Challenges and solutions for powder coating repair of metal can welds

All metal cans need to achieve the best non porous weld seam coating at the weld seam after welding. If the best coating effect is required, it can only be solved by applying powder coating. But what problems may arise during powder coating in production? Let's analyze them in detail below.

After entering the full spray coating process and curing in a large oven, the powder coating tape will wrinkle and become uneven. This phenomenon is generally observed in thermoplastic powder coatings, where the melting point of thermoplastic powder coatings is low. During the curing process of full spray coatings, after secondary melting, they solidify again. Due to the different thermal expansion coefficient from full spray coatings, shrinkage occurs during curing, forming wrinkles. This method can appropriately reduce the oven temperature of fully sprayed coatings, thereby reducing the secondary melting of thermoplastic powder coatings. If it cannot be solved yet, the only solution is to replace it with other types of powder coatings, which contain thermosetting coating components to increase the melting point of the coating during secondary melting.

After rolling and flanging, the powder coated tape broke. The reason is that the elasticity of the touch up tape is insufficient. Solution: If it is a thermoplastic coating, it may be due to excessive baking. You can lower the baking temperature or shorten the time appropriately, and accelerate the cooling after baking and curing to increase the elasticity of the touch up tape. If it is a thermosetting powder coating or a coating containing a high amount of thermosetting components, the temperature should be appropriately increased to increase the adhesion of the touch up tape.

After baking and curing the thermoplastic powder coating, a peeling test should be conducted. If the coating peels too much, exceeding 5mm, or if the adhesion test of the thermosetting coating is not effective, the baking and curing temperature should be increased to improve the adhesion of the touch up tape.

In addition, if the adhesion of the powder coating layer is very poor after baking and curing, and large pieces fall off from the overlapping area with the primer coating (printing iron coating yellow), and improving baking does not work, it may be due to too much wax in the solvent of the primer coating (printing iron coating yellow). After baking the primer coating, the wax precipitates and settles on the surface of the primer, causing a decrease in the adhesion of the powder coating. Solution: Improve the solvent of primer coating. Alternatively, before powder coating, use a strong organic solvent to scrub the weld repair area to remove the wax layer and improve adhesion.

Before the powder coating is baked and solidified, the powder coating tape is uneven and wrinkled, especially for powder coating that is compatible with low-speed welding machines. This phenomenon is due to the high charging voltage, which causes the powder to repel each other. It is necessary to reduce the static high voltage appropriately. In addition, this phenomenon can also occur when using powders that have been stored for too long (far beyond the supplier's shelf life).

After powder coating and before baking and curing, the powder is prone to fall onto the tank wall on the conveyor belt. The electrostatic voltage can be adjusted appropriately to increase the adsorption capacity of the powder. At the same time, check whether there is any collision, shaking or jumping phenomenon during the transportation of the can body, and try to make the can body move smoothly to reduce the falling of powder.

After the powder is sprayed, the powder coating becomes thinner, sometimes accompanied by shaking, and the powder spraying tube becomes blocked. Quickly clear the powder spray pipe or replace it with a new one. When installing the powder tube onto the welding arm, special attention should be paid not to collide with the copper wire guide wheel. The powder tube should be kept working at room temperature. Otherwise, if the melted powder gets stuck in the tube, it will be difficult to clear it.

The powder coating on both ends of the can is too thick, which affects the sealing effect of the subsequent process. The coating thickness should be reduced, or the suction force of the external powder suction port on the OHC should be increased to make the coating thinner at both ends.

Powder coatings may undergo changes in their performance during long-term storage or repeated use during production, such as charging performance, flowability, and the presence of some dirty dust. Therefore, attention should be paid during production. In addition, powder coatings generally contain a certain amount of moisture (0.6%~0.8%), which makes the flowability of the powder coating better. If the appropriate moisture is lost, the flowability of the powder will deteriorate, and the powder coating is prone to clumping, appearing damp. So, it is necessary to conduct a moisture content test. If there is too much difference, it is necessary to add water appropriately to make it contain the normal amount of water. Operate according to the methods and parameters provided by the powder coating supplier.