Having a means to evaluate and monitor the cleanliness of parts after cleaning is essential to assure the success and consistency of any cleaning process. Selecting and properly applying the appropriate cleanliness test for a particular application is a task that should not be taken lightly.
A cleaning test should be able to quantify the nature and amount of contaminant remaining on a part after cleaning with sufficient accuracy and repeatability to assure that the part being tested is clean enough for its intended use. A toy gyroscope, for example, does not need to meet the same cleanliness standard as a gyroscope used in a satellite guidance system. A simple visual evaluation is probably adequate for the toy while the guidance device both needs and merits a much more thorough evaluation. So first and foremost, a cleanliness test must make sense from both a practical and economic standpoint.
When establishing a cleanliness standard one should keep the following points in mind.
- The test should detect the contaminant of concern. For example, if particles are the problem, it should detect particles, if oil is the problem it should detect oil.
- The test should be adequately but not overly sensitive. Detecting residues of 1 part per million is not necessary when the process will tolerate up to 100 parts per million. Likewise, detecting a 1 micron particle is not necessary when the threshold particle is 100 microns. “Overkill” is usually expensive and may lead to unnecessary disputes.
- The test should be suitable for execution by the available personnel in the available environment. If the selected test requires a lab or other facility that is not available, you either need to add that facility or consider a different test.
- The test should support thorough documentation in a way that will minimize or eliminate the need for periodic supervisory intervention and prevent the inevitable effect of memory lapse.
- The test should provide results that unambiguous and can be quantified. There must be a clear definition between a “good” part and a “bad” part without the need for interpretation.
Once a cleanliness test is selected, it should be documented in detail. Documentation should include very specific information regarding the equipment to be used, how samples are to be taken, how the test is conducted and how the results are to be reported. The test should then be performed by several individuals previously unfamiliar with the testing procedure. The results should be the same independent of the person conducting the test on the same or similar samples from the same production lot. Again, nothing should be left to interpretation or “training” as over generations information will, inevitably, be lost or twisted.
Once correctly implemented, cleanliness testing helps assure process consistency by providing the necessary feedback to not only the cleaning process but processes preceding it as well. It provides early warning indicators that are invaluable to maintaining process control but only if properly selected and applied.