If achieving an optimal, pore-free weld seam coating on metal cans post-welding is only possible through powder coating, how do we determine the quality of powder coating in production? Today, we will discuss common inspection methods for seam welding powder coating:

Acetone Test:

• Test Solvent: Acetone

Test Procedure: Wet a cotton swab with acetone and gently move it back and forth over the cured liquid coating. Each back and forth motion counts as one stroke. Count the number of strokes until the bottom of the can is visible.

Evaluation: The number of strokes should exceed 50 before the liquid coating layer is completely dissolved. If not, the hardness of the coating needs further improvement, usually by increasing the curing temperature.

• Copper Sulfate Micropore Corrosion Test:

Test Solution: Copper sulfate (CuSO4.5H2O) 250g, distilled water (H2O) 875mL, hydrochloric acid (HCl) 100mL.
Test Procedure: Cut a section of the coating from the can and place it in a container filled with the test solution. Let it sit for about 2 minutes at an ambient temperature of approximately 20~25°C. Remove the sample, clean it with water, and observe the results under a magnifying glass.
Evaluation: For liquid coatings, a certain number of micropores are acceptable, depending on the contents. For thermosetting coatings, only 1~5 pores are acceptable. Thermoplastic coatings should ideally have no pores on the repair tape.

For further inspection of powder coating thickness and quality, apply a voltage of approximately 5VDC to the repair tape after the above test and observe for the formation of any pores. Powder coatings should not develop any pores after curing.

• Mechanical Strength Test of Coating:

Fill an empty can with saline solution (NaCl + H2O) to about 2~3mm below the top. Measure the current using an ammeter. A lower current reading indicates better resistance of the coating to mechanical stress, i.e., the coating is less likely to be damaged in subsequent processes.

• High-Temperature Boiling Test:

This test evaluates the coating’s repair effect under high-temperature conditions, simulating sterilization at 121°C (or up to 129°C under extreme conditions) for 90 minutes.
Cut a section of the repair tape from the can and place it in a sealed, pressure-resistant container filled with water or solvent. Heat the container to 121°C, simulating the conditions of high-temperature sterilization. After 90 minutes, remove the sample, let it cool, and observe the condition of the repair tape.
Evaluation: There should be no visible rust or corrosion, and for most liquid coatings, there should be no significant color change. If any issues are observed, such as whitening, the curing condition of the repair tape needs improvement. Additionally, this test can be used to assess the adhesion of the repair tape after high-temperature boiling, in combination with other methods.

• Thermoplastic Powder Coating Tensile Adhesion Test:

Cut a section of the repair tape from the can. Cut triangular notches on both sides of the repair tape and repeatedly bend it until the tinplate breaks, while the thermoplastic coating remains intact. Then, tear off the powder coating layer according to the method shown in the figure.
Evaluation: Measure the length of the peeled coating film. If it exceeds 5mm, the curing temperature is insufficient and needs to be increased. After high-temperature sterilization, the peeling length allowed in this test can be extended to 12~20mm.

• Cross-Hatch Cut Test (Powder Coating Adhesion Test):

Cut a section of the repair tape from the can and cross two straight lines with a sharp knife. Attempt to peel off the coating layer at the intersection of the lines using the same knife.
Evaluation: Excellent: No coating is detached. Good: Very few coating pieces are detached, 1~2 at most, not exceeding 5%. Acceptable: Approximately 1mm of coating detachment is acceptable.

• Thermosetting Powder Coating Adhesion Test:

Cut a section of the repair tape from the can and draw a set of intersecting lines on the repair tape. The line spacing should be approximately 5mm.
Stick transparent tape on the repair tape and press it tightly. Then quickly tear off the tape and evaluate:

Evaluation:
Excellent: No coating is detached along the cut lines.
Good: Very few coating fragments are detached, such as 1~2 pieces, maximum not exceeding 5%.
Acceptable: Up to 25% of coating detachment is acceptable, such as 2~3 pieces.
For post-high-temperature boiling test, up to 30% coating detachment is acceptable.

Additional Tests:
For Liquid Coatings:
Copper Sulfate Porosity Test: Determines corrosion resistance.
Coating Elasticity Test: Evaluates the flexibility of the coating layer.
Acetone Test: Measures the hardness of the coating layer.
Cross-Hatch Cut Test: Tests the adhesion of the coating layer.
High-Temperature Boiling Test: Evaluates the curing effect, adhesion, and corrosion resistance of the coating.

For Thermoplastic Powder Coatings:
Copper Sulfate Porosity Test: Determines corrosion resistance.
Coating Elasticity Test: Evaluates the flexibility of the coating layer.
Tensile Adhesion Test: Tests the adhesion of the coating layer.
High-Temperature Boiling Test: Evaluates the curing effect, adhesion, and corrosion resistance of the coating.
Powder Weight Test: Calculates the average thickness and weight of the powder.
Powder Thickness Test: Estimates the thickness and uniformity of the powder.

For Thermosetting Powder Coatings:
Copper Sulfate Porosity Test: Determines corrosion resistance.
Coating Elasticity Test: Evaluates the flexibility of the coating layer.
High-Temperature Boiling Test: Evaluates the curing effect, adhesion, and corrosion resistance of the coating.
Powder Weight Test: Calculates the average thickness and weight of the powder.
Powder Thickness Test: Estimates the thickness and uniformity of the powder.

In short, any detection method should be included in comprehensive quality control, the purpose of which is to predict the weak points of empty cans and prevent possible serious corrosion as early as possible.

Yixin Technology is a well-known manufacturer with 30 years of experience in providing turnkey metal can making services. Customers and friends from all over the world are welcome to consult us at any time on all professional issues related to metal can making!

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