Metal container welding plating

First, the galvanized sheet container coating process

Zinc easily forms a dense oxide film, so the zinc-plated steel plate is more corrosion resistant than ordinary steel plates. The galvanized container can be made of galvanized steel (tin plate). It can also be formed by galvanizing the bottom lid and can body of the container after cold rolling.

The zinc layer on the galvanized steel sheet is generally a hot-dip plating process. The zinc layer obtained by the hot-dip plating is thicker, and its structural layer is composed of a steel substrate, a neodymium-iron alloy layer, a tantalum layer, an oxide film, and an oil film. The containers made of hot-dipped iridium steel have good corrosion resistance.

After the container bottom cover and the can body are formed and galvanized separately, the electroplating zinc process is adopted. The zinc layer obtained by this process is zinc ions adsorbed by a grain of an electric current, and the formed zinc layer structure is formed. Because it is looser and thinner, and because there is no heat penetration, the electrogalvanized sheet lacks the zinc-iron alloy layer, and the oxide film formed by it is also thin, and there is no oil film layer protection. Therefore, the appearance is not as good as that of a container made of a plating template.

However, when the container body is made of galvanized steel sheet, the weld portion of the can body is ground because of welding and is oxidized after welding, so the galvanized layer here is no longer present. In order to maintain the galvanized layer of the galvanized container intact to ensure the corrosion resistance of the galvanized container as a whole, it is necessary to perform zinc treatment on the weld.

Currently, the zinc supplementation methods adopted by the container industry can be roughly divided into three types:

1. Hot-melt tinning method: The hot-melt tinning method is to remove the oxide layer at the weld seam and perform hot-melt tin addition. However, although tin has relatively stable chemical properties at room temperature and hardly reacts with air, it easily forms sub-salts in case of acid, and easily forms stannate when it encounters alkali. This has a great influence on the overall corrosion protection performance of galvanized containers.

2. Aluminum paste coating method: The aluminum paste coating method is a special zinc coating paint for galvanized containers after grinding the oxide layer at the weld seam. The paint is a two-component coating that is actually a paste of aluminum paste dissolved in a X-891 container. One component of the paint is a curing agent, and the other component is an anti-corrosion resin containing aluminum paste.

Metal aluminum can form an oxide film that is denser than zinc, so its corrosion resistance is better than that of zinc. For example, a container made of aluminum has 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 special galvanized special coating has no effect. However, although this kind of paint is dry in about ten minutes, it takes a long time to completely solidify. Therefore, after the inside and outside of the can body's weld joint is sprayed with the special zinc supplement paint for containers, it is better to carry out the roll again the next day. Packaged so that the coating is fully exposed to oxygen cure. Of course, if the can body can be baked at a temperature of 180°C for 15 minutes (this temperature has no effect on the galvanized layer), the paint can be completely cured, so that the can body can be packaged immediately.

Some container factories use silver powder paint to make up the weld seam. Although silver powder in silver powder paint is actually aluminum powder, first, the content of aluminum powder in silver powder paint is low, and secondly, the silver powder paint is mostly prepared with ordinary blended lacquer and aluminum powder. , Its anti-corrosion performance is poor, so it is relatively reluctant to make up the weld.

3. Spray zinc supplementation method: Spray zinc supplementation method is the best zinc supplementation method for galvanized containers. It is to use a special zinc welder for galvanized containers for zinc replenishment.

Galvanized container special welding machine consists of two parts: sandblasting and spraying.

Since the galvanized container weld area is oxidized due to welding, the oxide layer must be removed first to refill the zinc layer. The method of removing the oxide layer in the special zinc welder for the galvanized container is sandblasting.

When the can body comes down from the welder, it is lifted by the hoisting machine to the sandblasting box mouth (because the lower part is sanding, so the blasting box is made higher). At this time, the position of the supporting wheel in the blasting box is aligned with the tank opening. The can body is directly positioned on the supporting wheel when it enters the box. The operator will weld the sand blasting gun mouth and press the control button after positioning. The two sand blasting nozzles opposite to the spray hole start sand blasting both inside and outside the container. At this time, the supporting roller drives the can body to move parallel under the action of the air pump or the motor, so that the entire weld can of the can body is completely completed along the blasting gun mouth from beginning to end. Thus, the oxidation layer on the inner and outer welds of the galvanized container is Completely removed.

The blasting power may be the air pressure applied to the sand storage tank, or may be driven by the negative pressure generated by the compressed air flow. The sand particles are easily affected by the pressure difference during the spraying process, and the welds sprayed with the damp sand material are prone to produce rust in the rising temperature caused by the sand particles. Therefore, the galvanizing vessel welding zinc supplementing machine also needs the drying equipment support. To ensure that the sand used for sandblasting is dry.

After dust removal, it enters the zinc sprayer and the action is basically the same as that of sandblasting. The operator will press the control button after aligning the weld and positioning the zinc gun mouth. The two spraying nozzles are opposite to the zinc spray gun and the galvanized container at the same time. Zinc is sprayed inside and outside the weld. The zinc spray gun melts the zinc wire according to the high temperature generated by the combustion of the acetylene, and the liquid zinc is sprayed on the can body through the generated air pressure. The zinc wire automatically follows the melting speed. At the same time, the roller wheel drives the parallel movement of the can body. The entire weld of the can body is completed from the beginning to the end of the zinc spray gun, so that the entire damaged zinc layer inside and outside the galvanized container is completely filled.

Because the temperature of Zn-addition of zinc is high, zinc-iron alloy layer is easy to form, and a dense oxide film is easily formed on the surface. Therefore, the galvanization layer formed by Zn-addition of Zinc-addition welding machine for galvanized containers is closest to plating. Structure of zinc (white iron) board.

Second, tin (anchor) tank coating process

Tinplates and chrome plates are generally used for the production of small containers. The iron is printed before production. For food containers, it is generally required to print both inside and outside. When printing, in order not to affect the welding quality of the weld because of the coating, it is generally left blank at the overlapping part of the can body, ie no paint is printed. After seam welding, the weld seam is neither coated nor coated. The internal weld seam is in direct contact with the contents and is subject to rust. In particular, high temperature processing and long-term preservation of foods containing acids and sulfur, internal protective layers There will be more stringent requirements, so we must make up the weld.

The applicator is a device that coats the weld with a protective layer that is not detrimental to the contents and resists corrosion. The coating machine is generally composed of a can body conveying system and a supplementary coating system. The can body conveying system generally has two magnetic conveyor belts, which can continuously, stably and accurately send the can body after welding into and out of the supplementary coating area. Applicable equipment roughly classified as shown in Figure l.

Fig. 1 Classification of supplementary coating equipment

1. Inner coating machine

The inner coating machine is a device for applying the coating on the inner surface of the can's weld. The following are commonly used:

(1) Roller coating: The make-up motor is decelerated by the worm box, and the supplementary coating wheel with proper amount of paint is driven to rotate. When the can body passes through the device, the paint is coated on the inside weld, and the speed of the make-up wheel can be adjusted. The speed of the make-up wheel is consistent with the speed of the can body, the coating thickness can be fine-tuned, and the coating liquid level is automatically controlled. The roller coating form has the characteristics of low cost and thick coating film, but it 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 when dried.

(2) Air spray: It sprays the paint onto the can in the atomized compressed air. The spraying process is continuous. The characteristics are: thin coating film, but good protection effect; due to air mixing during the spraying process, it can drive off a large amount of solvent, and has special requirements on the quality of the weld seam and the shape of the can body, and the solid content is low; due to the continuous spraying, the coating Large consumption, heavier environmental pollution.

(3) Airless spraying: The paint does not mix air, uses high pressure to pass the small pipe, passes the heater to maintain the stable viscosity, and sends it to the spraying area. During the spraying process, when there is a can body passing through the nozzle, the sensor sends a signal to spray the paint, and it stops automatically when there is no can. Compared with gas spraying, the coating thickness is thick and the protection effect is better.

(4) Powder spraying: There are two types of vacuum feeding: low pressure vacuum conveying and air jet spraying. The feeding system will send polymer powder material with anti-corrosion performance continuously to the spraying area from the pipe on the lower arm of the welding machine, and it will be charged by high-voltage static electricity in the vicinity of the nozzle. The nozzle will spray the charged powder and make it adsorbed on the On the inner weld seam, a continuous coating strip is formed, which becomes a protective film after being heated and melted. Its biggest advantage is that it does not add any latent agent, the coating film has 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 expensive.

2. External coating system

The external replenishing coating system is a device for replenishing the external surface of the can body's weld. Commonly, there are two forms of roller coating and brush coating.

(1) Roller coating: The motor rotates with the outer coating wheel coated with a suitable amount of paint after deceleration. When the can body passes, a layer of transparent and appropriate coating is applied on the outer weld seam. Can body transport speed and make-up speed are adjustable.

(2) Brushing: When the can body passes, apply a brush to the outer surface of the weld with a fixed brush. The device is simple in structure, but the brush is susceptible to hair removal and affects the appearance of the can body.

3. Bake in equipment

The paint applied on the can body must be heat-cured in order to evaporate the solvent and allow the monomers to be polymerized to obtain corrosion resistance. Especially for foods that have been sterilized at high temperatures, their curing requirements are even higher. The temperature of the surface of the can body at the time of curing must not be higher than the melting point of tin, and the coating may have blistering or scorching, and the curing time must be sufficient. The drying equipment consists of a can body conveying system and a heating system. The conveyor system generally uses chains, steel belts, belts, etc., and the body is sent into and out of the heating area by means of magnetic adsorption. The conveying system is divided into straight line transmission and annular transmission, and the circular transmission saves the area.

Common drying equipments are as follows:

(1) Infrared heating dryer: The coating is heated and solidified with infrared rays. This method is difficult to control the temperature, prone to bake, scorch, produce bubbles and other phenomena, heating power, high power consumption.

(2) Electric air heating dryer: Air is blown through the resistance heating element with air to form hot air, then blown to the weld seam of the can body to prevent the surface coating from rapidly solidifying, and the internal solvent will not evaporate and generate air bubbles. The furnace temperature rises gradually with the conveyor line, the furnace temperature of each furnace can be controlled individually, and it is easy to achieve the requirement that the furnace temperature gradually increase from low to high.

(3) Gas-fired hot air heating and drying machine: Combine combustible gas such as natural gas and gas with air, and then distribute each burner through the pipeline network, and send 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 induction current generated in the magnetic field at high frequency AC. The temperature of the can body depends on the distance between the can body and the coil. By adjusting this distance, an appropriate can body temperature can be obtained, and the induction coil changes with the shape of the can body. Its characteristics are: the heating of the coating from the inside to the outside, is conducive to the evaporation of the internal solvent, the impact on the ambient temperature is small, it should adapt to the shape of the can body should be poor.