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Pad printing over nickel acetate-sealed anodized aluminum

Q. I am a manufacturing engineer working at a company that produces a variety of items with anodized aluminum housings, all of which are nickel acetate sealed before they arrive at our facility for assembly with the other components. Each housing requires a part mark applied after assembly; the housings cannot be marked prior to the nickel acetate sealing because the stocked housing is common to a variety of different individual made-to-order part numbers that use the same package.

In the process of attempting to implement a pad printing process to accomplish the part marking, we have encountered a problem with the adhesion of the printed ink onto the sealed anodized aluminum substrate. Parts that are simply cleaned with acetone prior to printing do not hold the ink very well; it can be easily wiped clean even after curing. Parts that are heated to 250 F for 4 hours then cooled to room temperature prior to printing, however, seem to hold the ink better.

What is going on during that preliminary baking process that enhances the ink adhesion? This is a project that I recently inherited, so I'm not quite sure where the idea to bake the parts before printing originated. Should I be concerned that the surface is somehow becoming unsealed? And, apart from that concern, does anyone have a suggestion for promoting an adequate bond between pad printing ink and sealed anodized aluminum?

(Our currently approved process involves a two-part epoxy ink and silkscreening. This yields a durable print but is somewhat more time consuming than pad printing, which is why I am looking at a different technology.)

Philip Brobeck
Manufacturing Engineer - Tucson, Arizona, USA
March 26, 2012