Metal printing is mainly widely used in packaging containers to "convey information about the contents of the package" and "increase added value through color and packaging effects". Metal printing is basically carried out in the form of offset printing, and the ink used for printing belongs to offset printing ink. In addition to the printing suitability and printing effect of offset printing inks, metal printing inks are also required to have corresponding suitability and tolerance in the treatment of canning and processing engineering contents after filling. In addition, since the printed body is different from paper, it cannot penetrate, so it needs to be heated to harden.
In order to protect the metal material, improve the strength of the coating film and the printing effect, metal printing is usually used in combination with fillers, white ink and other primers and primers and other surface coatings for cans. Depending on the application of the metal printing ink, it is necessary to have the performance of the coating film composition of the corresponding coating system. Since the introduction of the German-made platform printing machine in 1900, my country has started printing on metals. After that, although metal printing was localized for a time, it showed a temporary extreme decline during World War II.
Since then, the development of metal printing and iron printing ink is as follows:
(1950～) With the rapid expansion of the use of toys and food cans. Inks began to use alkyd resins instead of heavy-duty drying oils. And began to develop special high-performance inks that can be used for cooking.
(1960~) The use of beverage cans began to expand. Ink has started to improve mainly on high-speed rotary machines.
(1970~) The use of beverage cans was further extended. Correspondingly, the tinplate used in the past began to be replaced by chemically treated steel plates that omitted tinning, and resin-bonded cans began to be put into practical use. With the introduction of the DI can (Drawn and Ironed Can) production line, special inks have been developed.
(1980~) With the hydrogen filling method, the use of DI tanks has been expanded. The computer color matching system and its special ink were also developed accordingly.
(1990～) The can making method using thin steel plates was put into practical use. Coatings have begun to develop towards water-based systems, and inks corresponding to water-based systems have begun to improve.
Methods and uses of metal printing The most common printed matter used in sheet printing is the can body of a 3-piece can. In the past, it was mainly used as the outer packaging of general food, but with the popularity of beverage cans, the proportion of beverage cans is increasing. DR cans (Drawn and Redrawn Can), which are general food packaging, are two-piece cans that are formed after sheet printing. Therefore, the image formation in the printing stage requires special processing, and the ink used has to consider its processability. In addition, thin plate printing is used in a wide range of applications, such as film cassettes, dry battery packaging, toys, stationery and other diverse processed products. It is necessary to select the type of ink that corresponds to the application.
Curved surfaces on metals have traditionally been done with sprayers, as are aluminum impact cans, toothpaste-filled and pharmaceutical tubes. However, with the popularization of beverage cans and the addition of DI can production lines, the proportion of beverage containers printed on curved surfaces has increased. And with the emergence of 2-piece cans made of thin steel plates, beverage containers began to accelerate the transition from 3-piece cans to 2-piece cans.
1. Thin plate printing process
In thin plate printing, metal materials are selected according to the purpose of the printed matter, and printing and coating are carried out in the following order.
①Painting and baking of inner surface paint
②Coating and baking of the outside primer
③Outside printing, baking
④Coating and baking of the outer primer In the above-mentioned printing process, the inner surface coating of ① and the outer primer of ② can be omitted when unnecessary.
③The outer printing usually uses an offset rotary press with the same structure as paper printing, but because the solvent of the printed body does not penetrate, overprinting with more than 3 colors will have certain difficulties in overlapping printing, so a 2-color machine, 2 colors and 1 color can be used. The groups take turns to dry and print according to the required number of colors. When a coater for primer coating is installed behind the two-color machine shown in Fig. 1, a method of performing rotary printing with primer at the end of the final printing process is adopted.
In order to shorten this printing process, a combined UV curing system of UV ink and UV irradiation equipment can be used. Figure 2 is a UV oven-free system that uses a UV irradiation device instead of a heating oven to shorten the baking time and save oven space. This system is gradually being put into practical use in 1-3 color machines. Figure 3 shows the UV SET PASS system in which the lower layer is printed with UV ink hardened by UV radiation, and the upper layer is printed with bake-hardened ink, making multi-color printing possible. Usually, the primer coating of ④ is carried out after the printing and coating process is completed, but according to the application, when the inner surface needs to be printed, the primer coating process can be added.
2. Surface printing engineering
Curved surface printing refers to the method of printing and coating the can body after pressing and forming metal plates such as aluminum and steel. This method is rapidly increasing with the popularity of DI tanks. A representative DI tank production process is roughly as follows:
①→Pressing ②→Tempering, ironing ③→Toriming④→Washing ⑤→Coating and baking of outside primer ⑥→Coating of outside printing, primer and baking ⑦→Coating of inside paint , Baking⑧→Edge processing
When the DI tank printing machine is running, one blanket corresponds to 4 to 6 plates. After the ink is transferred from the plate in sequence, it is transferred to the outer surface of the tank for continuous varnish and rotary printing. In this way, it is impossible for a blanket to transfer full color at one time and perform overlapping printing. The dot control of color overprinting is also very difficult. Although the printing method is offset printing, there is no water supply device, and the plate uses a resin letterpress. Therefore, the dot gain is greater than that of thin plate printing using a flat plate. This DI can printing machine is shown in Figure-4.
In recent years, with the rapid development of printing machines, the printing speed has reached 1500 cans/minute. For the baking of ink and outer surface coating, compared with thin plate printing, it needs to be baked at high temperature and short time (200~220℃, 60~120 seconds).
