transfer time, ensure high-speed and accurate paper transfer, and ensure the smooth passage of paper; (2) In order to improve the automation degree of metal printing equipment, the equipment should introduce functions such as automatic feeding, automatic quick plate changing, quick ink changing, automatic clutch of ink roller, automatic plate washing and automatic paper size setting. (3) Adopt printing quality control and modular open system, automatically set and control ink quantity and water quantity, correctly adjust printing pressure, realize circumferential, horizontal and diagonal registration adjustment, and improve the accuracy and stability of multi-color registration. Printing on metal surface is only half the work of metal can processing, and printing and can forming are equally important in reflecting the value of metal packaging. The shape of metal can depends largely on the type of die and the conditions of stamping equipment. At present, the development and use of metal can molding equipment in China and even in the world are limited to cylinders and cubes, which are mainly used in cans, raw material barrels, food boxes and so on. Occasionally, some special-shaped cans appear, which often limit large-scale production because of their few uses, small processing capacity and high design and processing costs. This phenomenon greatly restricts the diversified development of the shape of cans, making many products reluctant to set foot in metal packaging because of the single shape of metal cans. Fashion products, such as candy, chocolate, coffee, decorations, cosmetics, jewelry and so on. In fact, it is very suitable for metal packaging. The exquisiteness of metal printing and the exquisite shape of metal box are undoubtedly a wonderful match with the contents. However, this kind of goods requires high appearance, appearance and durability of the packaging box, and the current types of metal boxes are difficult to adapt to the packaging requirements of this kind of products. In view of this situation, if more kinds of metal cans can be designed and equipped with corresponding molding equipment, more kinds of metal cans will be developed, which will fundamentally promote the development of metal printing and make metal can packaging have greater market prospects. Compared with ordinary paper packaging and plastic packaging, metal cans have the characteristics of good shape retention, durability and strong decoration. For ordinary consumers, people are used to reusing metal cans that are not easy to be damaged, such as moon cake boxes, jewelry boxes and other metal packaging, which is very suitable for reusing fruit bowls and sundries boxes. The use of old things greatly saves resources, which is consistent with the concept of saving society advocated today. However, there is a premise for the reuse of this metal can, that is, when using the built-in products, the metal packaging outside should be kept intact. As a metal packaging enterprise, this problem should be considered when processing metal cans. For those metal can products that will damage the packaging when used, such as cans, it is necessary to use easily recyclable metals and environmentally friendly printing ink as processing materials. Generally speaking, metals can be recycled, but printing has brought some obstacles to the recycling of metal cans. Paint and ink used in printing can only be removed by special methods such as high temperature and chemical reaction after metal recycling, which brings difficulties to metal reprocessing. At the same time, in the process of treatment, the resistance of some metals is not as good as that of coating, which makes the metal can have no recycling value at all. Therefore, in order to improve the reuse rate of metal cans, more efforts should be made to improve the reusability and recycling processability of metal cans.