Cleaning Q&A: Ultrasonic Cleaning for Plating

Q. We are in the plating-on-plastics business. We plate only silver on ABS. We mold these little “buttons,” tumble them with water and pumice to scratch the surface, then spray rinse the pumice off the parts, dip them in a chrome/sulfuric solution, neutralize with sodium sulfite, rinse, and then put them into an electroless coating of silver using tin for the “catalyst” and formaldehyde as the reducer.

My question to you is related to cleaning after the pumice step. How critical is it to have the surface free of any loose particulate to limit plating problems? I have found that, when I ultrasonically clean the parts, there is a considerable amount of “powder” that comes off the parts, making the solution cloudy. My point to management is that we need to add an ultrasonic cleaning step. What are your thoughts?

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A. It sounds like you have a well-established process, so one of my first questions would be: what are you targeting for improvement? From the basis of your question, it would appear that that the addition of ultrasonic cleaning to your process would be a no-brainer.

I would not expect the chemical processes you describe to completely dissolve the pumice, so the burden of removal would primarily fall to the spray rinse immediately following the abrasive. But you are completely reliant on this step to remove all the residual pumice, otherwise it will lead to coating defects at the plating step. In an idealized improvement pattern, the addition of the ultrasonic cleaning following the abrasive step would be a natural evolution of the process. Your management may be examining the short-term payoff, which is usually what it is their job to do. If the current process does not result in a significant reject rate (whatever that may be), then they may be reluctant to spend the capital for the ultrasonic installation. You may be able to review some of the intangible benefits along with the increase in process robustness to help your view.

Originally published in the November 2015 issue.