Problems in the production of metal shaped cans (2)
Fourth, the impact of welding quality on product quality
For the welding of shaped cans, resistance welding machines are basically used at present. Its characteristics are good sealing performance and high weld strength. When the tensile strength test is performed, the crack is often not on the weld. Second, the seam width is small, the raw material is saved, the weld seam is narrow, the color printing area is large, and the appearance is beautiful; the thickness of the weld is thin, which facilitates the subsequent operations such as flanging, necking, expansion and sealing. Third, due to the high degree of automation, productivity is high.
1. The basic principle of electric resistance welding
Resistance welding is to place two layers of metal sheets to be welded between two roller electrodes that are continuously rotated, energized and pressurized, and heat is generated by resistance, so that the welded sheets between the rollers advance, and numerous solder joints form welds ( See Figure 2).
The heat generated by the current thermal effect is Q=I2RT
In the formula I - current, A; T - power on time,
S; R - resistance of the weld zone, Ω. It is the contact resistance between the roller and the workpiece, the internal resistance of the two metal sheets and the contact resistance between the two workpieces.
It can be seen from the above formula that the most effective way to generate large heat in a short time is to increase the current. For this reason, the resistance welding machine is equipped with a special transformer, and the secondary coil has only about two turns so as to be in the secondary. Get low voltage and high current.
During seam welding, the roller as the electrode continues to rotate, so that the metal forming the weld is separated from the weld zone between the rollers, and at the same time the unwelded metal is brought into the weld zone. However, the seam welding process requires intermittent energization because if the energization is uninterrupted, the resistance at the formed solder joint will become the internal resistance. Due to the small internal resistance, most of the current will flow through the two pieces of metal to be welded from the solder joint, and the current between the rollers does not pass or only a small portion flows through the contact to be soldered to form a shunt phenomenon. The shunt phenomenon will affect the rise of the temperature in the weld zone, thus affecting the formation of new joints. In order to avoid this phenomenon, the energization time of the rolling electrode is continuously interrupted, and a lot of solder joints are formed to form a weld. However, the power-off time should not be too long, otherwise it will form an isolated solder joint and the weld seam will not be well sealed. Ideally, the two adjacent solder joints overlap slightly (see Figure 3).
2. Manufacturing process of resistance welding can
When the can body is welded, it is generally round. The manufacturing process of the can body is: printing iron → cutting (shearing) → winding round → welding → recoating → drying → bulging → flanging → sealing → inspection.
The tinplate used in the electric resistance welding tank must be left in the printing space with a certain width of the printing blank, called "left blank" to avoid the influence of printing ink on the welding quality. This process requires high precision because the unloading is inaccurate and directly affects the weld quality of the weld.
The flat sheet is fed by a transfer roller, and the sheet is repeatedly bent to eliminate internal stress by a bending device composed of a plurality of rollers and a guide block, which is called softening. The softened sheet is rolled into a cylindrical shape after entering the rounding device to the tip and the groove (see Fig. 4). The rounding device can be adjusted to form different can diameters.
The rolled cylinder is pushed by the conveyor chain into the beginning of the positioning rail (ie, the Z-shaped rod), see Figure 5. The Z-shaped rod guiding groove gradually transitions from the A in the figure to the state B at the exit of the Z-shaped rod. During the advancing process, the two sides gradually approach each other. When the welding roller is about to enter, the two sides of the tank overlap, the amount of overlap is a prescribed value, and then the chain is transmitted. The stopper pushes the can body into the upper and lower electrode rollers for welding.
When welding, the two overlapping sides of the can body to be welded are placed between the upper and lower welding rollers. In order to prevent the tin on the tin plate from sticking to the edge of the roller, a copper wire moving synchronously with the can body is used as an intermediate electrode to remove the tin splashed on the soldered plate. The copper wire is wound around the electrode roller groove (see Figure 6), and the welding current and voltage are transmitted to the can body weld. Therefore, the copper wire is required to have high electrical conductivity and thermal conductivity; the copper wire is subjected to tensile force and requires higher Tensile strength; also requires a clean surface, no oxide film and oil stains, otherwise, breakdown, copper wire sticking and other phenomena may occur. Used copper wire can be recycled and reprocessed. In order to make the roller in good contact with the power supply, the wheel is filled with mercury. In order to prevent the roller from overheating, it is dissipated by circulating water in the wheel (see Figure 7 for a short time).
In addition, welding parameters such as welding pressure, current and welding speed will affect the welding quality of the can body. For welded tanks, the surface of the weld is oxidized and blackened. To prevent corrosion from the contents and atmospheric corrosion, the tank should be immediately sent to the recoating mechanism and a layer of quick-drying paint should be applied to the weld. After the recoating, the can body is sent to a dryer for drying, and is cured at 280 ° C for 10 seconds.
1. Appearance quality check
There shall be no leakage or expansion of the tank;
The tin layer on the outer wall of the tank is intact and there is no obvious rust and scratch. The can body is smooth without obvious edges and corners;
The main pattern and text printed on the outside of the tank shall be clear and there shall be no serious abrasions and damages;
The whole weld seam is smooth and beautiful, the overlap is uniform, the weld joints are evenly connected, and there is no welding failure and breakdown phenomenon;
The welding recoating tape should be smooth and even, completely covering the weld and the blank of the paint, and the curing is complete, and there should be no large bubbles and iron spots.
The tank body should not have serious impact caused by mechanical collision;
The misalignment on both sides of the weld end shall be <0.5mm, and the end of the weld shall be <0.5mm;
The inner wall coating of the tank should not fall off;
Iron sulfide should not be produced on the inner wall of the tank.
2. Weld performance test
The tensile strength of the weld is required to be not lower than the tensile strength of the can body. The tear test is relatively simple, and is currently used in production. The method is to cut along one end of the weld seam of the tank, and bend the weld portion away from the tank body, and clamp it with the pliers to tear along the weld seam ( See Figure 8 for a sketch). Good weld seam, the weld bead should be smooth after tearing, and it should be qualified if the weld is not peeled off.
Fifth, the impact of the shape of the cans on the product quality
Before the crooked can is first turned over, the bulging process is first performed, and the approximate shape is swollen out, and then the cuff is trimmed. The burring plays an important role in the subsequent sealing (covering) process, because the tightness of the can is determined by the degree of sealing of the can body and the can lid. For high quality sealing, the burring of the can is particularly critical.
For the shaped can, the can edge is generally bent. When the can body enters the flanged die, the can body is hung by the outer die, and the can body is turned by the outer die. When the flange is turned, the four straight edges are bent and deformed, and the material does not have elongation and shortening. At the four round corners, the material belongs to the elongated flange, so the design of the flanged mold should pay attention to this problem. Under normal circumstances, the width of the flange at the rounded corner is less than the width of the flange at the straight edge of 0.8 to 1 mm, so that after the flange is turned, the wrinkles will not appear at the rounded corner, which is conducive to the smooth operation of the seal.
As can be seen from the production process of the raw materials, the thin plate is obtained by rolling. From the principle of plastic deformation of materials, it is known that the sheet has a certain directionality during rolling, and the mechanical properties of the longitudinal and transverse directions are different. Therefore, in the blanking layout, it is generally required that the can body has a circular direction and a rolling direction (ie, a longitudinal direction), and the burring direction is perpendicular to the rolling direction (see FIG. 9). Since the stress at the time of burring is large, such orientation allows the stress direction to follow the rolling direction of high mechanical properties, preventing the burring.
Sixth, the impact of the seal on product quality
Both the shaped can body and the bottom of the can (top) are double-edged. Since the distance from the center of the indenter to the edge of the can is changed, the can body is generally fixed, and the roller is rotated around the can to form a double bead. (The formation process is shown in Figure 10).
The key to the double crimping is the roller, which is divided into the first roller and the two roller. The outer dimensions are basically the same, only the groove shape of the roller is different. The roller groove curve is determined according to the need for the crimping operation. The head roller mainly bends the lid of the can lid downward to the side of the can body, and the cans are curled together to make them hook together, and the double crimping is basically shaped, so the upper curved circle of the groove curve is larger. It is convenient for the hook to continue to bend and gradually hook up and the side of the can. The two-way roller presses the crimped edge that has been initially shaped, and reaches the final shape of the curling edge, ensuring that the curling edge is smooth and beautiful, so the upper portion of the groove curve is curved larger and the lower portion is less curved.
The following defects are most likely to occur during sealing, which affects the intrinsic and extrinsic quality of the product.
1. False volume. Refers to a folded lid hook that presses the folded body hooks but not the hooks that are hooked to each other. As shown in Figure 11 (omitted).
2. Great collapse. It refers to the fact that when the can is sealed, there is a serious bump on the edge of the can body or the edge of the can lid, so that the can body and the can lid are not hooked to each other, and a clear cuff is exposed at the lower portion of the bead. As shown in Figure 12.
3. Sharp edge and fast mouth. The front edge on the inside of the top of the crimp is called a sharp edge. When the sharp edge reaches the degree of breakage of the tinned steel sheet, it is called a fast opening, as shown in Fig. 13.
4. The curling is not complete. Refers to the phenomenon that the partial curl is not fully pressed due to the slipping of the indenter or the poor rolling and rolling during the sealing process, as shown in Figure 14 (omitted).
5. Skip (skip). Refers to the phenomenon that the curling of the welded joint is skipped and the crimping is not pressed due to the thicker crimp at the weld, as shown in Fig. 15.
6. Crimping "tooth". It means that the sealing can is defective, the cover hook and the body hook are not overlapped partially, and the V-shaped protrusion formed at the lower edge of the curling edge is as shown in FIG.
7. Iron tongue and vertical lip. Due to the poor sealing, the iron tongue which is obviously exposed at the lower edge of the curling edge is called the iron tongue, as shown in Figs. 17 and 18.
8. The seal is broken. Refers to the poor sealing of the can, the fracture of the outer layer of the rolled edge, generally occurs at the seam of the can body, as shown in Figure 19.
9. Packing extrusion. Refers to the phenomenon that the lower edge of the rolled edge of the can is exposed to the sealant.
10. Double line. The double-line shape of the lower edge of the crimped edge which is rolled by the sealing roller significantly damages the rolling of the tin layer.
From the above analysis, these factors may directly affect the final quality of the product, so in the production process, only pay attention to each process step, according to the ISO9000 standard, strictly control the quality of the process, in order to obtain qualified products.
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