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Cascade Overflow Rinse
There is a simple truth in cleaning your manufactured parts: the parts are only as clean as your final rinse. The goal in any cleaning application is to make your final rinse as effective as possible. For many customers that means incorporating a cascade overflow rinse tank system.
Typical Wash-Rinse-Rinse Cleaning Line
In a typical aqueous cleaning line for medical device or aerospace parts, the parts will move through three separate tanks or baths as depicted above. The first is a cleaning solution – often an alkaline detergent – that is designed to loosen soils on the parts. Depending on the types of parts and soils, any of the following may be used in this tank to add a “scrubbing” action to the cleaning: ultrasonics, agitation, fluid circulation, and/or rotating parts baskets.
The second tank or solution will be the first rinse. In most applications this will be a tank of de-ionized water (DI water). But the truth is, it won’t remain de-ionized for long. As the parts are moved into this tank they will carry with them some of the wash solution from the wash cycle and some of the soils from the parts; loosened, but still present. Known as “drag out,” this solution will mix in with the DI water. The chemistry from the wash solution and the soils will interact in the DI tank and begin to re-ionize the solution.
This is why the second rinse is so important. This tank or solution (also DI) will receive the parts after the first rinse. As before, it will receive some drag out from the previous rinse tank, but by now the vast majority of the soils are long gone. As the parts come out of this bath, a final concern comes into play. Once the parts are dried you want to make certain there is no spotting on the surface of the parts. Typically spots are not caused by remaining soils, but by mineral content or other impurities in tap water or dirty rinse water. DI water helps, but just using a DI tank as a rinse may not be enough.
Cascade Overflow Rinse Tank – Stand-alone
The key that takes your cleaning line to the highest possible level of cleanliness is a cascade overflow rinse tank. The photo above shows a two-tank rinse unit. Near the center wall separating the two rinse tanks you can see a second wall that is shorter. In this system the tank on the right is the final rinse. Clean DI is added continuously – at a very low flow rate – from the bottom of the tank. As the tank fills it will eventually flow over the shorter wall into a weir which then cascades into the tank on the left. This process is depicted in the sketch below with the light blue stream moving from Rinse #2 to Rinse #1.
Cascade Overflow / Counterflow Concept
In time the left-hand tank – Rinse #1 – will overflow, too. This time the water flows to a weir (green in the image above) that directs the liquid to a drain – often a holding tank for later reclamation. In this type of system, the 2nd rinse is constantly refreshed with new DI water, and in turn it constantly refreshes the first rinse with cleaner water. The end effect is a a clean part with a spot-free rinse.
Cascade Overflow Rinse Tanks – Wet Bench Application
In the examples above we see tanks that Best Technology custom designed to fit specific customer requirements for their part configurations – but a cascade overflow rinse tank system can be added to virtually any aqueous system. Cascade overflow rinse is a best practice not only for rinsing parts after cleaning, but also after passivation. Another best practice? Contact the wet and dry finishing experts at Best Technology Inc.
