Lab Testing – Contaminant Loading

The risk of laboratory testing was discussed in the blog Beyond the Cleaning Lab Test Results.  This blog along with others that follow will explore these implications in more detail.

In developing a cleaning process in the test lab, a technician usually starts out by cleaning a small number of parts using freshly prepared and uncontaminated solutions of various cleaning agents in various concentrations using various cleaning mechanisms, temperatures and times to determine which is either effective or most effective.  In most cases, the “sample” represents only a small fraction of the number of parts that ultimately require cleaning.  This is especially true when the parts are costly or when the supply of sample parts is consumed by the necessity to explore several cleaning options.  In the real world, of course, the goal is not to clean just a few parts but to clean parts in large numbers.

I remember the old question, “How many babies can you get clean in a bath tub?” which I always found interesting as when I was growing up I often shared a bath with my siblings.  The inevitable answer, of course, is “it depends!” which leads to the followup question, “depends on what?”  Well, in parts cleaning, a lot!  The first and most obvious issue is that of contamination of the cleaning solution by whatever contaminant is being removed from the parts being cleaned.  Eventually, the cleaning solution will become so contaminated that parts may leave the cleaning solution even more contaminated then when they entered.  Basically like trying to “clean” the baby in mud.  The same effect happens with both insoluble and soluble contaminants.  In the case of insoluble contaminants, the effect may be overcome by filtration.  Soluble contaminants often present a greater challenge as they are typically more difficult to separate out for removal.  Partially soluble contaminants like emulsified oil, may be removed by gravity separation or coalescing but truly solubilized or “miscible” contaminants just continue to collect until re-deposition resulting in a failed cleaning process is inevitable.

In order for a cleaning process to be viable, the effect of cumulative effects of increasing contaminants in the cleaning solution over the long term must be taken into account.  In the laboratory testing stage where the sample size is limited, this may mean intentionally introducing contaminants to simulate what may be seen in cleaning over the long term and then extrapolating backward to anticipate the expected life of the process  under production conditions.  For some contaminants, coolants and oil for example, this exercise is relatively easy.  In the case of insoluble contaminants like buffing compound or metal fines, finding the appropriate source of contaminant to introduce is more difficult an may require assistance from the potential customer.  Whatever is used should be as “authentic” as possible.

Getting parts clean in the lab is not always a good indicator of how successful a process will be in the long run.  Barring those cases where a “clean” part is virtually impossible due to physical damage or a particularly impossible contaminant (cured epoxy or burned on carbon for example), a good lab tech can pretty much get a single part clean in the lab every time.  Finding a practical process that will work over the long term is where true process development is involved.

 –  FJF  –