Having a definition of a “clean” part and a means to quickly and effectively determine if the the criteria for cleanliness has been met is very critical to the success of any cleaning process. The problem, however, is that there are so many ways to define and detect cleanliness that the task can be a daunting one, especially for a new comer. But, even for us that have been in the cleaning business for decades there is the challenge of keeping up on the technologies available to detect cleanliness.
In this series, we will explore ways and methodologies to detect cleanliness. Things like sampling – how many sample parts need be tested from a given lot to assure with a target probability that the entire lot meets the cleanliness requirement? How do you deal with parts that have areas that are cleanliness critical and others areas that are not? How to avoid testing overkill that erodes profit and may not tell you much anyway. What cleanliness test works on what kind of parts and why. What is the effect of surface topography on cleanliness testing? When is destructive testing required and when might a passive test be just as effective and meaningful? And, finally, some things that have been proven NOT to work when it comes to cleanliness testing and why.
The evolution of cleanliness testing is pushed by improvements in cleaning technology overall because many cleanliness testing schemes rely on a better means of cleaning to dislodge and collect contaminants not removed by production cleaning. This is especially true in the case of particle residues. Today, we are testing for contaminating particles that are only a fraction of a micron in size. Under the best conditions the human eye can only detect a particle that is about 10 microns in size or greater. Up until recently, we always that “step ahead” cleaning technology to verify cleanliness of parts cleaned using conventional techniques. Hand cleaning with a solvent and a brush was used to collect particles not removed by water-based cleaning using agitation and spray. Eventually ultrasonic cleaning became the “gold standard.” But today, even ultrasonic cleaning requires a means to verify its effectiveness – what is that technology and what will be next.
Residues other than particles such as oil, grease and other films similarly push the limits of detection. OSEE and other technologies including electron microscopy (to name a few) although they have been around for a long time and are continually developing are beginning to be challenged by the demanding requirement for cleanliness detection.
This series, although not exhaustive in scope, will attempt to arm the reader with some basic information and tips to help direct the search for means to detect cleanliness. A few years ago, I was working with a major manufacturer of medical equipment that had invested thousands of dollars and countless man hours in an attempt to develop a cleanliness test for a part made of a graphite composite. The test was complicated and tedious and yielded a result that was indicative but not definitive. In simple terms, a “supervisor” still had to evaluate the results to give meaning to the test. On seeing the problem, I was able to recommend a test that takes less than a minute and uses common items available in any manufacturing facility, office or even at home. The “quick and dirty” test, as it turns out, was at least as sensitive as the test the manufacturer had labored over for years and had some distinct analytical benefits – but no one there had ever heard of it. I think I’ll tell you more about that in the next blog.
This series on cleanliness testing will be interesting. We’ll explore some tests many will find simple and even silly and others that are pretty sophisticated and stretch technology to the limits. The fact is that they are all real and they have all been effective in one situation or another and are worth knowing about!
As always, thanks for reading along and let’s have fun with cleanliness testing for a while.
– FJF –