The evaluation of cleanliness is often based on a quantification of the particles left on a surface following the conventional or production cleaning process. The first step in evaluating a part for cleanliness with regard to particles is to harvest those particles that remain after cleaning. Once harvested, the particles can be counted, weighed, microscopically examined or whatever to assign a measure of cleanliness. There are many procedures commonly used for harvesting particles from a cleaned surface. It is important that whatever procedure is used, it must remove particles more efficiently and completely than the cleaning process being evaluated. Manual Re-Washing With Solvent - Re-washing is one procedure commonly used to harvest particles. The procedure involves re-washing the part being tested for particle contamination using a solvent. Today, the solvent used is usually a hydrocarbon solvent such as mineral spirits or alcohol although there are numerous other solvent choices that are not uncommon. Clean solvent is either squirted on to the surface under test using a wash bottle (a plastic bottle with a bent tube through the cap) or worked over the surface with a brush. In either case, the solvent used for re-washing is collected along with the harvested particles for evaluation in a second step. Although it sounds rather simple, there are a lot of things to consider when using a re-washing procedure. Cleanliness of the Equipment - The solvent used for re-washing collects not only the particles from the part being tested. Also included are any particles initially present in the solvent, particles that were present on the collection pan prior to the start of the procedure, and particles present on or shed from the wash bottle or brush during the re-washing process. Most specifications for re-washing are quite specific in the procedures to be used to maintain cleanliness of the test equipment. They should be followed religiously. One way to assure that "rogue" contaminants are not included in the harvest is to, as a preliminary step, perform the washing procedure in every detail EXCEPT for the presence of a part. If a brush is being used, substituting a piece of glass or a stainless steel coupon that has been meticulously cleaned before performing the procedure helps to replicate any shedding of the brush that may occur during brushing. Once the procedure has been performed, it should be performed again (with the same "dummy" part used in the first test) using all of the same procedures used in the first re-washing but with fresh solvent. This is a kind of test of the test. Once evaluated and compared, the washings from the two re-washings give a good indication of the sensitivity of the test and establish a "background" for comparison when performing the test on real parts. If the second procedure yields significantly lower measured contamination, there is something wrong with the procedure. Consistency - The re-washing procedure should be thoroughly defined in a written specification. This specification should include, at minimum - -
- A description of the equipment to be used including the wash bottle or brush, collection pan dimensions and material (this is often glass or drawn stainless steel) and the procedures required to assure the cleanliness of this equipment prior to performing the re-washing procedure.
- A detailed description of the solvent used including its source and pedigree (ppm residue etc.) as well as the volume of solvent to be used in the test. There may also be a solvent preparation procedure such as filtration or (less frequently) distillation.
- A detailed description of the washing procedure itself including such things as where and how the solvent should be applied, the number and direction of brush strokes (if applicable) and any other detail that might have an effect on the efficiency of the washing procedure.
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