Weld seam coating plays a crucial role in achieving the best quality results, which depend not only on the coating reapplication method but also on the main components of the coating:

• Powder coatings are mainly composed of plastic, based on high molecular organic compounds, classified as follows:

a. Thermoplastic powder coatings: Similar to thermoplastic plastics, thermoplastic powder coatings can be repeatedly melted and shaped after heating for 10 minutes. The coating band formed by thermoplastic powder coatings has good elasticity and flexibility. To successfully reapply thermoplastic powder coatings to the weld seam area of the can body and form a thin coating layer without micropores (low cost), it is essential to choose the appropriate powder particle diameter. Experience shows that powder coatings with a maximum particle diameter of 100 μm can be applied to form a minimum thickness of 50 μm thin, micropore-free coating layer. When this powder coating melts at around 230°C, the powder coatings fuse together. The thickness and density of the coating layer have a certain relationship with the diameter distribution of the powder particles, directly affecting the cost of the powder reapplication process. Powder coatings with an average particle diameter of 50 μm can form a minimum thickness of 25 μm thin, micropore-free coating layer. However, to achieve this result, powder reapplication coatings with higher static charges must be used to form the best high-density coating layer in the weld seam area. Thermoplastic coatings have good plasticity and mechanical properties. Even if a thicker powder coating layer is used, the coating will not be damaged in subsequent processes such as flanging, necking, and rolling, even if there is no oxygen-free gas protection process on the weld seam. However, to save costs, powder coatings should be applied as thinly as possible under acceptable conditions. A thin coating, without any oxide at the weld seam, is preferable for subsequent processes. The baking and curing time of powder coatings should be 8-10 seconds to ensure that the powder coating layer is fully melted, forming a dense, micropore-free coating layer.

b. Thermosetting powder coatings: Thermosetting powder coatings cannot be reshaped repeatedly after heating. Generally, they are formed after one melting, and cannot be melted and reshaped again. The coating band formed by thermosetting powder coatings is not as flexible as thermoplastic coatings. This material is suitable for processing powder coatings with an average particle diameter of 30-40 μm, making the powder coatings have the necessary flowability during the reapplication process. Since this powder is a thermosetting coating, it undergoes a curing stage after the pre-polymer melts, that is, a polymerization reaction occurs, thereby forming a polymer compound. Because thermosetting coatings can achieve thinner reapplication layers, the cost of using powder reapplication is reduced to a level close to that of liquid coatings.

c. Mixed type powder coatings of thermoplastic and thermosetting: In order to improve the comprehensive properties of powder coatings and adapt to different contents, protective effects, and flexibility under different conditions, manufacturers often mix thermoplastic and thermosetting powder coatings in certain proportions to prepare different types of powder coatings to improve their mechanical properties and achieve good applicability and reapplication effects. Therefore, when using powder reapplication equipment, a small amount of mixing will not have a significant impact when changing different powder coatings.

• The baking and curing process of powder coatings is basically the same as the baking equipment used for liquid coatings, but the baking and curing process of powder coatings is different from that of liquid coatings because powder coatings do not contain solvents, so there is no need for solvent volatilization process in this process. Simply put, the baking and curing process of powder coatings is a plastic melting process. The powder coating layer needs to be rapidly heated and melted after entering the oven area, and then form a stable curing reapplication band. However, the baking curves are different. Due to the different characteristics of thermoplastic and thermosetting powder coatings, the baking and curing curves and principles are also somewhat different.

The baking and curing curve of thermoplastic powder coatings has a polymerization time of 5-8 seconds above the melting point temperature (232°C), and some powder coatings can last for 8-10 seconds to ensure that the reapplication band has uniform flowability, forming a micropore-free protective layer, and enhancing the adhesion of the coating. Thermoplastic powder coatings have a short curing time, so they have strong inclusiveness during curing.

Note: After the thermoplastic coating is baked and cured, the coating band requires rapid cooling as much as possible, and a cooling device can be added to improve the elasticity and flexibility of the reapplication band, which is beneficial for subsequent processes such as flanging, necking, and rolling. If the curing time is too long, the thermoplastic coating band will tend to recrystallize again, becoming relatively brittle and hard, which may cause problems in subsequent processes. Of course, if it is necessary to cut the cans, too much elasticity of the coating band will affect the cutting effect, and the curing time can be appropriately increased at this time.

The curing of thermosetting powder coatings is a chemical reaction, that is, a polymerization process occurs, forming a polymer compound, so a longer time and a higher temperature are required to complete this reaction. The temperature should be above 232°C, and the time should reach 7-10 seconds, and can last for 10-12 seconds, otherwise the adhesion will not be enough. Therefore, the baking and curing equipment for thermosetting powder coatings have higher requirements. In addition, because thermosetting coatings will not soften again after heating and curing, their flowability is better than that of thermoplastic powders after curing. Therefore, the coating layer formed after curing by thermosetting powder is thinner than that of thermoplastic powder, but the cost is lower, sometimes it is close to liquid coatings. However, the coating layer of thermosetting powder is relatively brittle, and there may be problems in subsequent processes.

Note: During baking and curing, whether it is thermoplastic or thermosetting powder coatings, there is a common problem, that is, when the baking temperature is high, the color printing coating on the outside of the can body will be baked to change color, so comprehensive consideration should be given when adjusting the temperature. After reaching the melting point temperature, various coatings need to be continuously maintained for a certain period of time.

• Commonly used powder reapplication baking and curing process equipment: Powder reapplication equipment, in the past 20 years, has mainly been produced and manufactured by two Swiss companies, as shown in Figures 3-104 and 3-105. In recent years, some new manufacturers have emerged. The equipment is mainly composed of powder processing system and pipeline system. The components of the pipeline system include powder spraying reapplication arms, powder spraying pipes, internal powder suction pipes, high-voltage wire pipes, clean high-pressure needle air pipes, and external powder suction devices on OHC.

The powder processing system includes powder raw material powder boxes, powder filtering powder boxes, cyclone powder processors, exhaust fans, reapplication powder boxes, high-voltage electrostatic systems, and PLC control monitoring systems.

Its working principle is as follows: the powder is drawn into the filter powder box by the vacuum negative pressure generated by the exhaust fan or the powder suction gun from the coating or raw material powder barrel, then passes through the powder filter screen and cyclone powder processing system to make the powder particles reach the appropriate size for reapplication and clean. To prevent the powder from being drawn into the production environment, a high-density filter is added in front of the exhaust fan to separate the air from the powder. At the same time, the powder is fluidized by a small airflow treatment in the filtering powder box. When powder is needed in the powder reapplication box (detected by a sensor), the processed powder is sent from the filtering powder box to the reapplication powder box by air flow. When the welder is running, a signal is sent to the powder reapplication equipment, which opens the powder spraying airflow and valve on the reapplication box simultaneously. The powder will be transported to the powder spraying arm under the action of airflow. There is a high-voltage needle at the outlet of the powder spraying arm, which emits high static electricity, about 20-40 kV. When the powder coating passes through the high-voltage area, the powder particles are charged. Under the action of airflow and high-voltage electric field, the powder particles are adsorbed on the tank wall in the weld seam area. At the outlet of the powder arm, there are a pair of lips on both sides, which play a sealing role, blocking the powder from spreading to both sides, forming a neat reapplication band. The higher the static high voltage, the better the adsorption of the powder coating.

During the reapplication, the powder between the two tanks is sucked back to the filtering powder box by the external powder suction device on OHC and reused. At the powder arm, some falling powder is sucked back to the filtering powder box by the bottom powder suction device for reuse. Therefore, the waste of powder reapplication equipment is minimal.