Metal containers are commonly used for the production of galvanized steel sheets, tin-plated steel sheets, and chrome-plated steel sheets. Since the steel sheets themselves have a plating layer, they are generally not required to be coated. Galvanized steel sheets are mostly used to produce containers with larger capacity, while tinplates and chrome plates are mostly used to produce containers with smaller capacities. In the production process, the seam welding of the coated steel sheet destroys the plating of the weld portion, thereby also destroying its corrosion resistance and aesthetics. Therefore, after the can body is welded, the weld bead must be recoated to restore its original function. For containers made of tinplate, it is generally pre-printed with iron, but for containers made of galvanized sheet, it is generally necessary to print signs and patterns on the container after the container is finished.
1. Recoating process of galvanized sheet container
Zinc is easy to form a dense oxide film, so zinc-plated steel sheets are more resistant to corrosion than ordinary steel sheets. The galvanized container may be made of galvanized steel sheet (white iron), and the bottom cover and the can body of the container may be formed by cold-rolling and then galvanized separately, and then packaged into a container.
The zinc layer on the galvanized steel sheet is generally hot-plated, and the zinc layer obtained by hot-plating is thick, and its structural layer is composed of a steel substrate, a ferroniobium alloy layer, a bismuth layer, an oxide film and an oil film. The container made of hot-dip bismuth steel plate has good corrosion resistance.
The bottom cover and the can body of the container are respectively galvanized, and the electroplating zinc process is adopted. The zinc layer obtained by the process is that zinc ions are adsorbed by one action on the current, and the zinc layer structure formed is formed. It is looser and thinner, and because of the lack of heat penetration, the electrogalvanized sheet lacks a zinc-iron alloy layer, and the oxide film formed is also thin and has no oil film layer protection. Therefore, it is not as good as the container made of the plate.
However, when the container body is made of a galvanized steel sheet, the weld portion of the can body is ground and welded after being welded, so that the galvanized layer here no longer exists. In order to keep the galvanized layer of the galvanized container intact to ensure the overall corrosion resistance of the galvanized container, the weld must be zinc-treated.
At present, the zinc supplementation methods adopted by the container industry can be roughly divided into three types:
1. Hot-melt tin-filling method: The hot-melt tin-filling method removes the oxide layer at the weld and then performs hot-melt tinning. However, although tin is chemically stable at room temperature and hardly reacts with air, it is easy to form a sub-salt when exposed to acid, and stannate is easily formed in the presence of alkali, which has a great influence on the overall corrosion resistance of the galvanized container.
2. Aluminum paste recoating method: The aluminum paste recoating method is a special zinc-filling coating for spraying galvanized containers after grinding the oxide layer at the weld. The two-component coating is actually composed of a special coating of the aluminum powder slurry dissolved in the X-891 container. One component of the coating is a curing agent, and the other component is a preservative resin containing an aluminum paste.
Metallic aluminum can form a denser oxide film than zinc, so its corrosion resistance is better than zinc. For example, containers made of aluminum have water, sulfide, concentrated nitric acid, any concentration of acetic acid and all organic acids. Very good corrosion resistance. Therefore, the overall corrosion resistance of the galvanized container treated with the special galvanizing coating has no effect. However, although the coating is dried in about ten minutes, it takes a long time to completely cure. Therefore, after spraying the zinc-plated coating on the inside and outside of the can seam, it is best to carry out the volume on the next day. Packaged so that the coating is fully exposed to oxygen cure. Of course, if the condition of the can is baked at 180 ° C for 15 minutes (this temperature has no effect on the galvanized layer), the coating can be completely cured, so that the can body can be packaged immediately.
Some container factories use silver paint to refill the weld. Although the silver powder in the silver paint is actually aluminum powder, firstly, the aluminum powder content in the silver paint is low, and secondly, the silver paint is mostly prepared by using ordinary blending varnish and aluminum powder. Its anti-corrosion performance is poor, so it is more reluctant to apply for the weld.
3. Spraying zinc supplementation method: Spraying zinc supplementation method is the best zinc supplementation method for galvanized containers, which is to use zinc-plated container special weld zinc-filling machine for zinc-zinc supplementation.
The special weld filling machine for galvanized containers consists of two parts: sand blasting and spray blasting.
Since the weld portion of the galvanized container is oxidized due to welding, the oxide layer is first removed by refilling the zinc layer. The method of removing the oxide layer by the special weld zinc filling machine for galvanizing containers is sandblasting.
When the can body is lowered from the welder, it is lifted to the sandblasting box by the hoist (because the lower part is sanded, the sandblasting box is made higher), and the position of the supporting wheel in the sandblasting box is aligned with the box opening. When the can body enters the box, it is directly positioned on the supporting wheel. The operator will spray the nozzle of the welding seam. After positioning, press the control button, and the two sandblasting guns opposite to the injection port start sandblasting at the same time inside and outside the container. At this time, the support wheel drives the tank body to move in parallel under the push of the air pump or the motor, so that the entire weld seam of the can body goes completely along the blasting gun mouth from the beginning to the end, so that the oxide layer on the inner and outer weld seams of the galvanized container is Completely removed.
The power of sand blasting can be the air pressure applied to the sand storage tank, or it can be driven by the negative pressure generated by the flow of compressed air. The sand particles are easily affected by the pressure difference during the spraying process, and the welds sprayed with the damp sand are easy to produce rust spots under the impact of the sand particles, so the galvanized container weld zinc filling machine also needs the sanding equipment. To ensure that the sand used for sandblasting is dry.
After dust removal, it enters the zinc spraying machine. The action is basically the same as that of sand blasting. The operator will weld the weld to the zinc spray nozzle and position it, then press the control button. The two spray nozzles are opposite to the zinc spray gun. Zinc is sprayed at the welds inside and outside. The zinc spray gun relies on the high temperature generated by the combustion of acetylene to melt the zinc filament, and the liquid zinc is sprayed onto the tank by the pressure generated thereby, and the zinc filament is automatically followed up in synchronization with the melting speed. At the same time, the support wheel drives the can body to move in parallel. The entire weld of the can body is completely moved from the beginning to the end along the zinc spray gun mouth, so that the damaged zinc layer on the entire weld seam inside and outside the galvanized container is completely completed.
Due to the high temperature of zinc-smelting zinc, it is easy to form a zinc-iron alloy layer, and the surface is also easy to form a dense oxide film. Therefore, the zinc-plated layer formed by zinc-zinc-filled zinc-plating container is the closest to plating. The structure of the zinc (white iron) plate.
Second, tin plating (anchor) tank recoating process
Tin plate and chrome plate are generally used to produce small containers, which are printed before the production. For food packaging containers, printing is usually required inside and outside. When printing, in order not to affect the welding quality of the weld due to the paint, it is generally left in the overlap of the can body, that is, the paint is not printed. After seam welding, there is no coating or coating at the weld, and the inner weld is directly in contact with the contents, which is easy to rust, especially for high-temperature processing and long-term preservation of foods containing high acid and sulfur. There will be more stringent requirements, so the weld must be recoated.
A recoating device is a device that applies a protective layer that protects the weld from both the contents and the corrosion. The recoating machine generally consists of a can body conveying system and a recoating system. The can body conveying system generally has two magnetic conveyor belts, which can continuously and smoothly and accurately feed the welded can body into and out of the recoating area.
1. Internal coating machine
The internal recoating machine is a device for recoating the inner surface of the can body weld. The common forms are as follows:
(1) Roll coating: The recoating motor is decelerated by the worm box to drive the recoating wheel with the appropriate amount of paint to rotate. When the can body passes through the device, the coating is applied to the inner weld seam, and the speed of the recoating wheel can be adjusted. The coating wheel speed is consistent with the can body speed, the coating thickness can be finely adjusted, and the coating liquid level is automatically controlled. The roll coating form has the characteristics of low cost and thick coating film, but has high requirements on the quality of the weld seam and the shape of the can body, and requires a long drying time, and is easy to foam during drying.
(2) Air-spraying: It sprays the paint onto the canister in an atomized compressed air, and the spraying process is continuous. The characteristic is: the coating film is thin, but the protection effect is good; due to the air mixed during the spraying process, a large amount of solvent can be removed, the weld quality and the shape of the can body are particularly required, and the solid content is low; due to continuous spraying, the coating The consumption is large and the environmental pollution is heavier.
(3) Airless spray: The paint is not mixed with air, and is passed through a small pipe with high pressure to maintain a stable viscosity through the heater and sent to the spray area. During the spraying process, when the can body passes through the nozzle, the sensor sends a signal to spray the paint, and automatically stops when there is no tank. Compared with air spray coating, the coating film is thick and the protection effect is better.
(4) Powder coating: From the feeding mode, there are two kinds of vacuum low pressure conveying and air jet conveying. The feeding system continuously feeds the polymer powder material with corrosion resistance to the spraying area by the pipe on the lower arm of the welding machine, and is charged by the high-voltage static electricity in the vicinity of the nozzle, and the nozzle sprays the charged powder and adsorbs it. On the inner weld, a continuous replenishing belt is formed, which is heated and melted to form a protective film. Its biggest advantage is that it does not add any submersibles, the coating film has a large curing thickness, strong corrosion resistance, short curing time, no special requirements on the quality of the weld, and the protection effect is very good, but the coating price is relatively expensive.
2. External recoating system
The external refilling system is a device for recoating the outer surface of the can body weld. Commonly, there are two forms of roll coating and brush coating.
(1) Roll coating: The motor is driven by a decelerating drive to rotate the outer coating wheel with a proper amount of paint. When the can body passes, a transparent, properly coated coating is applied to the outer weld. The conveying speed and the filling speed of the can body are respectively adjustable.
(2) Brushing: When the can body passes, the brush is applied to the outer surface of the weld by a fixed brush. The device has a simple structure, but the brush is easy to remove hair and affects the appearance of the can body.
3. Bake the equipment
The paint applied to the can body must be heat-cured to volatilize the solvent, so that the monomers can be polymerized to obtain corrosion resistance. Especially for foods that are sterilized by high temperature, the curing requirements are higher. When curing, the surface temperature of the can body shall not be higher than the melting point of tin. Otherwise, the coating may be foamed or charred, and the curing time must be sufficient. The drying device consists of a can body conveying system and a heating system. The conveying system generally adopts a chain, a steel belt or a belt, etc., and the body is fed into and sent out to the heating zone by means of the adsorption of the magnetic force, and the conveying system is divided into a linear conveying and a circular conveying, and the circular conveying saves the floor space.
Common drying equipment has the following types:
(1) Infrared heating dryer: heating and solidifying the coating with infrared rays. This method is difficult to control the temperature, and it is easy to cause the phenomenon of baking, burning, and generating bubbles. The heating power is large and the power consumption is high.
(2) Electric heating air heating dryer: the air is blown through the resistance heating element to form hot air, and then blown to the weld seam of the can body to prevent the surface coating from solidifying rapidly, and the internal solvent does not evaporate and bubbles are generated. The furnace temperature gradually increases with the conveying line, and the furnace temperature of each furnace can be controlled separately, which is easy to achieve the requirement that the furnace temperature is gradually increased from low to low.
(3) Gas hot air heating dryer: Mixing natural gas, gas and other combustible gases with air, then distributing each burner through the pipeline network, and simultaneously feeding air to adjust the temperature, the heating system can generally be automatically ignited and monitored.
(4) Induction heating dryer: The metal can body is heated by the induction current generated by the high frequency alternating current in the magnetic field. The temperature of the can body depends on the distance between the can body and the coil. By adjusting this distance, the proper can body temperature can be obtained, and the induction coil changes with the shape of the can body. The characteristic is that the heating of the coating from the inside to the outside is beneficial to the evaporation of the internal solvent, and the influence on the ambient temperature is small, and the shape of the can body should be poorly adapted.
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