What Is Clean? Part I

One of the most common phrases I hear from metal finishing managers when referring to their pretreatment cycle and working with them in choosing a cleaner for their process is, “We need the part to be clean.” But what is clean? How do you define clean? Can parts be too clean?  From parts washing between assembly steps at large automotive manufacturers, to clean assembly rooms, to parts involved in paint and plating pretreatment, the word “clean” takes on several definitions and concepts. In this two-part article we will describe:

  • Our definition of “clean” to meet the gambit of finishing operations – regardless of specificity
  • The importance of a clean surface
  • A common technique used to determine if your parts are “clean”

Part Washing Processes: Start Clean

It is said, “Cleanliness is next to godliness”, and this statement holds no truer in the metal finishing industry than in any other industry. Part washing is the first step in almost all metal finishing processes. If the first step is not done properly, or to expectations in any process, whether it is in athletics, sales, dating, or metal finishing, the odds increase exponentially that there will be failures further down the process. Unclean parts can cause a variety of issues: in the finish itself – adhesion (blisters, peeling, rubber bonding), spots of bare substrate (“fish eyes”, skip plating, exposed metal where plating went around the oil spot), unappealing final finish (dullness, streaks, patterns, etc) and weld failures (holes in the weld, “leakers”).

Performance failures of the final assembly – metal chips blocking orifices, coatings contaminating processes the parts may be used to transfer; contamination of other process tanks – build up in coating tanks can eventually destroy expensive plating baths, cripple plating efficiencies, and in worst cases scenarios, shut down entire process lines, requiring dumping of all tanks and power washing the line. Studies are going on now to determine the effects of more effective cleaning in the industries such as recycling and galvanizing to improve quality and process bath effectiveness. For decades, these industries have just used cheap and simple NaOH solutions for cleaning; early findings have been showing significant improvements further down the process from more effective cleaning, saving thousands of dollars in chemical use or earning thousands of dollars in quality improvement.

Clean is: A Contaminate-Free Surface

We are recognizing that a clean surface is a surface that will meet all the quality and specifications to accept the next process, free of contaminates that may affect later processes. So put another way, what is clean? A clean surface is a surface free of contaminates that directly affect the finish or the finishing operation.  As the rule of thumb goes with rinsing – use the cleanest possible water available and economical to rinse with; we believe the same holds true for cleaning – assure the surface is free from as many contaminates as possible before further processing.  You can rarely go wrong with a surface being “too clean”, but there is a significant amounts of what if questions if the surface is not clean enough. A majority of the above quality issues can all be avoided when the part is truly clean (free of all contaminates). Too many shops are accepting of unclean parts not realizing the long term affects on their finishes and/or process tanks. The metal finishing industry is seeing changes in soils, changes in product loads, or other changes within the process before or after the cleaning steps. Yet there is an expectation the cleaner used since the 1980’s should be sufficient to handle these changes and many times without considering the economical impact of increased usage, bath contamination, and increased re-work – seems logical?