Most cleaning processes are comprised of a number of discreet steps. A wash followed by one or more rinses and then a dry would be a typical cleaning process. As parts being cleaned are moved from one process step to the next, they are vulnerable to potential risks by conditions not directly related to cleaning. Whether related to time, workplace conditions or, believe it or not, even ambient conditions like the weather (yup, the weather), there are potential perils for parts moving through the cleaning process as they move from one step to the next that must be recognized. Consider, for example - Dry Down - Most cleaning chemistry is intended for use in its liquid form as diluted or dissolved in water. If, however, hot parts are removed from the cleaning solution and allowed to dry before being moved to the next cleaning process step (wash, rinse or whatever) there is a chance that the water diluting the chemistry will evaporate leaving a residue of the cleaning solution and removed contaminant it carries coating the part surface. This is a condition called dry-down and, depending on the temperature of the cleaning solution and the heat capacity of the part, can happen in seconds. Removing the residue resulting from dry-down is often a much greater challenge than the initial cleaning and, in fact, I have seen cases when, once dry-down occurs, the residue can not be removed even by re-cleaning the parts! Dry-down can be avoided or reduced by moving parts as quickly as possible from one cleaning step to the next, by reducing the temperature of the cleaning solution (if possible), or by providing a spray mist to keep the parts wet during transfer from one cleaning process to the next. Flash Rusting - Flash rusting is a condition that occurs primarily on steel parts. Many other metals, however, will oxidize in one form or another when exposed to moisture and oxygen. Many cleaning chemistries include as part of their formulation, ingredients which protect the surfaces of parts from oxidation while they are immersed as discussed in a previous blog. In most cases, carry - over of a small amount of cleaning chemistry from the cleaning process to the first rinse (immersion rinse) provides a small amount of rust protection at least through that first rinse. However, as the number of rinses is increased or the quality of the rinse water increases, the problem of flash rusting becomes more pronounced. As discussed earlier, the rusting of steel only occurs when both oxygen and water are present but increased temperature speeds up the process. Flash rusting is a condition that, once discovered, may require some creative solutions. Decresing rinse temperatures is sometimes helpful as is reducing the transfer time from one process to the next. Ultimately, however, most steel parts require a rust inhibitor in the rinse to eliminate the problem. Contamination by the Ambient Environment - It never ceases to amaze me that people expect to make super-clean parts in a dirty environment. Contaminants in the environment can include particles, oil (in a mist form), moisture in the form of humidity or atomized water and a myriad of other things. The most effective way to prevent contamination from the environment is to control and protect the environment by enclosing the process in a "clean room." Different levels of cleanliness are required for different applications. A "clean room" need not be the type requiring "bunny suits" etc. for most industrial cleaning processes but some sort of control is imperative. I will never forget visiting one customer's "clean room" (quite a high level clean room, in fact) and seeing a worker eating a peanut butter sandwich at her desk located within the room. The moral of this story, I guess, is that a "clean room" does not insure a clean environment without the proper attention to control. The above suggests some of the many conditions that can result in otherwise clean parts being contaminated or damaged during cleaning. In most cases, these problems are not identified in the laboratory during development of a cleaning process but, rather, once it is implemented in production. When troubleshooting a cleaning process, the process engineer needs to know that there are such variables and conditions which require recognition for successful cleaning.
- FJF -