Quality Control Analysis of Food Empty Cans Resistance Welding

Quality Control Analysis of Food Empty Cans Resistance Welding

Resistance welding food cans refer to food cans with resistance welding in the joints of the cans. They originated in Switzerland in the 1970s and were widely used in the world. In 1981, the first VAAK production line was introduced in the Canzhou Cannery. Up to now, it has been able to produce its own resistance welder.

Food canister resistance welding is a new canning technology that has replaced the production of solder cans. There are some technical issues in production that need to be further explored. The quality of welding is related to the proper use of the resistance welding machine. During the production process, there will be quality problems such as spatter, perforation, leakage welding, butterfly welding or irregularity of the welding wire, and the wrong angle or width of the overlap. Seriously caused large quantities of cans to leak. Therefore, it is important to study welding quality factors and find solutions to improve the quality of empty can products.

First, heat

Resistance welding is the use of heat generated by the current through the resistance of the workpiece, causing the metal on the contact surface to melt to form a solder joint. Good solder joints can only be formed by controlling the proper heat production. The heat generation is determined by the welding current (I), the workpiece resistance (R) and the welding time. The heat is proportional to the square of the current and proportional to the resistance time. The formula is expressed as follows: Q=I2Rt

1, welding current

During welding, the current path in the workpiece is very short. Only two tinplate thicknesses (about 0.4 to 0.5 mm) are welded to a solder joint for only half a cycle (0.01 s). Therefore, the welding current has the greatest influence on the heat generation, so it is particularly important to control the welding current. If the current is too large, the welding is excessive and splashed, the current is too small, the solder joint is not strong or there is no solder joint. The current adjustment is generally divided into coarse adjustment and fine adjustment. The coarse adjustment is adjusted with the tap changer of the transformer. Generally, the manufacturer has adjusted and fine-tuned to adjust the current phase. In the production process, it should be adjusted according to the actual situation.

2, welding force

The welding force is the force applied by the electrodes to the solder joints. The effect is to bring the two sheets into close contact, reduce the initial contact resistance between the interfaces, and finally compact the fusion. The initial resistance between the workpieces during welding has a large effect on the quality of the weld. If the surface contact is poor, defects such as arcing and splashing are liable to occur, and the welding force affects the heat through the resistor R. Therefore, when other conditions are constant, the welding current increases as the welding force increases.

3, the state of the welding surface

The state of the tinplate welding surface is very important for the welding quality. When the welding surface is covered with uneven coating such as oxide, rust, coating film or other impurities, the interface contact resistance changes, the heat generated by each solder joint is inconsistent, and the welding quality is inconsistent. Unstable, it is possible to make the weld irregular. Leave the iron and the electric iron, because the surface is smooth and soft, the contact resistance of the lap joint changes little, and the welding current is large. Although the coating iron is scraped yellow, the surface is not very smooth, and some damage the steel substrate. The contact resistance of the lap joint is relatively large, and the welding current is slightly smaller. Some such as scratching yellow or leaking scraping, due to the sharp increase in resistance during welding, the heat production rate is too fast, too high, causing overheating and violent liquid metal splashing, causing serious defects such as splashing, perforation, etc. Sometimes, because the heat is too high, the welder automatically stops.

4, electrode copper wire

The formed flat copper wire passes through the welding roller groove as an intermediate electrode, and the tin is deposited on the copper wire and is carried away without contaminating the welding wheel, thereby ensuring the cleaning of the welding wheel groove. The copper wire acts as the middle electrode in contact with the welding wheel and the tin plate. The resistance of the copper wire itself affects the welding quality. For example, the surface of the copper wire has oxides, oil stains, impurities, and the contact resistance between the copper wire and the tin plate changes. Each solder joint generates heat inconsistency, causing irregular weld quality defects in the weld. Therefore, the copper wire, the diameter tolerance is less than 0.02 mm, the purity is more than 99.9%, and the surface is smooth, clean, free of oxide film and coating contamination. In addition, the formed copper wire must be well matched with the wheel groove to reduce the contact resistance, otherwise the welding wheel groove should be repaired.

5, heat conduction

In resistance welding, part of the heat generated by the welding current is used to form the solder joint, so that the center temperature of the solder joint reaches about 1200-1300 °C, and part of it is diffused into the tinplate by the conduction of the tinplate, because the welding quality is related to thermal diffusion. The diffusion is fast, the welding current is larger, the diffusion is slower, and the current is smaller. Otherwise, the slow diffusion part is over-welded. Therefore, when the two solder joints start and end, the air heat diffusion is slow, and the welding current is low. To make a weakening adjustment. The adjustment is performed synchronously by the time controller and the current controller. The former controls the width of the current at both ends of the weld, and the latter controls the amount of current reduction at both ends. In addition, the domestic DODO-S welding machine uses the shunt method to reduce the current, and the adjustment requirements are high. Otherwise, it is difficult to ensure smooth production.
Second, mechanical positioning

The welding width of the welded joints of resistance welding food cans is generally 0.4mm, the large tank is 0.5mm, the manual welding machine is 0.5mm, and the deviation range is not more than ±0.05mm. Therefore, the can body must be accurately positioned during the welding process. If the positioning is not accurate, there will be a wrong angle at both ends of the weld seam of the can body, the weld lap overlap is inconsistent or does not meet the quality defects such as requirements, so the mechanical positioning is also an important factor affecting the welding quality.

1. Can body transfer pusher system

The can body transfer pusher is the part that pushes the rounded can body to the front of the welding wheel. If the adjustment is not correct, the wrong angle will appear at both ends of the can weld, and the original VAAK-100 adopts the pawl to push in parallel. Due to the adjustment and wear, the pawls on both sides are not parallel, and the wrong angle occurs. The unstable operation of the push rod will also affect the stability of the pawl. By adopting the Beijing Aopu company's never-wearing push system, the problem of pawl wear can be solved, and the non-parallel phenomenon of the pawl is basically overcome, and the possibility of occurrence of a wrong angle is small.
2, can body calibration ring

The can body calibration ring is the guiding and aligning device for the can body during welding, which has a great influence on the overlapping width of the welding seam. The calibration of the calibration ring seriously affects the welding amount of the can body welding, and it is easy to produce inconsistencies in the width of the two ends. Does not meet the requirements, even the can body guidance is not correct.

3, Z-shaped guide rod

The Z-shaped rod is a Z-shaped rail with a cross section, and each side has a V-shaped groove. The relative distance between the bottoms of the two grooves determines the amount of overlap of the can weld. Under the guidance and restriction of the calibration ring and the Z-bar, the amount of overlap is guaranteed. Under normal circumstances, the relative distance of the bottom of the Z-bar groove is 0.4mm for small cans, 0.5mm for manual cans, and when the distance is less than 0.4mm, production will be difficult to carry out. Domestic Z-bars have this possibility. The Z-bar affects the quality of the weld under the following conditions.

A, wear and tear

When the Z-bar wears, the relative distance of the bottom of the groove increases, and the amount of overlap increases, especially for the front end of the weld, which causes the overlap width at the beginning to be too large, and the welding is insufficient, resulting in false welding.

B, Z position

The Z-head head edge and the upper roller center-to-center spacing must be 17.0 mm. The center of the front end should be at the same height as the edge of the copper wire, so that the can body is horizontal when it is pushed from the Z-bar to the welding wheel. When the Z-bar is high or low, the cup body will be tilted or concave to affect the amount of overlap. , especially the front end, causing insufficient or too much overlap.
Third, other factors

1, the stroke exceeds

The stroke overrun is the overrun of the pusher when the can body is rounded and pushed by the pusher to the welding roller. If the can body is pushed to the welding roller and the pawl does not exceed the amount, the can body stops and cannot be carried away by the welding wheel for welding. Therefore, there must be an excess stroke amount. However, improper adjustment will affect the quality of welding. If the stroke is too large, the can is too fast, which will cause the weld to leak at the beginning of the weld and damage the can body due to excessive thrust. If the stroke is too small, the can is too slow, which will cause excessive welding at the beginning of the weld, causing perforation and even bonding with the copper wire. The stroke excess is proportional to the tank height, and the higher the tank, the greater the overrun stroke.

2, welding frequency and speed

In resistance welding, the welding current generates a solder joint every half cycle. The higher the frequency, the more solder joints are generated per unit time. The solder joint spacing = welding speed / (welding frequency × 2), the larger the solder joint spacing, the welding strength The worse the food can seams must be continuous airtight welds, therefore, the welds must be 15% to 20% overlap. When the spacing exceeds a certain range, the welds do not overlap each other, and the welds will Leaked unqualified. For example, the VAAK-100 type has a solder joint diameter of 2 mm and a maximum welding speed Vmax = 2 × 0.85 × 200 × 60 ÷ 1000 = 20.4 m / min. If it is greater than this speed, the quality of the weld will be affected. The DODO-S machine uses an adjustable frequency inverter with a frequency selection f(HZ)=1000×V/2×60×P P generally selected between 0.8 and 1.2 mm. Therefore, for the adjustable frequency resistance welding machine, the weld joints are increased, the welding strength is uniform, and the quality is good. Generally, the high speed welding machine adopts high frequency welding.

The production technology of resistance welding machine is only gradually explored in the continuous production. With the development of canned beverage industry, the development prospect of can manufacturing industry is broad. The canning industry in Ningbo has a early start and slow development and has a big gap with Guangdong Pearl River Delta. . Therefore, the discussion of resistance welding technology has great significance for the development of the can industry.