Affordable Cleaning Tests

“What I’d really like to have is a wealthy person’s approach to cleaning tests, but on a pauper’s budget.” This comment by an attendee at the Parts Cleaning Expo earlier this year reflects complaints common among part manufacturers: Analytical testing is costly, it is often time-consuming, and the answers these tests produce may not be useful.

We know of no test that absolutely guarantees a clean surface. Sometimes, sending a sample to a lab to be analyzed is the most direct way to solve a contamination problem. Before you call a lab, however, you can try simple, cost-effective tests to screen products for unwanted contaminants. Here are a few examples of such test measures.

Look at the part surface under standard conditions. “Standard conditions” are the kind of lighting or magnification that provide the most informative view of the surface of the part. Look at the surface using white light and a black UV light. Use a digital microscope and take photographs. What do you see? Does the surface look oily? Can you see particles? Some contaminants fluoresce. 

Drop water on the surface of the part. If you drop water on a freshly waxed automobile, it should bead up, indicating an organic (oily) residue. In such a water-break test on a part, water that spreads smoothly indicates a clean surface. Any fine particles or surfactants remaining on the part would interfere with this smooth spreading and show up as beading. Contact-angle tests allow users to assign a number to the degree of beading. A new, low-cost approach to contact angle measurement uses simple equipment and the camera on a cellphone.¹

Make the dirt easy to see. Use a “white glove” test to reveal hard-to-see dirt, wiping down the part with a clean white cloth. Or use clear tape to lift dirt off the surface. Do you see metal fines? Oil? Particles of lapping compound?

What if the dirt is stuck in blind holes and helicoils; what if the dirt is so diffuse you still can’t see it? Labs use techniques such as “extractive analysis,” a fancy term for removing soils by washing a surface with pure water or solvent (the extraction solution). 

There are three provisos for extraction. First, manage all chemicals safely; keep flammable solvents away from heat and combustion sources. Second, use extracting liquids that are residue-free; aqueous cleaning agents are likely to leave behind interfering residue. Third, select extraction fluids that pick up the soils of interest; water is not effective for picking up oil, for example. Test more than one extraction fluid. 

Once you have determined the appropriate extraction solution, you can place many small parts in a container with the solution and then slosh them around (or use an ultrasonic tank). With large parts, you might have to repeatedly run the solution over their surfaces. 

Next, concentrate the wash solution by evaporation, and deposit the concentrate onto a clean mirror for final drying. Once it has dried, look at the residue remaining on the mirror. Is it oily? Powdery? Does it contain particles? Maybe you can identify the particles as metal fines, skin flakes or fabric fibers. Further processing by gravimetric or chemical analysis may help further identity and quantify the residue.

The key to performing cost-effective tests is critical thinking and observation. Document everything you do and see so that any testing can be repeated.  Most companies, whether large or small, have limited budgets. They need to find effective and affordable methods for testing the quality of their products. Basic detective work like that described here can help identify potential contaminants and reduce the need for or at least the cost of complex tests. 

Reference
¹Williams, et al. “Contact Angle Measurements Using Cellphone Cameras to Implement the Bikerman Method,” Galvanotechnik. 102. 1718-1725 (2011).