Aluminum two-piece cans with easy-open lids are currently widely used in my country for holding beer and beverages. The quality of internal spraying directly affects the safety of the filled products, which is a problem that customers and consumers are extremely concerned about. All metal cans filled with products need to be sprayed internally to isolate the metal from the filled products. For two-piece cans, water-based paint is used for spraying to avoid direct contact between the metal and the contents, avoid corrosion of the can body, and also avoid the precipitation of aluminum ions. It is a safe spraying process.
There are two types of internal coating processes in the two-piece can industry: single spray gun and double spray gun. With the increasingly precise control of quality and consumption, new companies and new equipment mostly use double guns. This type of double-gun spraying is more flexible than single-gun spraying and can better complete the re-coating of key parts.
Internal spraying directly affects the coating quality of aluminum two-piece cans and is the key point that needs to be controlled for aluminum two-piece cans. We mainly start from several aspects that affect the quality control of internal spraying, including spraying time, overflow spray volume, nozzle model, paint temperature, spraying pressure, nozzle flow and other factors.
Different contents: Different spraying quality requirements. Due to different contents, filling temperature and storage period, different thicknesses of internal coating dry film weight are required. Generally, packaging tests are required before filling new products to confirm whether the production process under the test conditions meets the filling requirements.
Nozzle model and parameters: Different companies involve different can types and capacities, and different nozzles are selected for each can type. At present, the most commonly used nozzle models for producing ordinary 330ml can are 122004/122003 for small guns and 122000 for large guns. This nozzle is used as an example.
It can be seen from Table 1 that the flow rate of the large gun nozzle is significantly higher than that of the small gun, because the function of the large gun is to spray the cans as a whole, so the flow rate is relatively large, and it needs to cover the upper wall, middle wall, lower wall and bottom of the cans. The flow rate of the small gun is relatively small, mainly for repainting the lower wall and bottom of the cans.
| Nozzle Details | |||
| Application | Model | Flow Rate | Fan |
| Small Gun | 122003 | 0.04 | 2-inch fan |
| Small Gun | 122004 | 0.04 | 4-inch fan |
| Large Gun | 122000 | 0.09 | 4-inch fan |
For 330 mL cans, because the aluminum cans are relatively short, any nozzle of 122000, 122003 or 120004 can be selected to match it. Through adjustment, it can meet the requirements of can spraying and ensure the quality and conductivity of the coating.
However, for 500ml cans, because the can body is relatively long, if the small gun uses a 122004 nozzle, even if the spray gun is adjusted to the position closest to the can mouth, its spray coverage area cannot be less than 70% of the can body, as shown in the light blue part in Figure 1. This causes the spray volume of the small gun to be dispersed to various parts of the can body, and the small gun does not play the actual role of spraying the bottom of the can. In addition, the unit spray volume of the small gun is only half of that of the large gun. Therefore, the coverage ability of the lower wall of the can body is weaker than that of the large gun and cannot play the role of re-spraying. It is easy to cause uneven distribution of paint on the upper and lower walls of the can body, and more paint will be concentrated on the can mouth. If the lower wall is to achieve the required coating thickness, the amount of spraying will be very large, which will lead to unnecessary waste of paint on the one hand, and on the other hand, it is very easy to cause the paint on the bottom of the can to accumulate until bubbling and high conductivity occur.
Therefore, in this case, in order to ensure that the small gun can play a role in re-coating, it is necessary to reduce the spraying range of the small gun, so a nozzle with a relatively small fan surface should be selected. For example, the small gun nozzle model is changed to 122003. The difference between this nozzle and 122004 is that the fan surface is smaller, so that the small gun can play a role in re-spraying the lower part and bottom of the can body. The spraying effect is shown in the dark blue part in Figure 1, so that the small gun can focus on spraying the lower part and bottom of the can body. On the one hand, it reduces unnecessary consumption of paint, and on the other hand, it avoids the occurrence of bubbles and high conductivity at the bottom of the can that affect the quality of the coating.
Spraying time: The spraying time is related to the rotation speed of the suction can plate. There is a certain difference in the single-circle spraying time of different companies. If the rotation speed of the inner coating turntable is 2240~2248, it means that the time for spraying a circle of the can body is 26.69~26.79 ms. The spraying time can be set accordingly, generally increasing the number of full circles by 2~3ms.
Spraying pressure. Spraying pressure affects the spraying quality by affecting the flow rate of the nozzle. According to the information of the nozzle manufacturer, the spraying flow rate is proportional to the square root of the spraying pressure. Reducing the pressure while ensuring spraying is beneficial to reducing the spraying flow rate. The rated flow rate of the nozzle is measured at 500PSI. The greater the pressure, the more spraying per unit time.
Overflow spray volume: In order to meet the quality requirements of spraying the entire can body and ensure sufficient coverage of the can mouth to ensure that the mold is not scratched when shrinking and flanging, the spraying must have a certain amount of overflow spray. On the one hand, the overflow spray volume is unnecessary, and on the other hand, the overflow spray volume is a waste, so the size of the overflow spray volume must be weighed.
The inner coating process occupies an important position in the two-piece can. The quality of the inner spraying directly affects the interests of customers and consumers. This paper introduces the inner coating nozzle model, pressure control, spraying time control and overflow spray volume in detail, and analyzes the inner coating quality and control.
