Metal coating process

1, coating method

The coated and printed tinplate can be processed into three food cans, beverage cans and spray can containers. According to the characteristics of the substrate and the purpose of use, it should have a suitable corrosion resistance and printing, processing performance, for this purpose, the bottom coating should be carried out before printing. The coating processes related to printing mainly include prepress coating, surface white coating, edge seam coating, and coating (after-print coating).

Pre-press coating: coating with primer coating. The coated surface should be degreased prior to coating to remove oil from the surface. There are two types of precoating: inner coating and outer coating. The inner coating is applied on the inner side of the metal skin, that is, the molded product, and a layer of protective light oil is applied to protect the iron skin from corrosion of the contents, and also protect the contents. As the inner coating is in direct contact with the food, the coating must be non-toxic and odorless. After the inner coating, it should be dried in a desiccator. The outer coating is the coating of the printing surface, and its role is to improve the adhesion of the metal surface and the ink layer. Therefore, it is required that the coating has a strong binding force on the metal surface and also has a good affinity for the ink layer, which requires the coating itself to have good fluidity, water resistance and color rendering properties.

White coating: The metal surface is coated with a white paint or a colorless clear paint. It is often used as the background color for printed full-text graphics. The white paint shall have good adhesion and whiteness, and shall not yellow or fade under high-temperature baking conditions, and shall not peel off or peel during the canning process.

Edge seam coating: It refers to a kind of coating method in which after the resistance welding cans are formed, the edge seams can be applied to the seams of the cans to prevent rust.

Post-printing (glazing): Applying a layer of varnish to the surface of all graphic printing, its purpose is to increase the gloss of the printing surface, protect the printing surface, improve the light resistance and chemical resistance of the metal surface .

2, coating machine

As a separate metal printing auxiliary machine, the coating machine is also equipped with a feeding mechanism and a conveying mechanism and a tunnel oven. The automatic feed mechanism is similar to the metal printer feed. Before the tinplate enters the drum, it is necessary to adjust the position of the push plate and the conveyor belt to be connected, and to basically position it to prevent the printed sheet from being skewed. If the chain is found to be too loose, too tight, or the heads are not uniform on the two sides during operation, the center distance of the sprocket can be adjusted.

There are three types of rubber rollers commonly used in coating machines. The first one uses a thick blanket, that is, a solvent resistant blanket with a thickness of about 5 mm is wrapped around a hollow iron roll. This kind of rubber roller has good coating quality and is suitable for full-plate printing. The second method is to wrap the rubber blanket felt on the iron roller, which is suitable for full-size coating of general products. The third type is a rubber roller grooved according to the area of ​​the text and graphics, mainly used for resistance welding products such as food cans and aerosol cans.

The function of the scraper in the coating device is to scrape the paint and avoid the back material of the print sheet.

3, coating process

The gear pump pulls the paint in the oil pan through the conduit to the lower oil roller. The lower oil roller and the upstream oil roller gear mesh with each other and then roll. After the radial and axial forces are uniform, the paint is applied to the rubber roller. on. To control the flow of paint, the pipeline is equipped with a safety valve adjustment mechanism. The adjustment of the oil transfer roller is very important for the thickness and uniform distribution of the paint, which directly affects the product quality. The requirements for adjustment are that the axis (center line) of each roller is parallel and the contact pressure (ie, gap) is uniform. The size of the gap between the upper and lower transfer oil rollers needs to be adjusted according to the requirements of different product coating thicknesses. When the pressure of the oil roller and the rubber roller is adjusted, a thin steel sheet is used for pulling, and the pull rod is subject to elasticity. The adjustment of the rubber roller is also based on the thickness of the tinplate. The coating requirements are the same on the same sheet, and the amount of coating for each batch is the same.