Process and method of coating and printing paint in metal container

Process and method of coating and printing paint in metal container

The metal container made of thin steel plate usually needs to be coated with some kind of coating on its inner surface. and original flavor, so that the starting food can be stored, transported and sold for a long time without affecting its edible value.

01 Coating principle The coating on the inner surface of the metal container is to use a roller coating machine to print the appropriate coating on the smooth surface of the thin steel plate into a uniform and controllable thickness before the thin steel plate is cut into barrel blanks. Floor. The coating machine can independently carry out coating operation through its own feeding device, and can also be matched with an iron printing machine to form a printing and coating line.

The rotating stainless steel feed roller is half immersed in the feed tray, and the paint brought up is transferred to the rubber roller through the screed roller. There is an impression cylinder under the blanket cylinder, which is equipped with a gripper device. When the thin steel plate runs to the front of the blanket cylinder, the impression cylinder grips one end of the iron sheet tightly, and the blanket cylinder cooperates with the impression cylinder to push the iron sheet through to complete the coating operation. The thickness of the coating is controlled by the feed roll, the screed roll, and the gap between the blanket and impression cylinders. Due to the gap between sheets of sheet steel, a small amount of paint is carried away by the impression cylinder. Use a scraper to scrape off the impression cylinder (to avoid the back side of the iron sheet coating with material), the scraped paint flows into the receiving tray, and after collection, it is returned to the storage tank through the conduit and recycled.

The welding part of the side seam of the metal container should not be coated with paint, otherwise the barrel blank will easily break through the side seam during the high-frequency resistance welding process, resulting in waste barrels. Therefore, the operator needs to make grooves on the blanket cylinder according to the coating area of ​​the metal container. The paint is printed on the tinplate by the grooved rubber roller, and the blank iron left in the grooved part is the high-frequency resistance welding part of the empty barrel.

We have realized in practice that the size of the slot should be 1-2mm wider than the actual welding space, so that the size of the printed space can meet the welding requirements of the empty barrel.

The coated iron sheet is transported to the drying room by the conveyor belt, and is cured at high temperature in the drying room under the driving of the circulating chain. The baking room is generally divided into three parts: the first part is to preheat the iron sheet to the baking temperature; the second part is to bake at a constant temperature; the third part is to fully cool the iron sheet. The entire baking time takes 10 to 15 minutes. After that, the iron sheet is unloaded with an automatic plate unloader, and the coating is transferred to the barrel-making process after passing the inspection and physical and chemical tests.

02Process requirements

Sheet steel has a long history as the leader in metal container packaging materials, and its effectiveness should largely be attributed to the important protective effect of inner coatings. The performance of the coating should not only have corrosion resistance, but also have good adhesion, flexibility, non-toxic and tasteless. The coating must be able to withstand heating during high frequency resistance welding and autoclaving at 121°C after gridding without losing its properties. In addition, the type of coating and the thickness of the coating must be selected according to the contents of the container.

1. Coating Type and Suitability

Epoxy-phenolic resin coating: This coating has the widest application range. It has good corrosion resistance to acidic food, and its epoxy resin component makes it have good adhesion and flexibility. Its baking temperature is about 200~210℃/15min, it is mainly suitable for canned food such as soup, meat, fish, fruit and juice, and can also be used for the inner and outer surface coating of crown bottle caps and other kinds of caps. The content ratio of the two resins can vary widely. Although the adjusted formulation can provide resistance to phosphates, it will reduce the adhesion of phenolic resins. For foods with high sulfur content, zinc oxide and aluminum powder fillers can be added to prevent the formation of sulfurized spots on the inner wall of the barrel. Coatings with added zinc oxide are not suitable for acidic foods and can cause discoloration on individual green vegetables.
　　

Acrylic resin coating: usually added with titanium dioxide filler to make white, with good flexibility and excellent anti-sulfur performance, mainly used for vegetables, soups and foods containing substances that will generate sulfide spots. After opening, the inner wall of the container has a pleasing and attractive appearance like ceramics.

Phenolic resin coating: has good coverage against chemicals and has excellent corrosion resistance. It blocks sulfide ions and prevents them from penetrating the coating. It is suitable for meat, fish, vegetables and soups. But the adhesion to the tinplate surface is poor, especially when the coating thickness is greater than 3-4mm, the flexibility of the coating is poor. Its baking temperature generally does not exceed 190 ℃, otherwise the coating penetrates into the substrate through the pores of the tin-plated layer and thermal degradation occurs.

A sulfur-containing compound that occurs naturally in animal and plant foods. During the thermal processing of the container, these compounds will decompose to generate free sulfide and hydrosulfide ions, and will release hydrogen sulfide, forming sulfide spots on the scratched part (mechanical damage) of the inner wall of the barrel. When the pH value of hydrogen sulfide is less than 6, it will cause the production of iron sulfide in the headspace in the barrel and fall off to contaminate the contents of the container.

It should be pointed out that the phenomenon of vulcanization on the inner wall of the container only affects the appearance of the inner wall, and has little effect on the food. When consumers open the container for use, the vulcanization spots on the inner wall of the container will make consumers mistakenly think that the container has deteriorated and complain to the relevant departments. Therefore, when printing tinplate, the production technician should understand its use after printing, and select the type of coating according to the characteristics of the food to be bottled.

2. coating thickness

One of the main purposes of coating the inner wall of food containers is to prevent a large amount of tin from being dissolved. However, some foods require some tin in the barrel to dissolve to form free tin ions to maintain the appearance and flavor of the food in the barrel. Without free tin ions, the color of light-colored foods tends to darken, and the color of green vegetables changes. In the oxygen-deficient state in the barrel head, tin ions and food produce tin-chromium compounds to remove sulfides in food. These effects will allow the container to be stored, sold, and eaten over a long period of time.

A small amount of tin needs to be dissolved in food in metal containers, so the coating thickness of the inner wall of the container is generally required to be between 3.5 and 6.0g/m2. In this thickness range, the coating has a certain porosity, and the liquid in the container penetrates into the surface of the tin layer through these pores, and slowly dissolves the tin to form tin ions, thereby achieving the purpose of extending the shelf life of the can.

For acidic foods, such as asparagus, bamboo shoots in clear water, cauliflower, etc., the coating on the inner wall of the container can be printed by the "composite coating layer" process. The protein content of these plants is high, and in the process of heat sterilization by wrapping the container, more hydrogen sulfide is decomposed by the sulfur-containing amino saddle, which has strong sulfide corrosion on the barrel wall. The "composite coating layer" adopts the method of "three coats and two bakes" in the coating and printing process. That is, a primer with strong adhesion and high plasticity based on epoxy resin and xylene resin is firstly printed on the surface of the iron sheet. After baking and curing, the epoxy resin and phenolic resin type coatings are overcoated twice. (When the bottom of the bucket and the cover are applied for the second time, the position of the metal teeth should be reversed). After such "three coats and two bakes", the total thickness of the coating reaches 10-13g/m2, which increases the compactness of the coating and almost disappears 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

To achieve good coating quality, in addition to the anti-sulfur and anti-acid properties of the coating itself, the drying conditions of the drying room are closely related to the formation of the coating structure. The bake temperature and bake time depend on the curing conditions required for the paint coating. Therefore, the operator must fully understand the relationship between the temperature in the drying room and the surface temperature of the iron sheet during production to ensure that the coating printed on the iron sheet surface can be cured within the required temperature and time, so as to obtain a high-quality coating. Under normal circumstances, the difference between the temperature of the drying room and the surface temperature of the iron sheet is 20 °C. The temperature difference is closely related to the heat capacity of the drying room, the specification of the iron sheet, and the coating speed, and must be determined according to the actual situation. The commonly used test method is to select 5 fixed points on the surface of the tinplate, paste the temperature measuring paper on it, and then pass the iron sheet pasted with the temperature measuring paper through the hot drying room, and the maximum temperature difference of the five test points is required to be 10. within °C. 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. oven stand

The bracket scratches the tinplate surface coating, and even scratches the tin-plated layer, which is a serious quality problem in the coating process. Food is served in containers with scratched inner walls, where the iron base is exposed to the container contents and begins to corrode. During the corrosion process, a large amount of hydrogen is generated, which increases the pressure in the barrel, and the container is deformed (commonly known as expansion) and discarded. The main reasons for stent scratches are:
　　

(1) When the iron sheet is running in the drying room, due to the impact of the hot air, the iron sheet and the bracket are rubbed;

(2) The chain and the bracket shake during operation, causing the iron sheet and the bracket to collide with each other;

(3) The bracket is baked at high temperature for a long time, and the surface is rusted and oxidized. When the paint coating is not dry, the oxide adheres to the surface of the iron sheet to form sand grains, and the iron sheets are stacked to cause scratches.
　　

To avoid and reduce the traces of mechanical scratches, the oven rack must be kept clean and bright, and the rack must be cleaned regularly depending on the use of the rack. The sodium hydroxide solution for cleaning is heated to 80 ℃, automatically circulated and rinsed, then rinsed with clean water and dried. The chain and bracket of the drying room should be inspected at ordinary times to keep the operation straight and the speed balanced. If the vibration of the iron sheet is too large, the wind pressure should be appropriately reduced. The specification and load-bearing capacity of the bracket should be proportional to the specification and weight of the iron sheet to reduce the vibration amplitude during operation.