Ultrasonic cleaning is widely accepted in pharmaceutical, healthcare, electronics, medical device manufacturing, and related industries where product cleanliness is critical during and following production cycles. Powerful but gentle scrubbing action of imploding cavitation bubbles in benchtop and industrial ultrasonic cleaners is unsurpassed in removing the most tenacious of contaminants. The availability of biodegradable water-based ultrasonic cleaning formulations both broadens the application of sonic cleaning energy and removes disposal concerns associated with earlier used formulations such as trichloroethylene and perchlorethylene.

Correct cleaning solution formulations, along with applicable pre- and post-cleaning procedures, are an important part of compliance with QC and QA standards.

Using cleaning solution formulations designed for ultrasonic cleaners is important for other reasons, one of which is avoiding damage to the ultrasonic cleaning solution tank. Tanks can be exposed to frequencies typically ranging from 25 to 130 kHz and are exposed to strong bending stresses. Formulations not designed for these conditions may contain ions that can increase corrosion and wear on the tank. A tank failure will result in solution leaking into the electronics, and may necessitate a total replacement. 

Cleaning solution selection: The basics

Selecting correct ultrasonic cleaning solution formulations requires some advance thought to define what is to be accomplished. Before discussing formulations with suppliers, you should have a clear understanding or definition of the following points:

• Type of contamination to be removed. There’s a big difference between removing chips and coolant from machined parts, solder flux and residues from PCBs, 3D printing mold support, and blood and tissue residues from surgical instruments.
• The product composition. Virtually any product that can be safely immersed in an aqueous solution can be ultrasonically cleaned, but the wrong cleaning solution chemistry could cause damage.
• Post-cleaning steps, if any — i.e. sterilizing, plating, painting, and coating.
• How do you define “clean”?

Some advice up front: Chromium-plated parts should not be subjected to ultrasonic cavitation. Clean parts of the same composition in a cleaning cycle and avoid stacking parts in the cleaning basket.

Also note that manufacturers of ultrasonic cleaning solutions provide instructions on dilutions, suggested cleaning solution temperatures for best results, and disposal recommendations. Material safety data sheets (MSDS) include additional information on the formulations.

Primer on ultrasonic cleaning solution formulations

Most commonly used ultrasonic cleaning concentrates are formulated under four general classifications: acidic, neutral, mildly alkaline, and alkaline. They are usually supplied as concentrates so a little goes a long way.

Acidic solutions are specially formulated for removing mineral, rust, scale, and lime deposits from ferrous metals, but not for light alloys susceptible to corrosion.

Do not use strong acids such as nitric, sulfuric, formic, or hydrofluoric in an unprotected ultrasonic cleaner tank. An acid resistant plastic tub is needed to protect the tank. Fill the cleaner tank with water and a surfactant; place the plastic tub in the tank so its bottom is slightly immersed in the water and add the acid into the plastic insert. Cavitation passes through the plastic tub to act on the parts. Keep in mind that acids such as these require proper disposal.

As another note, temporary post-cleaning rust protection on ferrous materials can be achieved by using specially formulated rust inhibitors in the cleaning or rinse tank.  

Neutral solutions are used for delicate materials made of plastic, glass, all metals and rubber for removing impurities such as dust, grease, pigments and other organic compounds. Certain of these neutral formulations can be used either in the cleaning tank or as a spray with tap or deionized water. Low residue free rinsing formulations are available.

Mildly alkaline cleaning solutions are used for removing oil, grease, dust, soot, and other organic compounds from glass, plastic, ceramic, rubber, iron, and non-ferrous metals. Alkaline solutions are often specified for general all-purpose cleaning.

An ammonia-containing alkaline solution is a good choice for cleaning laboratory appliances, work pieces and products made of glass, ceramic and precious metals, and to brighten brass and copper implements as well as other non-ferrous metals. 

Specialized cleaning solutions are offered by manufacturers to handle specific cleaning tasks. A few of the many applications include cleaning precision optics, maintaining firearms and first-responder equipment, cleaning laboratory instruments, germicidal formulas for pre-soak and cleaning medical instruments, removing smoke and soot damage, and removing waxes and resins.

Emulsifying and demulsifying ultrasonic cleaning solutions

Ultrasonic cleaning solution formulations can also be broadly classified as emulsifying or demulsifying. The distinction applies to what happens to the contaminants removed by ultrasonic cavitation and especially to parts heavily contaminated by greases and oils.

These contaminants are held in suspension when using an emulsifying formulation and build up over several cleaning cycles until they begin to interfere with cleaning efficiency. They also can be “dragged out” when products are removed from the bath, perhaps necessitating a rinsing cycle, either with or without ultrasound, to achieve the desired level of cleanliness. For this reason emulsifying formulations are usually used for short (limited) batch cleaning cycles. When bath performance becomes unacceptable the tank should be drained, cleaned following the manufacturer’s instructions, then filled with a fresh solution. 

With a demulsifying formulation, oily contaminants rise to the surface of the bath where they should be skimmed off and set aside for later disposal. Industrial-sized ultrasonic cleaners may be equipped with spray bars or weirs that skim off the solution into containers to be held for later disposal. This extends cleaning solution performance but with continued use demulsifying formulations must also be drained, and the tank cleaned and refilled.

Metal fines and other hard particles that sink to the bottom of the tank must also be removed when solutions are replaced. This is because ultrasonic vibrations at the tank bottom can cause these particles to serve as a “drill” that eventually wears holes in the tanks, necessitating a replacement.

Manufacturers and distributors of ultrasonic cleaning systems offer products to help extend solution life, protect the ultrasonic cleaning tanks, and facilitate disposal of contaminants. For example, filter pumps can be attached to industrial tanks for separating dirt particles out of the ultrasonic cleaning solution, thereby extending bath life and reducing liquid waste. A dual-filter system can be employed, the first to remove particles that could cause wear on the pump itself; the second to remove smaller contaminants, returning clean solution to the ultrasonic tank. A pressure gauge indicates when filters are becoming saturated so they can be removed and cleaned or replaced.     

Ultrasonic cleaning with flammable solvents

Certain cleaning requirements call for the complete absence of cleaning residues. Examples include newly manufactured surgical implants, medical and surgical instruments, printed circuit boards, inkjet cartridges, and similar products. In these instances where a water-based solution is not acceptable flammable solvents may be called on to provide the required results. These solvents include acetone, IPA, MEK, and toluene, all of which have the distinct disadvantage of being highly flammable and likely to cause an explosion unless precautions are taken.

In such instances specially designed explosion-proof ultrasonic cleaners must be employed. These are designed in such a way to avoid solvent liquids or vapors from coming in contact with internal electronics that could serve as ignition points. Examples of such cleaners have internal electronics totally sealed in high density foam thereby fully isolating any spark source from contact with spilled solvent and solvent vapors.

Use of these solvents also creates what is called a hazardous location, where strict rules apply concerning ventilation and the absence of ignition triggers from nearby electrical systems. Otherwise stated operating an explosion-proof ultrasonic cleaner requires a controlled environment in compliance with National Electric Code (NEC) and National Fire Protection Association (NFPA) criteria, as well as local regulations. 

Other ultrasonic cleaning considerations

This article discusses some of the considerations that apply to selecting ultrasonic cleaning solution formulations. As suggested above, the use of flammable solvents for cleaning requires that special consideration be given to the local environment — the so-called “hazardous area.” Other environmental concerns may likewise apply depending on criteria governing the removal of surface contaminants and avoiding re-contaminating products after they are cleaned.

Further consideration should be given to the capabilities of the ultrasonic cleaning equipment in terms of ultrasonic power, ultrasonic frequency, what are called sweep and pulse modes, timers and temperature controls, and the size of cleaning tanks required.

Vendors of ultrasonic cleaning solutions and equipment are your best sources of information. Vendors are better able to help when you are fully informed on what is required in terms of degree of cleanliness, including industry and government regulations that may apply to your specific cases.  

Tovatech LLC director Dr. Bob Sandor has more than 15 years of experience managing high tech businesses. He led the start up and growth of 5 technology companies and has authored more than 40 patents and technical publications. Bob holds a B.S. degree in Chemistry from The University of Rochester and a Ph.D. degree in Inorganic Chemistry from Brandeis University. rsandor@tovatech.com; www.tovatech.com