Black Stains After Curing
Q. We are doing e-coating for small components like nuts and bolts, but are seeing black stains on the parts after curing. The last water drops that fall on the parts seem to leave white/yellow color spots that turn into black stains or spots after baking. Can you suggest a solution for this? R.A.
A. As you indicate, it seems like the problem is related to the water drops that always hang at the bottom of the racks or pieces. Both contamination and surface tension play a significant role in the effects of this defect.
As you can imagine, these drops develop and form on racks and parts after every stage of the entire cycle—after cleaning, rinsing, phosphate and passivation. Then this happens in the e-coat bath and on the permeate rinses. After each bath in which the drop forms, more and more chemicals and residues accumulate, leaving a poor chemistry at the spot where the drop hangs.
The problems associated with this phenomenon can be tackled in two ways:
1) Installing blow-offs after the cleaner and phosphate stages to blow off parts and racks. This will minimize the formation or the amount of time the drop has to form. In some cases it will be necessary for more stages to incorporate this technique.
2) Making sure the conductivity of the last DI water rinse of the system is well below the recommended range. Typically, this DI water should have a conductivity no greater than 25-30 microhms/cm. If you currently have permeate, try to change from permeate to DI or add an additional full or half DI stage.
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