Plastic Plating-Quality Evaluation of Coating
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Any type of part must undergo quality inspection after the electroplating process is completed. The quality inspection of the plating layer includes the appearance, thickness, adhesion with the base metal, ductility, microhardness, brittleness, corrosion resistance, wear resistance, solderability, etc. of the plating layer. Although the content of specific quality inspections varies from part to part and coating, the appearance, thickness, corrosion resistance and adhesion to the base metal of the coating are items that must be inspected for all coatings. Each country has its own national standards and unified international standards for quality inspection methods and evaluation of plated layers; different companies have also formulated corresponding corporate standards.
1) Appearance of the coating
The appearance of the coating is one of the items that must be inspected for any part and any plating type. Generally, the appearance of the coating is observed directly with the naked eye under natural light. Its contents include the macro uniformity, color, brightness, crystallization status and macro binding force of the forged layer. Generally speaking, in addition to its unique color and luster, the coating should also be uniform, meticulous and have good adhesion. No pinholes, streaks, blisters, peeling, burrs, nodules, pits, charred, cracking, peeling, abnormal color or missing plating are allowed. However, for hanging plated parts, slight defects at the hook are generally allowed.
2) Thickness of coating
To ensure the performance of the part, the coating on the surface of the part must reach a certain thickness. Commonly used coating thickness measurement methods include destructive and non-destructive. Destructive thickness measurement methods include anodic dissolution coulomb method, metallographic method, dissolution weighing method, liquid flow method, drop method, etc.; non-destructive thickness measurement methods include mechanical measuring method, magnetic method, eddy current method, beta ray back scattering method, X-ray spectroscopy, etc. Please refer to the corresponding national standards when using these methods to measure the thickness of the coating.
3) Corrosion resistance of coating
There are two main types of test methods for evaluating the corrosion resistance of coatings: natural environment test and artificial accelerated corrosion test. The former includes on-site tests in the use environment and atmospheric exposure tests under different climatic conditions. These methods can truly evaluate the corrosion resistance of the coating, but the shortcoming is that the cycle is too long; the latter includes neutral salt spray test (NSS) copper acetate accelerated test (CASS), corrosion paste test, electrolytic corrosion test, industrial gas corrosion test, damp heat test, etc. All these corrosion resistance tests have corresponding national standards, which stipulate the test conditions and evaluation methods.
4) Adhesion of coating
There are many methods to test the bonding force (also called the bonding strength) between the plating layer and the base metal, but they are all qualitative tests. Common testing methods for coating adhesion include friction polishing test, peeling test, file test, scribbled test, bending test, thermal vibration test, deep drawing test, etc. Different methods are suitable for different coatings, and there are also different evaluation standards. You can refer to the corresponding national standards for specific use.
