The production process of the bottom cover plate is: coating iron → wax spraying → cutting → punching cover → round edge → pouring → drying → inspection → packaging.
(1) Spray wax (oil-coated)
In order to avoid mechanical scratches, stripped wires and other defects during the capping process, which will affect the quality of the capping, a layer of protective oil should be applied before the coating iron is cut. Therefore, the painted iron must go through the wax spraying process before cutting. The oil used is mainly food grade paraffin oil.
The oiling is usually carried out on the paint line, that is, an oiling process is added after the inner and outer surfaces of the paint iron are painted. These are all done directly on the automatic assembly line. The amount of oil applied should be small and even. There are also some units that are specially equipped with a scouring machine, placing the integral coating iron on the feeding device, automatically conveying and scouring the oil, and forming an overall stack after completion. This method also increases the chance of mechanical scratching of the coating iron due to the additional transfer steps.
For the bottom cover punching, automatic CNC punching machines are used to complete equipment maintenance and maintenance, and some small can-making companies mostly use semi-automatic punching presses to punch the cover, which is slow and inefficient.
For punching, the blanking size must first meet the technical requirements. At the same time, the quality of the pre-press equipment must be strictly controlled during the production process of the punching to avoid missing corners, dragging tongues, and orders due to inaccurate feeding positions or defects of the coating iron itself. Edge defect. The expansion ring should be free of cracks, hard prints, and no scratches on the painted surface.
(3) Round edges
In the automatic capping assembly line, capping and rounding are done in one go. The rounded edge requires that the edge of the cover bottom should not be wrinkled, flat, or triangular. There is no squeezing, no greasy dirt, and the paint can not fall off when the bottom of the cover is punched. In large-scale production, the quality inspection department must frequently conduct random inspections, and the sample covers for random inspections are placed in the laboratory for necessary physical and chemical inspections. It is usually immersed in 5% copper sulfate solution for 30 minutes, and then the offset printing machine on the surface of the sample cover is observed, and the quality is qualified if there is no intensive pitting corrosion.
(4) Glue injection
In order to make the three-piece cans have good sealing performance and improve the shelf life of the stored foods, sealant liquid should be poured in the grooves around the lid circle. This is a food-grade water-based adhesive. It must be fully stirred before use, and the glue film should not be too little or too much when pouring, there should be no glue breaking, no bubbles, or wet glue. Strictly master and control the drying temperature and time of the film. Generally, the moisture content of the film after drying should be less than 3%. In order to achieve the best state of the sealant and reduce the residual moisture, place the dried lid for 24 hours before putting it into sealing use.