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Adhesion Test on small size part of dissimilar material

I am currently in China starting up a powder coating line. The main problem is that we are not getting good adhesion after the condensation test (8 cuts 2 mm apart cross hatched diagonally) . The part is a stainless steel screw surrounded by an aluminum knob. Specs are as follows: 2 stage wash with 3.5% soap solution @2 min each@ 65 C, 2 stage rinse ambient temperature, 3 stage cascaded rinse with D.I. water for 2 minutes each @ 1.4 microsec/cm.
The parts are in a stainless steel basket dried on an oven 120 °C for ten minutes and transferred to the fixtures using clean cotton gloves. Parts are then sprayed using Rhom and Haas epoxy resin Corvel solar black. Then transferred to a curing oven for 20 minutes at 210 °C. The manufacturer suggests 203 °C for 10 minutes given optimum conditions.
The parts have good adhesion before the condensate test but all but 2 test have failed after (31 total). I have downloaded and reviewed the ASTM spec for that test and am confident we are doing it correctly.
I have checked all grounds on system and did ohms check on both cable and guns. I haven't checked the multiplier and resistor due to not having a megohm meter, although I do have two separate control units and guns and both yield the same results. I have tried doing the test 24 hours later with no better results. The MEK / methyl ethyl ketone test shows a good cure.
The soap is the same type and supplier used in the states. Same with the booth, control unit and the gun. I have just sent some parts to home facility to coat and test there. I have also requested a sample of powder from that facility to try here. The powder was opened the on the 18th of Oct. and seems to spray in an even distribution. I have carefully recorded all of the variables in the process and cannot come up with a viable repeatable process. Tired of Chinese food... Help

James D. Braun
- Concordville, Pennsylvania, USA
2005

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2005

Is the problem limited to the stainless steel screw?
I would suggest that the concentration of your soap solution is too high and it's leaving a residue, and the parts are not getting adequate rinsing in the spots that touch together while they are in the basket. It's also very hard to get powder to stick to stainless anyway because of the nature of the steel. Is there a way that you could use a low RPM spinning basket while the parts are in the tanks? You would definitely get a better wash and rinse. Make sure that all of your process controls are EXACTLY the same as your operation in the states, if they are and you are not having the same problem here, you might have to call in Mulder and Scully. Don't they sell Philly cheese steak in China? Tsk,Tsk.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina