After welding, achieving the best pore-free weld seam repair coating on metal cans can only be achieved by using powder coating. However, there are several issues that may arise during powder coating in production. Let’s analyze them in detail below:

  • Wrinkling and unevenness of the powder coating repair band may occur after entering the full-spray process and curing in a large oven. This is typically observed with thermoplastic powder coatings, where the melting point is low. During the curing process, secondary melting and subsequent curing may lead to shrinkage and wrinkling due to differences in thermal expansion coefficients between the full-spray coating and the repair band. To solve this issue, the oven temperature for the full-spray coating can be reduced to minimize the secondary melting of thermoplastic powder coatings. If the problem persists, switching to other types of powder coatings containing thermosetting components can increase the melting point of the secondary melting.
  • Fractures may occur in the powder coating repair band after flanging and seaming. This is often due to insufficient elasticity of the repair band. To address this issue, if using thermoplastic coatings, it may be due to excessive baking. In such cases, reducing the baking temperature or shortening the baking time and accelerating cooling after baking and curing can increase the elasticity of the repair band. If using thermosetting powder coatings or coatings containing a significant amount of thermosetting components, increasing the temperature appropriately can enhance the adhesion of the repair band.
  • After baking and curing thermoplastic powder coatings, if the coating peels off too much during the peel test, exceeding 5mm, or if the adhesion test of thermosetting coatings yields poor results, the baking and curing temperature should be increased to improve the adhesion of the repair band.

Challenges and Solutions in Powder Coating Repair for Weld Seams on Metal Cans

Furthermore, if the adhesion of the powder coating layer is very poor after baking and curing, with large pieces peeling off at the overlap with the primer coating (tinplate coating), and baking has no effect, it may be due to excessive wax content in the primer coating. After baking the primer coating, wax precipitates and deposits on the surface, causing a decrease in adhesion of the powder coating layer. To solve this issue, improve the solvent of the primer coating, or before powder coating repair, wipe the weld seam repair area with strong organic solvents to remove the wax layer and improve adhesion.

  • Before baking and curing the powder coating repair, the repair band may be uneven and wrinkled, especially when matched with low-speed welding machines. This phenomenon occurs when the static high voltage is too high, causing repulsion between powder particles of the same charge. To address this issue, the static high voltage should be appropriately reduced. Additionally, if using powder that has been stored for too long (far beyond the supplier’s shelf life), this issue may also occur.
  • Before baking and curing the powder coating repair, the powder may easily fall onto the can wall on the conveyor belt. To prevent this, the static electric voltage can be adjusted to increase the powder’s adsorption capacity. Additionally, check whether there is any collision, shaking, or jumping of the can body during the conveying process to keep the can body movement stable and reduce powder falling.
  • After powder spraying, the powder coating layer may become thin, sometimes accompanied by shaking and clogging of the powder spray pipe. In such cases, the powder spray pipe should be unclogged as soon as possible or replaced with a new one. When installing the powder pipe on the welding arm, special attention should be paid to ensure that the powder pipe does not collide with the copper wire guide wheel. The powder pipe should be kept working at room temperature. Otherwise, if melted powder blocks the pipe, it will be difficult to unclog.
  • The powder coating layer at both ends of the can may be too thick, affecting the sealing effect of subsequent processes. To address this, reduce the coating thickness or increase the suction power of the external powder suction port on the OHC to make the coating thinner at both ends.
  • During long-term storage of powder coatings or continuous recycling during production, the performance of the powder coatings may change, such as charging performance, fluidity, and contamination with some dirty dust. Therefore, attention should be paid during production. Additionally, powder coatings generally contain a certain amount of moisture (0.6%~0.8%), which improves the fluidity of the powder coatings. If the powder loses the proper amount of moisture, its fluidity will decrease, and it may agglomerate, appearing damp. Therefore, moisture content should be tested. If it is significantly lower than normal, appropriate water should be added to achieve the normal water content. Then, follow the methods and parameters provided by the powder coating supplier for operation.

Yixin Technology is a well-known enterprise with 30 years of professional experience in metal can making equipment. Feel free to consult us with any professional questions related to metal can making!