Study on Printing and Processing of Metal Cans (Part 2)

2.3 Printing

Metal printing refers to the process of printing a designed layout on the surface of a metal plate or a metal molding after it is determined. The production of metal cans can be printed and then processed, or they can be formed and then printed. Therefore, the printing method of metal cans differs depending on the printing material, the style of the can body, and the shape of the substrate. Metal is a non-elastic material. Although it has been painted on its surface, it cannot fully improve its full contact with the printing plate, and the image transfer effect is not good. In order to improve the ink transfer performance, no matter which printing method is selected, there is a common principle that a printing plate or an indirect printing plate directly in contact with a metal surface should have flexibility to compensate for the printability of the rigid metal material. The lack of, thus ensuring the transfer of ink. For this reason, commonly used metal printing methods mainly include lithographic offset printing, gravure offset printing, letterpress offset printing, and screen printing. Lithographic offset printing cylinders and screen-printed thick ink layers facilitate the transfer of ink, and both gravure and letterpress are hard plates that are not conducive to ink transfer, so an eraser is installed between the plate cylinder and the substrate. The roller can solve the problem of ink transfer. Therefore, this type of printing method with rubber rollers added is gravure offset printing and letterpress offset printing.

1) Printing of metal plates. Metal plate printing can use a variety of printing methods, such as gravure offset printing. Its working principle is shown in Figure 2.

P in FIG. 2 is a gravure plate, and I is an impression cylinder. In order to allow the ink to be transferred to the metal plate more fully, a blanket cylinder B is added to the printing system. In this way, the ink on the gravure plate is first transferred to the eraser. On the drum, the secondary transfer of the blanket cylinder then transfers the ink to the metal plate. The presence of a blanket cylinder greatly improves the adhesion of the ink to the metal surface. The squeegee and the cleaning roller in the figure can clean the ink on the squeegee to ensure the clarity of printed graphics.

2) Printing of metal cans. Printing after metal forming does not have absolute adaptability, but can only be performed on metal cans with a regular shape and a large number of production. The regular shape mainly refers to cylindrical metal cans, such as cans and beverage cans packaging is the most typical representative. Unlike metal plate printing, metal can printing is done by automated production lines, where coating and printing are part of the production line. Generally, coating is done before molding, while coating and printing are After molding. Pipeline production generally has higher speed requirements, so the use of letterpress (metal letterpress can also be flexographic) offset printing, printing color number to 4 color machine as the standard model. The working principle of the four-color letterpress offset press is shown in Figure 3. Cylindrical metal cans are smoothly conveyed to the assembly line by the rotation of the star wheel; the printing plate is a relief plate, and there are four cylinders to achieve four-color printing; the ink on the relief plate is first transferred to the blanket cylinder and then transferred by the blanket cylinder to the metal can The surface; After the printing is completed, the surface of the can has a glazing treatment, which can improve the fastness of the ink and improve the surface gloss of the can body. Through the device of Fig. 3, the surface of the metal can is printed and polished, and the finished metal can is output. 3) Polishing after printing. Whether it is metal plate printing or metal molding printing,

The process of printing a layer of varnish on the surface after printing is called coating. The glazing can increase the brightness of the color, increase the luster of the metal surface, increase the strength of the ink film, and thereby increase the value of the product itself. The coating machine is similar to the coating machine (see Figure 3). After the metal is printed, the surface of the metal can is coated with varnish by the coating machine. Therefore, the post-print coating is also called post-coating.

There are two types of varnish commonly used for varnishing, one is an organic solvent varnish and the other is a varnish varnish. The former can meet the requirements of coating from bright, beautiful and firm, but this type of varnish is based on aromatic hydrocarbons as the main solvent J. It is volatile, flammable, explosive, toxic, and environmentally friendly, so it is not suitable. The use of food packaging, pharmaceutical packaging, and beverage packaging is subject to greater restrictions. The water-based varnish replaces or partly replaces the organic solvent with water, and increases certain additives and connecting materials, so that in addition to the basic properties of the varnish, it also has the characteristics of being clean, non-toxic, environmentally friendly, and resistant to baking. The use of water-based varnish broadens the application of metal prints, increases the development potential of metal cans, creates more business opportunities, and promotes the development of metal printing.

3 Development direction of metal can printing and processing

3.1 Metal Printing Equipment

Metal-printed substrates are different from paper. The weight and thickness of the metal itself have certain restrictions on printing efficiency and print quality. Therefore, efforts should be made in the aspects of supply, delivery, and automatic control in the updating and design of the equipment.

(1) Magnetic intelligent tooth transfer device, double-diameter impression cylinder and transfer roller can be used to extend the time of tooth transfer, ensure accurate transfer of printed sheets at high speed, and ensure that printed sheets pass smoothly;

(2) The equipment should be equipped with automatic feeding, fully automatic quick plate change, quick ink change and automatic ink roller clutching, automatic plate washing, and automatic setting of print sheet size functions to improve the automation of metal printing equipment;

(3) Adopt print quality control and modular open system to automatically set and control the amount of ink and water, correctly adjust the printing pressure, realize the adjustment of circumferential, horizontal and diagonal lines, and improve the accuracy of multi-color registration. stability.

3.2 Metal can forming

Finishing the printing of the metal surface is only half of the work done for metal can processing. Printing and can forming are equally important in reflecting the value of metal packaging. The shape of the metal can largely depends on the style of the mold and the conditions of the stamping equipment itself. At present, the development and use of the metal can forming equipment at home and in the world is also limited to the two can shapes of cylinder and cube. Applied to cans, raw material barrels, food boxes and so on. Occasionally, there are some special-shaped cans, often because of their small application surface, the small number of processing, high design and processing costs, which limits the large-scale production. This phenomenon greatly restricts the development of the variety of pots, making many products do not want to get involved in metal packaging due to the single shape of metal cans. Fashion products such as candy, chocolate, coffee, accessories, cosmetics, jewelry, etc., are actually very suitable for metal packaging. The exquisite combination of metal printing with metal box shape is undoubtedly a wonderful match with the content. However, these types of goods have high requirements for the shape, appearance and durability of the packaging box. The current type of metal box is difficult to adapt to the packaging requirements of such products. In view of this situation, if more types of metal cans can be designed and matched with corresponding molding and processing equipment, more types of metal cans will be developed, and the development of metal printing will be fundamentally promoted. Can packaging has a greater market prospect.

3.3 Improve the reuse of metal cans

Compared with general paper packaging and plastic packaging, metal cans have good shape retention, strong durability, and strong decorative features. For ordinary consumers, people are accustomed to reusing metal cans that are not easily damaged in appearance. Metal packaging such as moon cake boxes and jewelry boxes are very suitable for reusing fruit dishes and junk boxes. The use of old materials has greatly saved resources and is in line with the thinking of a conservation-oriented society advocated today. However, there is a precondition for the repeated use of such metal cans, that is, the external metal packaging should be kept intact when using the internal products. As a metal packaging enterprise, this problem must be taken into account when processing metal cans. For metal cans such as cans, etc., which are damaged during packaging when used, the use of easily recycled metal and environmentally friendly printing inks should be used as processing materials. In general, metals are recyclable, but printing brings certain obstacles to the recycling of metal cans. The paints, inks, etc. used in printing must be removed by special methods such as high temperature and chemical reaction after the metal is recycled, which brings difficulties to metal reprocessing. At the same time, some metals are not as resistant to the coating layer during the treatment process, resulting in no recycling value for the metal cans. Therefore, in order to increase the recyclability of the metal cans, more efforts must be made to increase the reusability of the metal cans and recycle the processability.

4 Conclusion

In the process of printing and processing metal cans, on the one hand, we must focus on improving production processes and updating processing equipment. On the other hand, we must put more effort on the shape of metal cans and product adaptability, and strive to develop more types of products. In order to make metal cans have a broader market prospects.