After the metal substrate is printed, it usually undergoes two processes: glazing and molding. The purpose of the varnish is to protect the ink film, increase the gloss of the printed matter, make the product more beautiful, and enhance the ability to withstand the bending and mechanism impact during can making. For the glazing principle and process method, please refer to Chapter 12 of this book. This section focuses on the forming process technology and main equipment of metal packaging products.
It should be pointed out that since the manufacturing of metal packaging products is carried out in an automatic assembly line, printing is one of the key processes, and the substrate is often a molded product. In the molding process of the product, a considerable part of the process is actually It was done before printing. In order to ensure the integrity of the system, this section does not make a strict distinction between the pre-press and post-press processes in the narrative.
1. Molding process
1. Stamping process. The metal packaging container, whether it is a box or a can, is formed from two major processes of separation and plastic deformation using the principle of metal stamping.
The separation process is a stamping method that separates the stamped part and the sheet material from each other along the required contour line and obtains a certain cross-sectional quality. The separation process often includes operations such as cutting, blanking, punching, cutting, trimming, and trimming.
The plastic deformation process is a stamping processing method to make the stamping blank plastically deform without breaking, so as to obtain the required shape and dimensional accuracy. There are usually three types of bending, stretching and forming. Bending includes operations such as bending, curling, twisting, bending, rolling, bending, and drawing; drawing is mainly drawing and thinning; there are many forming methods, including hole turning, flanging, flaring, Shrinking, forming, curling, bulging, spinning, shaping, and leveling operations.
2. Can-making process. The traditional manufacturing method of metal packaging cans is: first cut the sheet metal blank into squares, and then roll the blank into a cylinder (that is, the cylinder), and then solder the longitudinal bonding wire formed to form a side seal, one of the cylinders The end (that is, the bottom of the can) and the round end cap are mechanically formed with flanges and rolled and sealed (this is the double crimped seam) to form the can body; the other end is sealed after the product is loaded. Since the container is composed of three parts: the bottom of the tank, the body of the tank, and the lid of the tank, it is called a three-piece tank. This can-making method has basically not changed much in the past 150 years, but the degree of automation and processing accuracy have been greatly improved. In recent years, the welding seam of the side seal has been changed to fusion welding.
A new can-making principle appeared in the early 1970s. According to this principle, the can body and the can bottom are a whole, stamped from a round slab blank, filled with the product and sealed, which is a two-piece can. There are two forming methods for this can: stamping-thinning drawing method (ie drawing method) and stamping-re-stamping method (ie deep drawing method). These technologies are not new in themselves. The drawing method has been used to manufacture cartridge cases as early as the First World War. The difference between can making is the use of ultra-thin metal and the high production speed (annual output can reach hundreds of millions).
(1) Manufacture of three-piece cans. The production process is: using a shear to cut the coil into rectangular plates; painting and decoration printing; cutting into long strip blanks; rolling into a cylinder and welding the side seam; repairing the joint and coating; cutting the cylinder; forming the concave Grooves or corrugations; flanges are pressed out at both ends; back cover is rolled; inspection and code are placed on the tray.
â‘ Processing of cylinder. The key processes are crimping and welding side seams. There are three sealing methods for the side seam: soldering, fusion welding and adhesive bonding.
Tin solder in tin solder cans is generally composed of 98% lead and 2% tin. A cylinder making machine that rolls a slab into a cylinder is used in pairs with a side seam sealing machine for soldering. In the cylinder making machine, the edge of the slab is cleaned and bent into a hook shape, so that it is easy to fix when forming the cylinder. Then the cylinder passes through the side sealing machine, adding solvent and solder, preheating the sealing area with a gas spray gun, and further heating the longitudinal soldering roller to make the solder flow to the seam, and then using the rotary scraper roller will mainly exist in the form of drops The solder is cleaned.
Fusion welding is based on the principle of consumable wire electrode and adopts resistance welding process. Earlier fusion welding systems used large laps to raise the temperature of the steel to the melting point and welded at lower roller pressure. The latest welding machine uses a small overlap (0.3 ~ 0.5mm), the metal temperature is just below the melting point, but to increase the pressure of the welding roller, forging the two overlapping surfaces together.
The weld seam destroyed the original or painted smooth surface of the inner surface, resulting in the presence of exposed iron, iron oxide, and tin on both sides of the weld seam. In order to prevent the product from being contaminated and the welding seam from being corroded by the product, in most cases, the side seal needs to be protected by a coating.
The side seal of cans containing only dry products can be glued. That is, the longitudinal seam is pasted with nylon tape, which is melted and coagulated after the cylinder is formed. The 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.
â‘¡Simplified post-processing. Flanges must also be machined on both ends of the simplified Chinese in order to install the end cap. For processed food cans, the cans may be exposed to external pressure during processing, or the interior is under vacuum during storage. In order to increase the strength of the tank, the simplified surface may also need to be made of ribs. This process is called crimping. In order to improve production efficiency, the round length of shallow containers is often two to three cans long. At this time, the first process is to cut the cylinder. The traditional method is to cut the slab before the forming (scoring machine) Make an unbroken cut. However, due to the recent appearance of the tank trimming shear developed for the production of two-piece cans, it has been possible to use this shear for cutting cylinders that are not pre-scored.
(2) Manufacture of two-piece cans. The two forming methods for manufacturing the two-piece can adopt the metal plate forming method. This method is based on the "fluidity" of the metal exhibited by the rearrangement of the crystal structure under the action of compound stress, and the material should not break during this process.
â‘ Stamping. That is, a punch of a punching 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 re-pressing process, a punching sleeve is used to replace the punching die, which is installed between the punch and the inner diameter of the punching cup. The equal area rule determines that the increase in height is accompanied by the reduction in diameter. The stamping process can be repeated again. The book diameter gradually decreases within a certain limit and avoids the occurrence of metal fracture.
â‘¡ Thinning and stretching of the cup wall. The punched cylindrical cup is placed on the punch, and the punch is squeezed into a die in the axial direction. The gap between the die and the punch is smaller than the thickness of the cup wall. In this way, the wall thickness is maintained while the diameter remains unchanged. Get thinned. The metal volume of the barrel 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. The punch with the cup passes through a series of dies successively, one stroke 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.
â‘¢Manufacture of cupping. The manufacturing process of the drawing cup is roughly as follows: unfolding ordinary ribbon coil; Xu lubricant; blanking and punching; re-punching; thinning and stretching of the side wall of the cup; bottom shaping; simplified trimming at the correct height; cleaning and processing . For dry beverage cans, outer surface coating, printing and packaging, inner surface coating, flange forming and closing of the open end are also required. For food cans, outer surface coating, flange forming at the open end, barrel pressure ripples and inner surface coating are performed during the cleaning process.
1- The end cap is crimped and folded on the flange of the cylinder 2- Sealing packing 3- Sealing overlap amount 4- The cylinder 5- End cap
The lid or can end is made of pre-coated plates on a multi-die press, and the can lid is corrugated (reinforcement ribs) to bear internal and external pressure. The final process here is hemming to form a double hemming seam (see Figure 17-1). Place a filler on the final seal to act as a sealing gasket.
â‘£Manufacture of deep-drawn cans. The steps of manufacturing deep-drawn cans are as follows: coiling thin sheet strip into vortex-shaped sheets; coating and finishing plates (also coated coils); blank blanking and punching cups; re-punching (one time Or secondary, depending on the size of the tank); forming the bottom of the tank; cutting the flange to the correct width; inspection and sterilization.
As with the drawing process, a multi-mold cup punching machine is used to load and unload wide rolls of thin sheets or plates and punch them into shallow cups.
The shallow cup gradually reduces the diameter and increases the height through one and two re-punching. When the final stamping is stopped to a certain extent, a flange will be left on the can, like all other metal sheet forming processes. Irregular edges should be cut off, and the bottom of the tank should be processed into the required shape. [next]
2. Processing equipment
The main equipment for can manufacturing includes various stamping and forming machines, shear blanking machines and other auxiliary equipment. There are so many kinds of equipment that it is impossible-one introduction. In view of the characteristics of the modern can making industry that uses automated production, this chapter only uses the composition of the drawing and can production line
The sheet rolls are transported to the roll unrolling machine (roll unrolling rack) by a transport vehicle. The unwinding machine generally has two arms, one arm toward the thin plate coil transport vehicle, and the other arm feeding the press. The thin plate coil can be replaced by rotating the arm. The thin-plate coil is coated with lubricant on both sides through a lubricant coating device, and passes through a pair of rubber rollers to remove excessive lubricant. On the older production line, the lubricant is mixed to the correct concentration and held in one Refill the lubricant tank when needed. In newer production lines, an external circulation system is generally used, which can perform temperature control, continuously filter and keep the lubricating oil quality constant.
The punching press is a specially designed double-action press. It first punches out several round slabs, and then punches the round slabs into a shallow cup shape. These two actions are completed in one trip.
The function of the cup wall thinning and drawing machine is to transform the cup into a cylinder with the correct thickness distribution of the side wall, 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 passes through three thinning pull rings to thin the cup wall. At the end of the trip, enter the auxiliary press, forming the round arch and cup bottom. At the beginning of the return trip, with the aid of compressed air from the punching tube, the spring ejector rod removes the can body from the punch, and the canister is sent out of the machine by a conveyor belt.
Each cup wall thinning and drawing machine works in conjunction with an edge cutting machine. There are two types of edge trimmers: rail type and roller type. A lot of cores are installed on the rotating frame of the rail-type edge trimming machine, and the edge of the can is cut between the rotating core and the stationary guide rail. On the roller-type edge trimming machine, one end of the tank is fed between two rollers and rotates around its own axis, and the two rollers are close to each other to complete the trimming action.
After the tank is trimmed at a fixed height, the open side of the tank is facing down, upside down on the mesh conveyor belt, and it is transferred to a series of cleaning chambers equipped with spray nozzles in large quantities. The lubricant used at the time is washed away, and finally rinsed in deionized water to ensure no stains and dry.
Retracting a flange forming machine performs the closing and flange forming process of the beverage can, while the food canister is corrugated and flange shaped. In order to avoid roughening of the axial surface of the metal during thinning and drawing, the flange forming generally uses a spinning process. The neck of the top of the beverage can can be retracted and formed with a mold after spinning the flange.
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