When Aqueous Cleaning Isn’t Enough, What Should You Look At?

Blind holes, threads and internal channels aren't just hard to clean, they're also hard to rinse and dry. Hubbard-Hall's Mike Valenti explains why geometry, soil type and equipment matter as much as chemistry when troubleshooting aqueous cleaning.

As manufacturers move more cleaning processes from solvent to aqueous systems, complex part geometries are creating new challenges. Threads, blind holes, internal channels, rough surfaces and recessed areas can make it difficult for cleaning chemistry to reach contamination. It is equally difficult for cleaner and rinse water to get back out.

When an aqueous cleaning process isn’t delivering the expected results, the instinct may be to change the chemistry, increase concentration or turn up the temperature. But the root of the problem may have as much to do with the part, soil and equipment as it does with the cleaner.

Here are some of the questions we commonly encounter when manufacturers are troubleshooting aqueous cleaning of complex parts.

Q: Why are complex part geometries so difficult to clean with an aqueous process?

A: One of the biggest things people underestimate is geometry. With aqueous cleaning, we’re not just concerned with what is on the surface. We have to think about whether the cleaning solution can physically reach every area where the soil is located, and whether it can get back out again.

Threads can hold lubricants and particulates. Sharp edges and ledges trap particles. Surface roughness creates additional places for contamination to remain. Then you have bends, internal diameters, recesses and blind holes, which make solution transfer and soil displacement even more difficult.

I think of it as a “tortuous path.” The more twists, turns and difficult-to-reach areas you have, the more you have to think beyond simply choosing a cleaner.

Q: What role does water itself play in the challenge?

A: Surface tension is a big part of it. The cleaning solution has to wet the surface and get chemistry into all the areas where soils reside. Water has a surface tension of about 72 dynes/centimeter, compared with approximately 30 dynes/centimeter for an aqueous cleaning solution. Lower surface tension helps the solution wet the part and penetrate difficult geometries.

We can formulate the cleaner to improve wetting, but we also have to remember that ultimately we’re rinsing with water. Getting rinse water into and back out of very small or blind areas can still be difficult. So, sometimes the challenge isn’t simply, “Can I clean the part?” It is “Can I successfully clean, rinse and dry the entire part?”

If a part isn’t getting clean, why not increase the cleaner concentration or temperature? Because more isn’t necessarily better. Many nonferrous alloys are sensitive to aggressive chemistry and process conditions. Caustic chemistry, for example, reacts aggressively with aluminum and zinc. That limits how assertive we can be when trying to solve a cleaning problem.

There are practical limits to other process parameters as well. Excessive cleaner concentration may create more residue without providing additional cleaning benefit. Higher temperatures can cause cleaner residues to dry quickly if the parts aren’t rinsed immediately. And excessive heat can potentially damage parts. Extending cleaning time doesn’t necessarily solve the underlying problem, either.

Therefore, before turning up the temperature, concentration or time, we need to understand why the soil is remaining on the part.

Q: How important is understanding the soil being removed?

A: It’s critical because not all oils behave the same way in an aqueous cleaning process.

Water-based coolants and soluble oils generally can be removed with milder cleaners and will incorporate into the cleaner bath. Hydrocarbon lubricants typically require more aggressive cleaners and may split or emulsify in the bath.

That difference also affects bath maintenance. Depending on the soil, it may be filtered from the cleaner bath or separated from it. And telling a manufacturer to simply change the lubricant isn’t always realistic. Once a machining or forming process has been established around a particular lubricant, changing it can affect other aspects of production.

That’s why we have to understand the alloy, soil, geometry and existing manufacturing process together.

Q: When does equipment become part of the cleaning solution?

A: There are cleaning problems where equipment can be just as important — and sometimes more important — than chemistry.

We have several tools available to us, including solution impingement through pressure spray or eductors, ultrasonic cavitation, vacuum-assisted cleaning and newer technologies such as cyclic nucleation.

Spray, for example, improve solution contact and physically dislodge soils. Ultrasonics create a scrubbing action that reaches tight crevices and detailed surfaces. Vacuum-assisted systems help move solution into and out of recesses during washing and rinsing.

Ultimately, the objective is pretty simple: Get the chemistry where it needs to go, remove the soil and then get everything back out of the part.

Q: When should a finisher consider ultrasonic cleaning?

A: Ultrasonics can be very effective when you have contamination in tight crevices, along edges or in other difficult-to-reach areas.

Ultrasonic cleaning uses high-frequency sound waves to create microscopic cavities in the cleaning solution. When those cavities collapse, they form an intense scrubbing action that helps lift contamination from the surface.

Frequency matters. Higher frequencies — around 40 kilohertz and above — produce smaller, more numerous cavities and are better suited for delicate parts and fine details. Lower frequencies, roughly 20-40 kilohertz, create larger cavities and a more aggressive cleaning action useful for heavier contamination.

We had one application involving a highly machined 6000-series aluminum part where small particulates were adhering to edges. In that case, ultrasonics provided the additional cleaning action needed to address the problem.

The key is matching the equipment to the part and the contamination, rather than assuming one technology will solve every cleaning problem.

Q: What about blind holes and internal passages where cleaner or rinse water becomes trapped?

A: Those are particularly difficult because you have to solve two problems: getting solution into the area and getting it back out.

Vacuum-assisted cleaning can be useful in these applications. Modulating vacuum within the wash chamber helps move solution into and out of recessed areas during both the wash and rinse cycles. It also assists with drying at lower temperatures.

Another technology is cyclic nucleation, which uses rapid, repeated pressure changes to create and collapse vapor cavities. That action helps dislodge and flush contamination from complex geometries including blind holes, internal channels and porous surfaces.

These technologies give us another way to attack the problem without continually trying to make the chemistry more aggressive.

Q: As manufacturers evaluate alternatives to traditional solvent degreasing, what should they keep in mind?

A: We’re going to see more aqueous cleaning, but that doesn’t mean solvent cleaning disappears completely.

Traditional solvent options are becoming more limited, while some newer solvent alternatives come at significantly higher costs. As a result, more manufacturers are looking at transitioning processes to aqueous cleaning, and finishing operations may begin receiving more parts that haven’t already been solvent pre-cleaned.

The important thing is not to assume that replacing a solvent process with an aqueous process is simply a matter of choosing a new chemical. We have to look at the entire process: What is the alloy? What soil are we removing? Where is that soil hiding? Can the chemistry reach it? Can we rinse it? Can we dry the part? And what equipment do we have available to help us?

Sometimes chemistry is the answer. Sometimes equipment is the answer. Often, it’s about finding the right combination of both.

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