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How Clean Is Clean? Cleaning Validation in the Aerospace Environment

airplane wing

In our previous cleaning validation post, we discussed creating a validation program by choosing a cleanliness standard, creating a plan which accounts for all your challenges and processes, choosing the best measurement method, and putting regular reviews in place. In this post, we'll take a closer look at choosing a measurement method. We'll review the methods that fit the cleanliness standards most commonly seen in aerospace, covering how they work and what kinds of soils they are best at detecting. Armed with this information, you can begin deciding what will fit your particular operation.

How clean?

When we talk about cleanliness‚ and when upper management or a customer sends you a cleanliness spec to meet‚ the standard is usually expressed as a certain weight of contaminant substances over a certain area, such as .01 grams per square centimeter. To comply, you'll need to be at or below that weight. In aerospace, the required cleanliness usually falls between between 0.01 grams and 0.001 grams per cm2. The validation methods required at this level tend to be technically sophisticated, but can still be simple to perform. Lower levels of contamination than 0.001 grams can be achieved, but tend to provide diminishing returns.

Which method?

So, how do we ensure we're hitting the target? Let's look at the available methods:

  • Extraction‚ This is a high tech version of a white glove test. A swab or filter paper is used to wipe a surface, then is analyzed with a process such as UV visible spectrophotometry, atomic absorption or liquid chromatography. Extraction is especially well suited to detecting detergent residues, and it can give you a quantitative analysis of your remaining soils if the sample is always taken from a known area. It can also detect contamination levels below a microgram per cm2.
  • Gravimetric Analysis‚ You might say this process is dirt simple. It also allows you to not only evaluate the cleanliness of parts, but evaluate your basic cleaning process as well. In the first case, small parts (or batches of them) can be weighed, and their weight compared to the clean weight‚ of a properly cleaned part. If the item weighs more, contamination is present. In the second case, a coupon‚ or other sample item can be weighed clean, weighed dirty, then cleaned and weighed a third time. When the three weights are compared, you'll have clear data points telling you exactly how much dirt is being removed.
  • In-Situ Particle Monitoring (ISPM)--The ISPM method allows users of ultrasonic cleaning to monitor effectiveness in real time by collecting samples of their cleaning medium upstream of their filtration setup. These samples can then be examined for soil concentration. This allows you to refine your ultrasonic cleaning process as frequently as you wish, and also allows you to perform A/B testing of different detergents (or concentrations thereof), cleaning times, preparation methods, etc.
  • Millipore Filter Measurement‚ Parts are sprayed with a solvent at a pressure of 60 to 80 psi, and the resulting stream is vacuumed into a filter membrane. The membrane can then be inspected by microscope or weighed to detect contaminants.
  • Optical Microscopy‚ While we're on the subject, examining sample parts with high powered microscopes is a simple and effective option, especially for electronics. The drawback? The operators need careful training and the equipment tends to be expensive.
  • Oil Evaporation‚ Another simple, low tech method, but only useful to detect oil residues. A few drops of solvent are placed on the part's surface, then removed and placed on a watch glass to dry. If oils are present, they will leave a visible residue when they dry.
  • Surface Energy Testing‚ If a material is hard and flat, it's possible to predict the size a droplet of a given liquid will form on its surface. Hydrophobic soils will cause the droplets to be smaller; hydrophilic soils will produce larger droplets. The one drawback? It's ineffective on parts that have numerous crevices and holes.

Ultrasonic cleaning delivers unparalleled results, and with ultrasonic cleaners, the advanced SonicTouch II control system and the right validation methods, you can deliver a perfectly spotless finished product.