Cleaning, as we’ve all known it for years, is a matter of taking a “dirty” part and making it a “clean” part using whatever means is necessary and feasible. Typically, the cleaning process involves mechanical removal of contaminants or the use of a chemistry to dissolve and dilute the contaminant. In the case of an insoluble contaminant, chemistry is used to dissolve whatever is holding the contaminant in place and prevent its re-attachment to the substrate so it can be flushed away. These concepts have served us well since the beginning of cleaning history.
As cleanliness specifications tighten to meet the needs of advancing technology, we are pushing closer and closer to allowing ABSOLUTELY NOTHING to remain on a “clean” part! As we approach the ultimate in cleanliness, the concepts of cleaning as defined in our opening paragraph start to lose meaning. Parts that were considered absolutely clean a few months ago are all of a sudden unable to meet tightening cleanliness standards. And, try as we might, tweaking of the old standby variables of time, temperature, chemistry and agitation can’t get us to the required cleanliness. Clearly, either our concept of “clean” and/or the whole discipline of cleaning requires a re-think.
Although I would take this opportunity to suggest that cleanliness specifications are often aggressive to the point that they can not be properly administered, it’s not likely that they will be relaxed or substantially restructured in the foreseeable future. In some cases, in fact, the cleanliness testing process itself has been shown to generate the very contaminant it is meant to detect (a subject for a future blog). So, having “tweaked” traditional cleaning parameters to the limit without achieving the required result, we are now forced to look elsewhere for ways to further improve cleanliness.
In an increasing number of cases, sources of “permanent” contamination are being traced to the origin of the part or the processes used in its manufacture. In fact, the cleaning process itself can be a significant source of contamination! Cleaning must start on the drawing board!
Take, for example, recent trials conducted on a part made of nodular steel cast in green sand. This part, which is a component of an internal combustion engine, is cast and then machined, in part, to a precision finish. The customer’s cleanliness specification requires that no particles greater than 75 microns in size be found using a re-wash and filter technique after manufacturing and cleaning. In fact, laboratory tests using the most aggressive practical cleaning technologies currently available were found unable to achieve this cleanliness level!
In an attempt to establish a suitable cleaning technology, a single part was re-cleaned as many as ten times with a cleanliness test performed after each cleaning. After ten cleanings, the specification was still not met as the part continued to shed particles with dimensions in excess of 75 microns. The “as cast” surface was suspected as the source of the particulate contamination. To eliminate the “as cast” surface from consideration, various masking techniques including TI coating were tried as methods to contain the offending particles. Re-washing continued to reveal particles. In one final attempt, the entire part was finished to a precision surface. But, even the precision-machined part was found to shed particles larger than those allowed after up to ten successive washings. In the end, it was determined that this part, because of its composition and method of manufacture, will not likely ever meet the customer’s specification for cleanliness without a major change in the material and perhaps the manufacturing process as well.
In another case, parts made of stainless steel, titanium and various “super alloys were found to fail clean room cleanliness testing due to particulate contamination described as “metal flakes.” In this case, many of these small parts were machined from solid blocks of metal so the nature of the substrate was not an issue! On researching the reason for these failures, two things were discovered. First, in the course of the manufacturing process, many surfaces of the parts were ground and polished frequently for purely aesthetic reasons. This polishing, of course, caused smearing of the surface of the metal creating layers of loosely adhered “metal flakes.” Additionally, to collect re-wash samples for cleanliness testing, the parts were processed in an ultrasonic cleaner. Ultrasonic energy was effective in breaking free the loosely adhering flakes. In this case, the parts were re-cleaned until no further metal flakes were found. In fact, the final machining process was performed as part of the testing when the parts were ultrasonically cleaned until they no longer shed flakes. A better solution was to eliminate the process steps that created the flakes in the first place.
One can not overlook the cleaning equipment itself as a potential source of contamination. Many cleaning machines and associated cleaning hardware including material handling systems, baskets and fixtures may introduce contamination in excess of that found on the “dirty” parts. Take, for example, the familiar plastic-framed basket with metal side and bottom inserts. Although this widely used basket is acceptable for the vast majority of cleaning applications, it may serve as a significant source of contamination in critical cleaning applications. Particles of plastic or the fiberglass or other materials used to strengthen the plastic, as well as metal chips generated as parts rub against the metal basket sides and bottom, are both found in examinations of washings from parts cleaned using these baskets. Welds and fasteners are also potential sources of contamination. In some cases, improperly designed baskets or fixtures trap and harbor contamination transferring it through the cleaning process and eventually into the rinses where it is re-deposited on the cleaned parts.
Finally, the cleaning machine itself may generate contamination. Obvious sources may include lubricants and particles generated by wearing parts including bearings and sliding surfaces. Again, these sources are of little or no concern in run-of-the-mill cleaning, but may emerge as significant as cleaning specifications tighten. Hidden sources include pumps, pipe fittings, nozzles, valves, and filter housings. Any of the former which start life as castings are particularly worthy of careful scrutiny. Basically any surface in contact with the part being cleaned or the liquid contacting the part must be considered as a source or purveyor of contamination.
And so –
Parts makers have been telling me quietly for years that they are able (apparently at will) to make parts that can not be cleaned! I suspect that many of the parts I received for laboratory testing through the years were prime examples of these although they were always cleverly presented as “worst case” parts. It appears that now may be the time to explore that art in reverse to make parts that can be cleaned to meet new, more critical cleanliness requirements. It’s also time to look beyond the self-limiting parameters of time, temperature, chemistry and agitation to further advance the case for part cleanliness. Better answers may come from the consideration of different materials of construction, new and better “clean” fabrication techniques, and equipment designs and cleaning technologies which promote and protect cleanliness. Both manufacturing and cleaning methods will require significant change in the very near future to keep up with the increased cleanliness demands of advancing technology. With apologies to the US Navy, then, – – “Let The Adventure Begin!”
– FJF –