Ultrasonic Rinsing?

“Enjoying your newsletter. Wondering if you can comment on using ultrasonics during rinsing.” Clean Source subscriber

Our short answer is: All other things, being equal, use ultrasonics not only during cleaning but during rinsing. Ultrasonics are necessary to access and remove soil during the wash (or clean) step; and ultrasonics are also a good idea during the rinse step. Here’s why ultrasonics in the rinse tank can improve process effectiveness and product quality.

Cleaning with ultrasonics, high frequency sound waves, is effective in accessing and removing soil from complex, ornate surfaces and from blind holes. In spray systems, the nozzle is aimed at the target; the soil to be removed must be in the line of fire.  Ultrasonic cavitation is omnidirectional as opposed to line-of-sight. This means ultrasonics effectively go around corners that a spray system can’t “see.”

The functions of the rinse step are to continue the removal of residual soil and of cleaning agent from the surface, to keep the material away from the surface, and to prevent redeposition of soil.  All of this has to happen without damaging the surface or structure of the part.  You may have carefully selected the cleaning agent because it removes soil effectively without damage to the product or component.  However, at the rinse stage, that same cleaning agent becomes a soil – you don’t want it on the part any more.

Winning an Uphill Battle
The thing is, particularly with aqueous systems, rinsing away that cleaning agent is an uphill battle against the forces of nature. Aqueous cleaning agents contain surfactants to enhance wettability and therefore allow access to blind holes and closely-spaced components; and using ultrasonics often improves accessibility. During rinsing, you are trying to remove the cleaning agent As cleaning agent is removed, wettability of the rinse tank gradually decreases. Any residual soil, including cleaning agent residue, is likely to remain trapped.  During rinsing, you don’t have the cleaning chemistry to help remove soil and improve wettability. Therefore, using ultrasonics is perhaps at least as important during rinsing as it is during initial cleaning. 

As in the wash step, other factors are important. You also need to consider the temperature of the rinse tank and the time allotted to rinsing.   Remember that with ultrasonics, there is a point beyond which raising the temperature can be counterproductive. This is because near the boiling point the ultrasonic bubbles or tears in the liquid become increasingly filled with vapor and become“cushy,”  so the ultrasonic implosions may not be effective.

Here’s the full inquiry
Each cleaning process has many variables and considerations. Here’s the full question.

“Enjoying your newsletter. Wondering if you can comment on using ultrasonics during rinsing.” [We are] in the process of shopping and comparing different automated clean lines and one vendor (of the 4 we’ve surveyed so far) does not recommend using ultrasonics on the rinse baths. This was news to us and a very different story from what other vendors have recommended. Would be interested to hear your take.” Clean Source subscriber

Why?
In preparation for writing this newsletter, we contacted the engineer who asked the original question. We suggested contacting the clean line vendor and asking why he or she does not recommend ultrasonics in the rinse tanks. The vendor’s response was that they get “the best cleaning” without ultrasonics.  This response does not make intuitive sense. Frankly, I would want to know more about the vendor’s rationale. Is there a subtle technical consideration that would preclude the use of ultrasonics in the rinse tank? Is there a general reluctance to use ultrasonics (some people still have ultrasonophobia)? Is there a lack of knowledge about ultrasonics? Is the cleaning equipment so costly that adding ultrasonic transducers would result in sticker shock?

All other things being equal
In general, if ultrasonics make the wash step better, ultrasonics will also make the rinse step better. Aspects of better include faster, more effective, more reliable production, lower defect rate, and higher quality product.

Notice that we said that ultrasonics in the rinse tank is a good idea, all other things being equal. However, in cleaning, some things are more equal than other things. For example, with some metals, corrosion or erosion may be exacerbated by ultrasonic cleaning in water, especially highly purified water.  While using tap water is not advisable, highly purified water is looking for ions and can pull ions off of the part being rinsed, resulting in erosion or corrosion. Elevated temperature in combination with ultrasonics could exacerbate the impact of deionized water.

In general, ultrasonics in the rinse tank improve the effectiveness of the cleaning process.  However, each cleaning process has a particular set of considerations. Part of critical cleaning involves looking at your process with a critical eye. Ask questions; and question the answers. Contact us and take advantage of our experience. As always, we’re here to help.