The traditional production method of metal packaging cans is: first cut the iron flat blank into rectangular squares, then roll the blank into a cylinder (i.e. cylinder), and then solder the formed longitudinal joint lines to form a side seal, one of the cylinders. The end (that is, the bottom of the can) and the round end cap are flanged mechanically and rolled and sealed (this is the double crimp seam) to form the can body; the other end is sealed with the lid after the product is loaded. Since the container is composed of three parts: the bottom, the body, and the lid, it is called a three-piece can . This can-making method has remained basically unchanged for more than 150 years, except that the degree of automation and processing accuracy have been greatly improved. In recent years, the welds of the side seals have been changed to fusion welding. In the early 1970s, a new can-making principle emerged. According to this principle, the can body and the bottom are a whole, stamped from a round flat blank, and are sealed after the product is loaded. This is a two-piece can . There are two forming methods for this kind of can : stamping-thinning and drawing method (ie punching method) and stamping-re-stamping method (ie deep drawing method) . The technologies themselves are not new. The stamping method has been used to make bullet casings as early as World War I. What makes cans different is the use of ultra-thin metal and the high speed of production (annual output can reach hundreds of millions).

Manufacturing of three-piece cans

The production process is: use a shearing machine to cut the coil into rectangular plates; paint and perform printing; cut into long blanks; roll into a cylinder and weld the side seams; repair the joints and coating; cut the cylinder; form a concave groove or corrugation; press out flanges at both ends; roll back cover; inspect and stack on pallet. ①Cylinder processing. The key processes are crimp forming and side seam welding . There are three ways to seal side seams: soldering, fusion welding and glue . The tin solder of tin soldering cans is generally composed of 98% lead and 2% tin. A cylinder making machine that rolls flat blanks into a cylinder is used in pairs with a side seam sealing machine that performs soldering. In the cylinder-making machine, the edges of the slab are cleaned and bent into hook shapes to facilitate fixation when forming the cylinder. Then the cylinder goes through the side sealing machine, adds solvent and solder, uses a gas spray gun to preheat the sealing area, and further heats it through the longitudinal soldering roller to make the solder flow into the seam, and then uses a rotating scraper roller to remove the excess that mainly exists in the form of drops. The solder is removed cleanly. Fusion welding uses the principle of consumable wire electrodes and uses resistance welding technology. Earlier fusion welding systems used large laps, raised the temperature of the steel to its melting point, and welded at lower roller pressures. The latest welding machines use a small overlap amount (0.3~0.5mm), and the metal temperature is just below the melting point, but the welding roller pressure needs to be increased to forge the two overlapping surfaces together. The weld destroys the original or painted smooth surface of the interior surface, leaving exposed iron, iron oxide, and tin on both sides of the weld. In order to protect the product from contamination and the weld seam from being corroded by the product, the side seals need to be protected by coating in most cases. The side seals of cans containing only dry products can be glued . That is, the longitudinal seams are pasted with nylon tape, which melts and solidifies after the cylinder is formed. Its advantage is that the original edge can be completely protected, but it can only be used for tin-free steel (TFS) because the melting point of tin is close to the melting temperature of plastic. ②Simplified post-processing. Flanges must also be machined at both ends of the barrel to facilitate the installation of end caps. For processed food cans, the can may be subject to external pressure during processing or be subjected to a vacuum inside during storage. In order to increase the strength of the tank, reinforcement ribs may be made on the surface of the cylinder . This process is called corrugation. In order to improve production efficiency, the length of the cylinder for making shallow containers is often two to three can lengths. At this time, the first process is to cut off the cylinder. The traditional method is to cut the blank on a cutting machine (scoring machine) before forming. Make non-breaking cuts. However, recently, can trimming shears developed for the production of two-piece cans have become available.

Manufacturing of two-piece cans

Both forming methods for making two-piece cans use sheet metal forming. This method is based on the "fluidity" of metals through the rearrangement of the crystal structure under the action of compound stresses, and the material should not break during this process. ① Stamping forming. That is, a punch of the stamping machine is used to punch a flat plate into a cylindrical die, thereby deforming the flat plate into a cylinder. The diameter of the cup formed after the initial stamping can be reduced by applying a re-stamping process. In the stamping process, a stamping sleeve is used instead of the die and is installed between the punch and the inner diameter of the punching cup. The equal area rule determines that as the diameter decreases, the height increases. The stamping process can be repeated again to gradually reduce the diameter within a certain limit and avoid metal breakage. ②Thinning and stretching of the cup wall . The stamped cylindrical cup is placed on the punch, and the punch is squeezed into a mold along the axial direction. The gap between the mold and the punch is smaller than the thickness of the cup wall. In this way, the wall thickness remains unchanged while the diameter remains unchanged. Get thinned. The metal volume of the cylinder after thinning is equal to the metal volume of the cup during the thinning and stretching process, and is also equal to the metal volume of the original slab. In the manufacture of cans, this process is repeated two or three times, with the cupped punch passing through a series of dies, one die at a time. The most convenient way to install the stamped forest on the punch is to perform a re-stamping operation before the first thinning and stretching. ③Manufacture of cupping cupping. The manufacturing process of punching and cupping cans is roughly as follows: unrolling ordinary strip coils; adding lubricant; blanking and punching the cup; re-stamping; thinning and stretching of the side wall of the cup; forming the bottom; trimming at the correct height; cleaning and processing . For dry beverage cans, outer surface coating, printing and decoration, inner surface coating, flange forming and closing of the open end are also required. For food cans, the outer surface coating, the flange forming of the open end, the barrel corrugation and the inner surface coating are carried out during the cleaning process or the can ends are manufactured from pre-coated sheet metal on a multi-mold press, the can lid press There are corrugations (reinforcing ribs) to bear internal and external pressure. The final step here is hemming to create a double rolled seam (see Figure 1). Place packing over the final seal to act as a sealing gasket. ④Manufacturing of deep-drawn cans . The steps for manufacturing deep-drawn cans are as follows: roll and cut thin strips into swirl-shaped sheets; coat and finish the sheets (coils can also be coated); blank the wafer blanks and punch cups; then punch out (one time or twice, depending on the size of the can); forming the bottom of the can; trimming the flange to the correct width; inspection and sterilization. As with the punching process, a multi-die cup punching machine is used to load and unload wide sheet rolls or sheets and punch them into shallow cups. The shallow cup gradually decreases in diameter and increases in height through one and two re-punches. When the final stamping is stopped at a certain point, a flange is left on the can, like all other sheet metal forming processes. Irregular edges need to be cut off and the bottom of the tank needs to be machined into the required shape. Processing equipment The main equipment for canning processing includes various stamping machines, shearing and blanking machines and other auxiliary equipment. There are so many types of these devices that it is impossible to introduce them all in one place. In view of the characteristics of modern can-making industry that mostly adopts automated production, this chapter only takes the composition of the cupping can production line (Figure 2) as an example for a brief explanation. l- The thin plate rolls are sent to the unwinding machine (unwinding rack) by the transport vehicle. The unwinding machine generally has two arms, one arm faces the sheet roll transport truck, and the other arm feeds the press. The sheet roll can be replaced by rotating the arm. The sheet roll passes through the lubricant coating equipment, is coated with lubricant on both sides, and passes through a pair of rubber rollers to remove excess lubricant. In older production lines, the lubricant is mixed into the correct concentration and then stored in a container and refill the lubricant tank when necessary. In newer production lines, an external circulation system is generally used, which can control temperature, continuously filter and maintain constant lubricant quality. The cup punching press is a specially designed double-action press that punches down a number of round slabs first and then punches the round slabs into shallow cup shapes. These two actions are completed in one stroke. The function of the cup wall thinning and drawing machine is to transform the cup into a cylinder with the correct distribution of side wall thickness, and to change the bottom of the cup into a design shape that can resist internal pressure during use. Here, the punch cup is re-punched and left on the punch, and then passed through three thinning drawing rings in sequence to thin the cup wall. At the end of the stroke, an auxiliary press enters, forming the dome and cup base. At the beginning of the return journey, with the help of compressed air sent from the stamping pipe, the spring ejector rod removes the can body from the punch, and the can is sent out of the machine by the conveyor belt. Each cup wall thinning and drawing machine works in conjunction with an edge trimmer. There are two types of edge trimming machines: guide rail type and roller type. The rotating frame of the rail-type edge trimmer is equipped with many cores, and the can edges are cut between the rotating cores and the stationary guide rail. On a roller-type edge trimmer, one end of the can is fed between two rollers and rotates around its own axis. The two rollers move closer to each other to complete the edge trimming action. After the cans are trimmed to a fixed height, the cans are placed upside down on a mesh conveyor belt with their open sides facing down, and are transported in large quantities to a series of cleaning chambers equipped with spray nozzles, where they are stamped, thinned and stretched using detergent. The lubricants used are washed off and finally rinsed in deionized water to ensure no stains and dried. The closing-flange forming machine performs closing and flange forming processing on beverage cans, while it performs corrugation and flange forming on food cans. In order to avoid roughening the axial surface of the metal during thinning and drawing, the flange forming process generally uses spinning. The necking at the top of the beverage can can be retracted using a mold after spinning the flange.