Painting and printing of the inner paint of the ho

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Coating and printing of can inner coating (Part 2)

sulfur containing compounds naturally exist in animal and plant foods. During the hot processing of cans, these compounds will decompose to produce free sulfide and hydrogen sulfide ions, and will release hydrogen sulfide, forming sulfide spots on the scraped part (mechanical injury) of the inner wall of the can. When the pH value of hydrogen sulfide is less than 6, it will cause the production and falling off of iron sulfide at the top gap in the tank and pollute the contents of the can

it should be pointed out that the vulcanization phenomenon on the inner wall of cans only affects the appearance of the inner wall, and has little impact on food in fact. When consumers open the can for eating, the vulcanization spots on the inner wall of the can will make consumers mistakenly think that the can has deteriorated and complain to the relevant departments. Therefore, when painting tinplate, the production technologist should understand its purpose after painting and choose the coating type according to the characteristics of the canned food

2. coating thickness

one of the main purposes of coating the inner wall of food cans is to prevent a large amount of tin from dissolving. However, some foods need some tin in the can to be dissolved to form free tin ions to maintain the appearance and flavor of the food in the can. If there is no free tin ion, the color of light colored food tends to darken, and the color of green vegetables will also change. Under the condition of lack of oxygen in the can, tin ions form tin chromium compounds with food to remove sulfides in food. These functions will enable cans to be stored, sold and eaten for a long time

tinned food requires a small amount of tin to be dissolved, so the coating thickness of the inner wall of the can is generally required to be between 3.5 ~ 6.0g/m2. Within this thickness range, the coating has a certain porosity, and the liquid of the contents of the can penetrates into the surface of the tin layer through these pores, slowly dissolving the tin to form tin ions, so as to extend the shelf life of the can

for acidic foods, such as asparagus, clear water bamboo shoots, coconut, etc., the coating on the inner wall of cans can be painted by the "composite coating layer" process. The protein content of these plants is high. During the heating and sterilization process of can sealing, there is more hydrogen sulfide resolved by sulfur-containing amino saddle, which has strong sulfide corrosion on the tank wall. "Composite coating layer" refers to the method of "three coating and two baking" in the coating and printing process. That is, first paint a layer of primer with strong adhesion and high plasticization based on epoxy resin and xylene resin on the surface of the sheet. After baking and curing, re paint the epoxy resin and phenolic resin coating twice (immediately after the second coating of the can bottom and cover, it is necessary to transfer the position of the mouth iron mouth of the investment guide for key areas formulated by MA). In this way, after the "three coating and two baking" coating, the total thickness of the coating is 10 ~ 13g/m2, which increases the compactness of the coating and almost eliminates the porosity. The coating not only has good adhesion, but also has good flexibility and ductility, and has good can making performance

3. Baking conditions

in order to obtain good coating quality, in addition to the sulfur and acid resistance of the coating itself, the drying conditions of the drying room have a close relationship with the formation of the coating structure. Power battery giants represented by Panasonic, 3-star SDI, LG chemistry, etc. have successively entered China to build power battery factories. The baking temperature and baking time depend on the curing conditions required by the coating. Therefore, the operator must fully understand the relationship between the temperature in the drying room and the surface temperature of the sheet during production, so as to ensure that the coating painted on the surface of the sheet can be cured at the required temperature and time, so as to obtain a high-quality coating. In general, the temperature difference between the drying room and the surface of the sheet is 20 ℃. The temperature difference is closely related to the heat capacity of the drying room, the specification of tinplate and the printing speed, and must be measured according to the actual situation. The commonly used test method is to select five fixed points on the surface of the tinplate, paste the temperature measuring paper on it, and then pass the iron sheet with the temperature measuring paper through the hot drying room. It is required to constantly strive to keep the maximum temperature difference of the five test points within 10 ℃. The temperature and heat distribution of the drying room can be judged from the temperature measuring paper, so as to know whether the coating is heated evenly

4. Drying room bracket

the bracket scratches the surface coating of the sheet, and even the tin coating, which is a serious quality problem in the process of printing. Use containers with scratched inner walls to contain food, and the iron base of the scratched area is exposed to the contents of the can and begins to corrode. A large amount of hydrogen is produced in the corrosion process, which increases the internal pressure of the tank, and the can is deformed (commonly known as the expansion tank) and discarded. The main reasons for the scratch of the support are:

(1) when the iron sheet is running in the drying room, due to the impact of hot air, the iron sheet and the support friction

(2) the chain and support vibrate during operation, which makes the iron sheet and support collide with each other, and the control circuit is highly integrated

(3) the support is baked at high temperature for a long time, and the surface is rusted and oxidized. When the coating is not dry, the oxide adheres to the surface of the iron sheet to form sand particles, which will cause scratches after the sheet is stacked

to avoid and reduce the traces of mechanical scratches, the drying room support must be kept clean and bright, and cleaned regularly according to the use of the support. Heat the sodium hydroxide solution for cleaning to 80 ℃, wash it automatically, rinse it with clean water, and then dry it. The chain and support of the drying room should be overhauled at ordinary times to keep the operation straight and the speed balanced. If the iron sheet jitters too much, the wind pressure should be appropriately reduced. The specification and bearing weight of the support shall be in direct proportion to the specification and weight of the iron sheet to reduce the vibration amplitude during operation

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