Prevention of E-Coat Cracking on Bent Sheet Metal
Why is the electrocoat cracking on sheet metal bent at 90 degrees?
Q. I electrocoat sheet metal parts that are 1.2 mm thick. On some parts, the electrocoat cracks and delaminates when we bend them at 90 degrees. The film thickness of the electrocoat is 18-20 microns. Can you tell me what I can do to solve this problem? –G.P.
A. The overall bending performance of the parts is not only influenced by the outer layer (e-coat) but by the overall number of layers, thicknesses and qualities of each layer between the base metal substrate and the electrocoat.
For epoxy electrocoat systems, the variable that plays the biggest role on film flexibility is the cure temperature. Low cure temperature and cure profile produce films with low degree of crosslinking/polymerization and typically high film flexibility. Reducing cure temperatures in your electrocoat cure oven will improve your film flexibility and bending results.
E-coat operations with low p/b formulations can also provide additional film flexibility because of low pigment levels in the film. High-flexibility electrocoat resin formulations have also been employed in other cases to provide additional film flexibility.
Another variable that can contribute to film flexibility and bending is the overall thickness and quality of pretreatments between the e-coat layer and the metal substrate. The more layers (or the thicker the layers are) between the base metal and the electrocoat, the worse the system’s flexibility. Heavy phosphate weights or large phosphate crystal size can also contribute to poor flexibility and poor bending performance. Remember that electrocoat paints pretreatment, not metal.
If the metal substrate is galvanized steel, then the additional layer of zinc is very critical for good overall system flexibility and bending. If the base metal is aluminum or zinc that has received a chromate treatment for passivation (such as to extend storage or shipping conditions), those systems would exhibit poor flexibility because of the increased number of layers and its overall thickness.
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