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Aluminium sheet powder paint cracking while bending at 90 degree

We have done powder coating of Aluminium sheet on only one side. For pretreatment we have used Chromating process. When we do post operation on painted Aluminium sheet, the paint is cracking. Our pretreatment process and baking time is OK but still Paint is cracking mainly where bend is 90 degree. Kindly suggest what will be the reason ? what corrective action we should take? We are using Nerolac make Pure polyester powder for this job.

Nilesh Kale
Engineering dept - India
September 30, 2011

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October 3, 2011

One gathers from your post it is only when you incorporate a 90îÛ bend onto the PC item you obtain cracking?
If adhesion on the flat surfaces after carrying out a cross hatch test is OK I would surmise that that aluminium substrate is cracking under stress off the bending action and taking the powder with it.
However if after carrying out a cross hatch and you discover that the overall adhesion is suspect then it may be:
a) Poor pre-treatment?
b) Some sheet Al is supplied with a protective strippable coating. Once removed this can leave a residue behind that most pre-treatments will not remove -- resulting poor top coat adhesion. Remedy -- use a ketone solvent to clean the surface prior to your normal pre-treatment process.

Terry Hickling
Birmingham, United Kingdom