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The importance of precision cleaning and verification

Microlevel cleanliness is essential in some industries

As instruments and systems in some industries become more sophisticated and sensitive, the cleanliness levels for both solid contaminants and residues are being driven to the micro level.

[Editor's Note: The following comes from installments of the Astro Pak blog.]

As the name implies, precision cleaning involves removing all possible contaminants from the surface of an object. This process is vital to keeping machinery running for critical operations in the aerospace, defense, medical, computer tech, and pharmaceutical industries. As instruments and systems in these industries become more sophisticated and sensitive, the cleanliness levels for both solid contaminants and residues are being driven to the micro level.

What’s so important about meeting industry and client standards for cleaning in these industries? If cleaning is lax and a machine stops running, the best-case scenario is that the company will face slowed production, incur added expenses, and its reputation might suffer. In the worst case, though, people could die.

The medical industry needs its machinery to work at full capability 100% of the time to ensure accurate diagnoses and treatment. Any less than 100% could compromise important work and, ultimately, people’s lives. And of course, medical equipment must be properly sanitized to prevent the transmission of infectious diseases. And in the aerospace industry, landing gear, for example, could fracture or fail to work if a speck of dirt got caught in the inner machinery.

Cleaning Methods

The most basic way to achieve precision cleaning is manually with brushes, waterjets, and rinses. Using these tools, operators scour visible surfaces, but they might miss deep holes and crevices that are difficult to reach. In addition, this method requires a lot of time and manual labor.

Chemicals and solvents also are used widely, but they can be expensive to purchase and difficult to dispose. Corrosive, toxic chemicals also might have special storage requirements.

Ultrasonic cleaners can be controlled precisely to remove contamination from various equipment and surfaces. However, since they are machines themselves, they also are susceptible to damage, especially if the room or equipment has high temperature.

Verification of Cleanliness

Since none of the cleaning methods is foolproof, surfaces must be measured after cleaning to ensure they can be certified as meeting required cleanliness levels. These levels are defined in IEST-STD-CC1246E, Product Cleanliness Levels – Applications, Requirements, and Determination, which provides methods for specifying and determining product cleanliness levels for contamination-critical products, with an emphasis on contaminants that can affect product performance.

The requirement for precision cleaning is that postcleaning contaminants, both solid and residue, must be verified to be less than one micron in size.

Solid Contaminants. Particles and fibers are solid contaminants of metal, plastic, and silica-based compounds often introduced during a manufacturing process or maintenance shift when critical surfaces are open to the environment.

A chart of particulate cleanliness levels.

Figure 1. This table shows typical sizes ranges for solid contaminants during postcleaning inspection, along with the allowable number of particles per 0.1 m2 of critical surface area.

During postcleaning verification, these contaminants usually are collected by rinsing the critical surface with a measured amount of finely filtered solution, which is captured in a clean container. This captured solution containing particles and fibers is filtered through a membrane filter rated at 0.45 µm, and the filter is examined under a microscope. The operator then counts the contaminants down to a size of 5 μm and measures them individually, grouping the data into “bins” of a defined range of sizes (see Figure 1).

In addition, if the filter membrane used for the particle is of a known tare weight, it can be dried in a vacuum oven and weighed again to determine the collective mass of the particles and fibers.

Residues. Residues are shapeless contaminants typically comprising hydrocarbon oils and greases. Because of their sticky and semiliquid nature, they must be dissolved in a suitable solvent for collection from a critical surface.

Once the solvent and residue sample are captured in a clean container, they are passed through a membrane filter rated at 0.45 µm, which removes the solid contaminants from the solvent. This solids-free solvent then can be evaporated down to a few milliliters, which concentrates the residue picked up during the surface wash.

The final few milliliters are evaporated to dryness in a weighing dish of known tare weight. This dish with the dried residue in it then is weighed on an analytical balance, and the mass of the residue is determined through subtraction. This process is called gravimetric nonvolatile residue mass determination (NVR) (see Figure 2).

Long-wave ultraviolet (UV) light inspection can be an alternative to NVR measurement if no fluid can touch the surface. It also can be used as a supplemental test. With this method, the critical surface is inspected in a dark room or chamber, and the long-wave UV light source is aimed at the critical surface. Most hydrocarbon-based oils and greases will fluoresce under this light (see Figure 3). This test is pass/fail only and does not give metric measurements.

Precision cleanliness matters and has an impact on the real world. While the effects and importance of precision cleaning varies by industry, it’s undeniable that removing contaminants down to the microscopic level and verifying the results ensures that equipment functions as intended and minimizes potential damage to the machine.

Astro Pak, 270 E Baker Street, Suite 100, Costa Mesa, CA 92626, 949-518-1440, astropak.com