Ultrasonic cleaning is a well-defined, understood, and categorized industrial cleaning process widely employed in medical device manufacturing. It is particularly effective in cleaning small intricate parts and hard-to-reach blind holes and in accelerating surface treatment processes. Ultrasonic precision-cleaning systems set the standard for cleaning medical devices during manufacture. Both the equipment used and the chemistry of the cleaning solution must meet the FDA’s stringent cGMP guidelines. Additionally, the cleanliness results of processes must satisfy various industry quality and safety standards.

Ultrasonic cleaning incorporates high-frequency sound pressure waves to agitate a liquid or solvent. Cavitation “bubbles” induced by this agitation produce high forces on contaminants adhering to substrates such as metals, plastics, glass, and ceramics. Cavitation, in combination with heat and detergents, break the contaminants’ surface tension, allowing the contaminant to dislodge from the surface being cleaned.

Power of Ultrasonic Cavitation

The cavitation bubbles are oscillated in the liquid as the sound wave passes. This causes a buildup of positive pressure, which causes the cavitation bubbles to grow and become unstable. Eventually the positive pressure causes the violent collapse of the cavitation bubbles, which results in implosions, and shockwaves radiating from the bubbles collapse. It is this collapse and implosion of the millions of cavitation “bubbles,” throughout the ultrasonic tank, which, when combined with detergents, solvents, and water, create the foundation of ultrasonic cleaning technology. The combined action also penetrates blind holes, cracks, and recesses. Aqueous deionized (DI) water-based solvents or heated vapor can be used, depending on the type of contamination needing removal.

Effective Contaminant Removal. Manufacturing process contaminants such as hydrocarbon lubricants including PE waxes, paraffin, metal soaps, esters (high esterification), amides, and fatty acids can be effectively removed using precision ultrasonic cleaning. Common process lubricants, polishing compounds, flux agents, and mold release agents are also commonly removed. Biological contaminants can also be effectively removed, such as algae, fungus, bacteria, and viruses.

Ultrasonic cleaning is an effective method not only for removing organic contaminants but also for reducing bacterial and viral loads. Ultrasonic cleaning can be used for a wide range of workpiece shapes, sizes, and materials, and often disassembly of the part is not necessary prior to cleaning.

Cavitation Process: Ultrasonically induced compression waves tear the liquid apart, leaving behind many millions of microscopic “voids”/partial vacuum “bubbles” (cavitation). These bubbles/voids in the microsecond timescale grow until they become unstable, collapse, and release enormous energy; temperatures and pressures on the order of 5,000 K and 135 MPa are achieved (5,000 °C/19,500 PSI).

Efficient Process Ultrasonic Cavitation and Vapor Cleaning. In an ultrasonic cleaner, the substrate or parts to be cleaned are placed into a tank containing a suitable solution (aqueous, organic, solvent, or vapor, depending on the application). Typically, in ultrasonic vapor cleaning, the solvent is brought to boil to create and contain vapors in the vapor zone. Substrate or parts to be cleaned are held in a basket and then transferred to the vapor zone.

Comparison of Ultrasonics and Other Cleaning Methods

With ultrasonic cavitation, the harsh and costly chemicals used as cleaners in many industries are not needed or are used in much lower concentrations. The usefulness of ultrasonic cleaning in regard to biological pathogens like bacteria and virus has been known for some time.

Ultrasonic cleaning delivers significant cost savings over hand washing in most situations. Ultrasonic cleaning is quick and easy, and often it takes a fraction of the time of hand washing medical devices or parts. Even when cleaning a small number of high-value parts, the labor savings can be significant. But when it is necessary to wash hundreds or thousands of parts, the savings can be significant.

Hand-washing solvents can include a variety of toxic chemicals, such as methanol, mineral spirits, petroleum distillates, turpentine, benzene, toluene, xylene, perchloroethylene, trichloroethylene, methyl ethyl ketone (MEK), gasoline, and kerosene. These commonly used chemical solvents are hazardous, not environmentally friendly, not employee friendly, and require toxic vapor mitigation.

Important Considerations for Ultrasonic Precision Cleaning

In ultrasonic aqueous cleaners, surfactants (detergent) are often added to permit dissolution of nonpolar compounds such as oils and greases. An ultrasonic transducer, typically built into the tank, produces ultrasonic waves in the fluid.

There are five important considerations with precision-cleaning medical devices to meet the quality standards of manufacturers, the performance needs of users, and strict guidelines of the FDA.

  • Ultrasonic Cleaning: Use of ultrasonic cavitation at the right frequency and in combination with the right cleaning chemistries to remove contaminants and comply with FDA guidelines.

  • Ultrasonic Rinsing: Overflow rinsing with DI water optionally coupled with ultrasonic cavitation is provided at appropriate points in the process.

  • Passivation: Nitric or (more recently) citric acid chemistries can provide maximum corrosion resistance of passive oxide film to meet ASTM A967 or A380 standards.

  • Drying: Recirculating hot air drying is typically required. Additionally, HEPA filtration may aid in restricting any airborne contaminants.

  • Data Acquisition: A control system can be incorporated to capture key operating parameters and provide an audit trail to ensure that each part or batch went through the process properly.

Systems can be configured to communicate directly with Emerson’s DeltaV distributed control system technologies. DeltaV history data is available anywhere, including integrated batch, continuous, and event data, with an easy-to-use data search engine, improved data viewing, and analysis with detailed diagnostics via AMS Device Manager asset management software.

Experience and expertise in precision cleaning can provide the ideal solution for the intricate cleaning demands of medical implants. Emerson’s portfolio of Branson™ aqueous cleaners and solvent vapor degreasers, for example, offers state-of-the-art quality and capabilities. Benchtop, stand-alone consoles, multistage systems, components, and accessories can be used as stand-alone units or fully automated, computer-controlled systems configured to meet specific cleaning application needs. Ultrasonic cleaning systems offer a variety of options to customize the installation to meet the needs of even the most challenging cleaning applications, including:

  • Ability to automate all or part of the cleaning process.

  • Custom tank sizes.

  • Options for monitoring and maintaining proper liquid levels and chemical detergents concentrations.

  • Sanitary plumbing.

  • HEPA filtering for hot air drying.

  • On-site water DI capabilities.

  • Cleanroom compatibility.

This article was written by Tom Hoover, Sr., Medical Business Development Manager – Americas, Assembly Technologies, at Emerson, Danbury, CT. For more information, visit here .


Medical Manufacturing and Machining Magazine

This article first appeared in the September, 2020 issue of Medical Manufacturing and Machining Magazine.

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